call.go

     1  // Copyright 2013 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file implements typechecking of call and selector expressions.
     6  
     7  package types
     8  
     9  import (
    10  	"go/ast"
    11  	"go/token"
    12  	. "internal/types/errors"
    13  	"strings"
    14  )
    15  
    16  // funcInst type-checks a function instantiation.
    17  // The incoming x must be a generic function.
    18  // If ix != nil, it provides some or all of the type arguments (ix.Indices).
    19  // If target != nil, it may be used to infer missing type arguments of x, if any.
    20  // At least one of T or ix must be provided.
    21  //
    22  // There are two modes of operation:
    23  //
    24  //  1. If infer == true, funcInst infers missing type arguments as needed and
    25  //     instantiates the function x. The returned results are nil.
    26  //
    27  //  2. If infer == false and inst provides all type arguments, funcInst
    28  //     instantiates the function x. The returned results are nil.
    29  //     If inst doesn't provide enough type arguments, funcInst returns the
    30  //     available arguments; x remains unchanged.
    31  //
    32  // If an error (other than a version error) occurs in any case, it is reported
    33  // and x.mode is set to invalid.
    34  func (check *Checker) funcInst(T *target, pos token.Pos, x *operand, ix *indexedExpr, infer bool) []Type {
    35  	assert(T != nil || ix != nil)
    36  
    37  	var instErrPos positioner
    38  	if ix != nil {
    39  		instErrPos = inNode(ix.orig, ix.lbrack)
    40  		x.expr = ix.orig // if we don't have an index expression, keep the existing expression of x
    41  	} else {
    42  		instErrPos = atPos(pos)
    43  	}
    44  	versionErr := !check.verifyVersionf(instErrPos, go1_18, "function instantiation")
    45  
    46  	// targs and xlist are the type arguments and corresponding type expressions, or nil.
    47  	var targs []Type
    48  	var xlist []ast.Expr
    49  	if ix != nil {
    50  		xlist = ix.indices
    51  		targs = check.typeList(xlist)
    52  		if targs == nil {
    53  			x.mode = invalid
    54  			return nil
    55  		}
    56  		assert(len(targs) == len(xlist))
    57  	}
    58  
    59  	// Check the number of type arguments (got) vs number of type parameters (want).
    60  	// Note that x is a function value, not a type expression, so we don't need to
    61  	// call Underlying below.
    62  	sig := x.typ.(*Signature)
    63  	got, want := len(targs), sig.TypeParams().Len()
    64  	if got > want {
    65  		// Providing too many type arguments is always an error.
    66  		check.errorf(ix.indices[got-1], WrongTypeArgCount, "got %d type arguments but want %d", got, want)
    67  		x.mode = invalid
    68  		return nil
    69  	}
    70  
    71  	if got < want {
    72  		if !infer {
    73  			return targs
    74  		}
    75  
    76  		// If the uninstantiated or partially instantiated function x is used in
    77  		// an assignment (tsig != nil), infer missing type arguments by treating
    78  		// the assignment
    79  		//
    80  		//    var tvar tsig = x
    81  		//
    82  		// like a call g(tvar) of the synthetic generic function g
    83  		//
    84  		//    func g[type_parameters_of_x](func_type_of_x)
    85  		//
    86  		var args []*operand
    87  		var params []*Var
    88  		var reverse bool
    89  		if T != nil && sig.tparams != nil {
    90  			if !versionErr && !check.allowVersion(go1_21) {
    91  				if ix != nil {
    92  					check.versionErrorf(instErrPos, go1_21, "partially instantiated function in assignment")
    93  				} else {
    94  					check.versionErrorf(instErrPos, go1_21, "implicitly instantiated function in assignment")
    95  				}
    96  			}
    97  			gsig := NewSignatureType(nil, nil, nil, sig.params, sig.results, sig.variadic)
    98  			params = []*Var{NewParam(x.Pos(), check.pkg, "", gsig)}
    99  			// The type of the argument operand is tsig, which is the type of the LHS in an assignment
   100  			// or the result type in a return statement. Create a pseudo-expression for that operand
   101  			// that makes sense when reported in error messages from infer, below.
   102  			expr := ast.NewIdent(T.desc)
   103  			expr.NamePos = x.Pos() // correct position
   104  			args = []*operand{{mode: value, expr: expr, typ: T.sig}}
   105  			reverse = true
   106  		}
   107  
   108  		// Rename type parameters to avoid problems with recursive instantiations.
   109  		// Note that NewTuple(params...) below is (*Tuple)(nil) if len(params) == 0, as desired.
   110  		tparams, params2 := check.renameTParams(pos, sig.TypeParams().list(), NewTuple(params...))
   111  
   112  		err := check.newError(CannotInferTypeArgs)
   113  		targs = check.infer(atPos(pos), tparams, targs, params2.(*Tuple), args, reverse, err)
   114  		if targs == nil {
   115  			if !err.empty() {
   116  				err.report()
   117  			}
   118  			x.mode = invalid
   119  			return nil
   120  		}
   121  		got = len(targs)
   122  	}
   123  	assert(got == want)
   124  
   125  	// instantiate function signature
   126  	sig = check.instantiateSignature(x.Pos(), x.expr, sig, targs, xlist)
   127  	x.typ = sig
   128  	x.mode = value
   129  	return nil
   130  }
   131  
   132  func (check *Checker) instantiateSignature(pos token.Pos, expr ast.Expr, typ *Signature, targs []Type, xlist []ast.Expr) (res *Signature) {
   133  	assert(check != nil)
   134  	assert(len(targs) == typ.TypeParams().Len())
   135  
   136  	if check.conf._Trace {
   137  		check.trace(pos, "-- instantiating signature %s with %s", typ, targs)
   138  		check.indent++
   139  		defer func() {
   140  			check.indent--
   141  			check.trace(pos, "=> %s (under = %s)", res, res.Underlying())
   142  		}()
   143  	}
   144  
   145  	// For signatures, Checker.instance will always succeed because the type argument
   146  	// count is correct at this point (see assertion above); hence the type assertion
   147  	// to *Signature will always succeed.
   148  	inst := check.instance(pos, typ, targs, nil, check.context()).(*Signature)
   149  	assert(inst.TypeParams().Len() == 0) // signature is not generic anymore
   150  	check.recordInstance(expr, targs, inst)
   151  	assert(len(xlist) <= len(targs))
   152  
   153  	// verify instantiation lazily (was go.dev/issue/50450)
   154  	check.later(func() {
   155  		tparams := typ.TypeParams().list()
   156  		// check type constraints
   157  		if i, err := check.verify(pos, tparams, targs, check.context()); err != nil {
   158  			// best position for error reporting
   159  			pos := pos
   160  			if i < len(xlist) {
   161  				pos = xlist[i].Pos()
   162  			}
   163  			check.softErrorf(atPos(pos), InvalidTypeArg, "%s", err)
   164  		} else {
   165  			check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
   166  		}
   167  	}).describef(atPos(pos), "verify instantiation")
   168  
   169  	return inst
   170  }
   171  
   172  func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
   173  	ix := unpackIndexedExpr(call.Fun)
   174  	if ix != nil {
   175  		if check.indexExpr(x, ix) {
   176  			// Delay function instantiation to argument checking,
   177  			// where we combine type and value arguments for type
   178  			// inference.
   179  			assert(x.mode == value)
   180  		} else {
   181  			ix = nil
   182  		}
   183  		x.expr = call.Fun
   184  		check.record(x)
   185  	} else {
   186  		check.exprOrType(x, call.Fun, true)
   187  	}
   188  	// x.typ may be generic
   189  
   190  	switch x.mode {
   191  	case invalid:
   192  		check.use(call.Args...)
   193  		x.expr = call
   194  		return statement
   195  
   196  	case typexpr:
   197  		// conversion
   198  		check.nonGeneric(nil, x)
   199  		if x.mode == invalid {
   200  			return conversion
   201  		}
   202  		T := x.typ
   203  		x.mode = invalid
   204  		switch n := len(call.Args); n {
   205  		case 0:
   206  			check.errorf(inNode(call, call.Rparen), WrongArgCount, "missing argument in conversion to %s", T)
   207  		case 1:
   208  			check.expr(nil, x, call.Args[0])
   209  			if x.mode != invalid {
   210  				if hasDots(call) {
   211  					check.errorf(call.Args[0], BadDotDotDotSyntax, "invalid use of ... in conversion to %s", T)
   212  					break
   213  				}
   214  				if t, _ := T.Underlying().(*Interface); t != nil && !isTypeParam(T) {
   215  					if !t.IsMethodSet() {
   216  						check.errorf(call, MisplacedConstraintIface, "cannot use interface %s in conversion (contains specific type constraints or is comparable)", T)
   217  						break
   218  					}
   219  				}
   220  				check.conversion(x, T)
   221  			}
   222  		default:
   223  			check.use(call.Args...)
   224  			check.errorf(call.Args[n-1], WrongArgCount, "too many arguments in conversion to %s", T)
   225  		}
   226  		x.expr = call
   227  		return conversion
   228  
   229  	case builtin:
   230  		// no need to check for non-genericity here
   231  		id := x.id
   232  		if !check.builtin(x, call, id) {
   233  			x.mode = invalid
   234  		}
   235  		x.expr = call
   236  		// a non-constant result implies a function call
   237  		if x.mode != invalid && x.mode != constant_ {
   238  			check.hasCallOrRecv = true
   239  		}
   240  		return predeclaredFuncs[id].kind
   241  	}
   242  
   243  	// ordinary function/method call
   244  	// signature may be generic
   245  	cgocall := x.mode == cgofunc
   246  
   247  	// If the operand type is a type parameter, all types in its type set
   248  	// must have a common underlying type, which must be a signature.
   249  	u, err := commonUnder(x.typ, func(t, u Type) *typeError {
   250  		if _, ok := u.(*Signature); u != nil && !ok {
   251  			return typeErrorf("%s is not a function", t)
   252  		}
   253  		return nil
   254  	})
   255  	if err != nil {
   256  		check.errorf(x, InvalidCall, invalidOp+"cannot call %s: %s", x, err.format(check))
   257  		x.mode = invalid
   258  		x.expr = call
   259  		return statement
   260  	}
   261  	sig := u.(*Signature) // u must be a signature per the commonUnder condition
   262  
   263  	// Capture wasGeneric before sig is potentially instantiated below.
   264  	wasGeneric := sig.TypeParams().Len() > 0
   265  
   266  	// evaluate type arguments, if any
   267  	var xlist []ast.Expr
   268  	var targs []Type
   269  	if ix != nil {
   270  		xlist = ix.indices
   271  		targs = check.typeList(xlist)
   272  		if targs == nil {
   273  			check.use(call.Args...)
   274  			x.mode = invalid
   275  			x.expr = call
   276  			return statement
   277  		}
   278  		assert(len(targs) == len(xlist))
   279  
   280  		// check number of type arguments (got) vs number of type parameters (want)
   281  		got, want := len(targs), sig.TypeParams().Len()
   282  		if got > want {
   283  			check.errorf(xlist[want], WrongTypeArgCount, "got %d type arguments but want %d", got, want)
   284  			check.use(call.Args...)
   285  			x.mode = invalid
   286  			x.expr = call
   287  			return statement
   288  		}
   289  
   290  		// If sig is generic and all type arguments are provided, preempt function
   291  		// argument type inference by explicitly instantiating the signature. This
   292  		// ensures that we record accurate type information for sig, even if there
   293  		// is an error checking its arguments (for example, if an incorrect number
   294  		// of arguments is supplied).
   295  		if got == want && want > 0 {
   296  			check.verifyVersionf(atPos(ix.lbrack), go1_18, "function instantiation")
   297  			sig = check.instantiateSignature(ix.Pos(), ix.orig, sig, targs, xlist)
   298  			// targs have been consumed; proceed with checking arguments of the
   299  			// non-generic signature.
   300  			targs = nil
   301  			xlist = nil
   302  		}
   303  	}
   304  
   305  	// evaluate arguments
   306  	args, atargs := check.genericExprList(call.Args)
   307  	sig = check.arguments(call, sig, targs, xlist, args, atargs)
   308  
   309  	if wasGeneric && sig.TypeParams().Len() == 0 {
   310  		// Update the recorded type of call.Fun to its instantiated type.
   311  		check.recordTypeAndValue(call.Fun, value, sig, nil)
   312  	}
   313  
   314  	// determine result
   315  	switch sig.results.Len() {
   316  	case 0:
   317  		x.mode = novalue
   318  	case 1:
   319  		if cgocall {
   320  			x.mode = commaerr
   321  		} else {
   322  			x.mode = value
   323  		}
   324  		x.typ = sig.results.vars[0].typ // unpack tuple
   325  	default:
   326  		x.mode = value
   327  		x.typ = sig.results
   328  	}
   329  	x.expr = call
   330  	check.hasCallOrRecv = true
   331  
   332  	// if type inference failed, a parameterized result must be invalidated
   333  	// (operands cannot have a parameterized type)
   334  	if x.mode == value && sig.TypeParams().Len() > 0 && isParameterized(sig.TypeParams().list(), x.typ) {
   335  		x.mode = invalid
   336  	}
   337  
   338  	return statement
   339  }
   340  
   341  // exprList evaluates a list of expressions and returns the corresponding operands.
   342  // A single-element expression list may evaluate to multiple operands.
   343  func (check *Checker) exprList(elist []ast.Expr) (xlist []*operand) {
   344  	if n := len(elist); n == 1 {
   345  		xlist, _ = check.multiExpr(elist[0], false)
   346  	} else if n > 1 {
   347  		// multiple (possibly invalid) values
   348  		xlist = make([]*operand, n)
   349  		for i, e := range elist {
   350  			var x operand
   351  			check.expr(nil, &x, e)
   352  			xlist[i] = &x
   353  		}
   354  	}
   355  	return
   356  }
   357  
   358  // genericExprList is like exprList but result operands may be uninstantiated or partially
   359  // instantiated generic functions (where constraint information is insufficient to infer
   360  // the missing type arguments) for Go 1.21 and later.
   361  // For each non-generic or uninstantiated generic operand, the corresponding targsList and
   362  // elements do not exist (targsList is nil) or the elements are nil.
   363  // For each partially instantiated generic function operand, the corresponding
   364  // targsList elements are the operand's partial type arguments.
   365  func (check *Checker) genericExprList(elist []ast.Expr) (resList []*operand, targsList [][]Type) {
   366  	if debug {
   367  		defer func() {
   368  			// type arguments must only exist for partially instantiated functions
   369  			for i, x := range resList {
   370  				if i < len(targsList) {
   371  					if n := len(targsList[i]); n > 0 {
   372  						// x must be a partially instantiated function
   373  						assert(n < x.typ.(*Signature).TypeParams().Len())
   374  					}
   375  				}
   376  			}
   377  		}()
   378  	}
   379  
   380  	// Before Go 1.21, uninstantiated or partially instantiated argument functions are
   381  	// nor permitted. Checker.funcInst must infer missing type arguments in that case.
   382  	infer := true // for -lang < go1.21
   383  	n := len(elist)
   384  	if n > 0 && check.allowVersion(go1_21) {
   385  		infer = false
   386  	}
   387  
   388  	if n == 1 {
   389  		// single value (possibly a partially instantiated function), or a multi-valued expression
   390  		e := elist[0]
   391  		var x operand
   392  		if ix := unpackIndexedExpr(e); ix != nil && check.indexExpr(&x, ix) {
   393  			// x is a generic function.
   394  			targs := check.funcInst(nil, x.Pos(), &x, ix, infer)
   395  			if targs != nil {
   396  				// x was not instantiated: collect the (partial) type arguments.
   397  				targsList = [][]Type{targs}
   398  				// Update x.expr so that we can record the partially instantiated function.
   399  				x.expr = ix.orig
   400  			} else {
   401  				// x was instantiated: we must record it here because we didn't
   402  				// use the usual expression evaluators.
   403  				check.record(&x)
   404  			}
   405  			resList = []*operand{&x}
   406  		} else {
   407  			// x is not a function instantiation (it may still be a generic function).
   408  			check.rawExpr(nil, &x, e, nil, true)
   409  			check.exclude(&x, 1<<novalue|1<<builtin|1<<typexpr)
   410  			if t, ok := x.typ.(*Tuple); ok && x.mode != invalid {
   411  				// x is a function call returning multiple values; it cannot be generic.
   412  				resList = make([]*operand, t.Len())
   413  				for i, v := range t.vars {
   414  					resList[i] = &operand{mode: value, expr: e, typ: v.typ}
   415  				}
   416  			} else {
   417  				// x is exactly one value (possibly invalid or uninstantiated generic function).
   418  				resList = []*operand{&x}
   419  			}
   420  		}
   421  	} else if n > 1 {
   422  		// multiple values
   423  		resList = make([]*operand, n)
   424  		targsList = make([][]Type, n)
   425  		for i, e := range elist {
   426  			var x operand
   427  			if ix := unpackIndexedExpr(e); ix != nil && check.indexExpr(&x, ix) {
   428  				// x is a generic function.
   429  				targs := check.funcInst(nil, x.Pos(), &x, ix, infer)
   430  				if targs != nil {
   431  					// x was not instantiated: collect the (partial) type arguments.
   432  					targsList[i] = targs
   433  					// Update x.expr so that we can record the partially instantiated function.
   434  					x.expr = ix.orig
   435  				} else {
   436  					// x was instantiated: we must record it here because we didn't
   437  					// use the usual expression evaluators.
   438  					check.record(&x)
   439  				}
   440  			} else {
   441  				// x is exactly one value (possibly invalid or uninstantiated generic function).
   442  				check.genericExpr(&x, e)
   443  			}
   444  			resList[i] = &x
   445  		}
   446  	}
   447  
   448  	return
   449  }
   450  
   451  // arguments type-checks arguments passed to a function call with the given signature.
   452  // The function and its arguments may be generic, and possibly partially instantiated.
   453  // targs and xlist are the function's type arguments (and corresponding expressions).
   454  // args are the function arguments. If an argument args[i] is a partially instantiated
   455  // generic function, atargs[i] are the corresponding type arguments.
   456  // If the callee is variadic, arguments adjusts its signature to match the provided
   457  // arguments. The type parameters and arguments of the callee and all its arguments
   458  // are used together to infer any missing type arguments, and the callee and argument
   459  // functions are instantiated as necessary.
   460  // The result signature is the (possibly adjusted and instantiated) function signature.
   461  // If an error occurred, the result signature is the incoming sig.
   462  func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type, xlist []ast.Expr, args []*operand, atargs [][]Type) (rsig *Signature) {
   463  	rsig = sig
   464  
   465  	// Function call argument/parameter count requirements
   466  	//
   467  	//               | standard call    | dotdotdot call |
   468  	// --------------+------------------+----------------+
   469  	// standard func | nargs == npars   | invalid        |
   470  	// --------------+------------------+----------------+
   471  	// variadic func | nargs >= npars-1 | nargs == npars |
   472  	// --------------+------------------+----------------+
   473  
   474  	nargs := len(args)
   475  	npars := sig.params.Len()
   476  	ddd := hasDots(call)
   477  
   478  	// set up parameters
   479  	sigParams := sig.params // adjusted for variadic functions (may be nil for empty parameter lists!)
   480  	adjusted := false       // indicates if sigParams is different from sig.params
   481  	if sig.variadic {
   482  		if ddd {
   483  			// variadic_func(a, b, c...)
   484  			if len(call.Args) == 1 && nargs > 1 {
   485  				// f()... is not permitted if f() is multi-valued
   486  				check.errorf(inNode(call, call.Ellipsis), InvalidDotDotDot, "cannot use ... with %d-valued %s", nargs, call.Args[0])
   487  				return
   488  			}
   489  		} else {
   490  			// variadic_func(a, b, c)
   491  			if nargs >= npars-1 {
   492  				// Create custom parameters for arguments: keep
   493  				// the first npars-1 parameters and add one for
   494  				// each argument mapping to the ... parameter.
   495  				vars := make([]*Var, npars-1) // npars > 0 for variadic functions
   496  				copy(vars, sig.params.vars)
   497  				last := sig.params.vars[npars-1]
   498  				typ := last.typ.(*Slice).elem
   499  				for len(vars) < nargs {
   500  					vars = append(vars, NewParam(last.pos, last.pkg, last.name, typ))
   501  				}
   502  				sigParams = NewTuple(vars...) // possibly nil!
   503  				adjusted = true
   504  				npars = nargs
   505  			} else {
   506  				// nargs < npars-1
   507  				npars-- // for correct error message below
   508  			}
   509  		}
   510  	} else {
   511  		if ddd {
   512  			// standard_func(a, b, c...)
   513  			check.errorf(inNode(call, call.Ellipsis), NonVariadicDotDotDot, "cannot use ... in call to non-variadic %s", call.Fun)
   514  			return
   515  		}
   516  		// standard_func(a, b, c)
   517  	}
   518  
   519  	// check argument count
   520  	if nargs != npars {
   521  		var at positioner = call
   522  		qualifier := "not enough"
   523  		if nargs > npars {
   524  			at = args[npars].expr // report at first extra argument
   525  			qualifier = "too many"
   526  		} else {
   527  			at = atPos(call.Rparen) // report at closing )
   528  		}
   529  		// take care of empty parameter lists represented by nil tuples
   530  		var params []*Var
   531  		if sig.params != nil {
   532  			params = sig.params.vars
   533  		}
   534  		err := check.newError(WrongArgCount)
   535  		err.addf(at, "%s arguments in call to %s", qualifier, call.Fun)
   536  		err.addf(noposn, "have %s", check.typesSummary(operandTypes(args), false, ddd))
   537  		err.addf(noposn, "want %s", check.typesSummary(varTypes(params), sig.variadic, false))
   538  		err.report()
   539  		return
   540  	}
   541  
   542  	// collect type parameters of callee and generic function arguments
   543  	var tparams []*TypeParam
   544  
   545  	// collect type parameters of callee
   546  	n := sig.TypeParams().Len()
   547  	if n > 0 {
   548  		if !check.allowVersion(go1_18) {
   549  			switch call.Fun.(type) {
   550  			case *ast.IndexExpr, *ast.IndexListExpr:
   551  				ix := unpackIndexedExpr(call.Fun)
   552  				check.versionErrorf(inNode(call.Fun, ix.lbrack), go1_18, "function instantiation")
   553  			default:
   554  				check.versionErrorf(inNode(call, call.Lparen), go1_18, "implicit function instantiation")
   555  			}
   556  		}
   557  		// rename type parameters to avoid problems with recursive calls
   558  		var tmp Type
   559  		tparams, tmp = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
   560  		sigParams = tmp.(*Tuple)
   561  		// make sure targs and tparams have the same length
   562  		for len(targs) < len(tparams) {
   563  			targs = append(targs, nil)
   564  		}
   565  	}
   566  	assert(len(tparams) == len(targs))
   567  
   568  	// collect type parameters from generic function arguments
   569  	var genericArgs []int // indices of generic function arguments
   570  	if enableReverseTypeInference {
   571  		for i, arg := range args {
   572  			// generic arguments cannot have a defined (*Named) type - no need for underlying type below
   573  			if asig, _ := arg.typ.(*Signature); asig != nil && asig.TypeParams().Len() > 0 {
   574  				// The argument type is a generic function signature. This type is
   575  				// pointer-identical with (it's copied from) the type of the generic
   576  				// function argument and thus the function object.
   577  				// Before we change the type (type parameter renaming, below), make
   578  				// a clone of it as otherwise we implicitly modify the object's type
   579  				// (go.dev/issues/63260).
   580  				asig = clone(asig)
   581  				// Rename type parameters for cases like f(g, g); this gives each
   582  				// generic function argument a unique type identity (go.dev/issues/59956).
   583  				// TODO(gri) Consider only doing this if a function argument appears
   584  				//           multiple times, which is rare (possible optimization).
   585  				atparams, tmp := check.renameTParams(call.Pos(), asig.TypeParams().list(), asig)
   586  				asig = tmp.(*Signature)
   587  				asig.tparams = &TypeParamList{atparams} // renameTParams doesn't touch associated type parameters
   588  				arg.typ = asig                          // new type identity for the function argument
   589  				tparams = append(tparams, atparams...)
   590  				// add partial list of type arguments, if any
   591  				if i < len(atargs) {
   592  					targs = append(targs, atargs[i]...)
   593  				}
   594  				// make sure targs and tparams have the same length
   595  				for len(targs) < len(tparams) {
   596  					targs = append(targs, nil)
   597  				}
   598  				genericArgs = append(genericArgs, i)
   599  			}
   600  		}
   601  	}
   602  	assert(len(tparams) == len(targs))
   603  
   604  	// at the moment we only support implicit instantiations of argument functions
   605  	_ = len(genericArgs) > 0 && check.verifyVersionf(args[genericArgs[0]], go1_21, "implicitly instantiated function as argument")
   606  
   607  	// tparams holds the type parameters of the callee and generic function arguments, if any:
   608  	// the first n type parameters belong to the callee, followed by mi type parameters for each
   609  	// of the generic function arguments, where mi = args[i].typ.(*Signature).TypeParams().Len().
   610  
   611  	// infer missing type arguments of callee and function arguments
   612  	if len(tparams) > 0 {
   613  		err := check.newError(CannotInferTypeArgs)
   614  		targs = check.infer(call, tparams, targs, sigParams, args, false, err)
   615  		if targs == nil {
   616  			// TODO(gri) If infer inferred the first targs[:n], consider instantiating
   617  			//           the call signature for better error messages/gopls behavior.
   618  			//           Perhaps instantiate as much as we can, also for arguments.
   619  			//           This will require changes to how infer returns its results.
   620  			if !err.empty() {
   621  				check.errorf(err.posn(), CannotInferTypeArgs, "in call to %s, %s", call.Fun, err.msg())
   622  			}
   623  			return
   624  		}
   625  
   626  		// update result signature: instantiate if needed
   627  		if n > 0 {
   628  			rsig = check.instantiateSignature(call.Pos(), call.Fun, sig, targs[:n], xlist)
   629  			// If the callee's parameter list was adjusted we need to update (instantiate)
   630  			// it separately. Otherwise we can simply use the result signature's parameter
   631  			// list.
   632  			if adjusted {
   633  				sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(tparams[:n], targs[:n]), nil, check.context()).(*Tuple)
   634  			} else {
   635  				sigParams = rsig.params
   636  			}
   637  		}
   638  
   639  		// compute argument signatures: instantiate if needed
   640  		j := n
   641  		for _, i := range genericArgs {
   642  			arg := args[i]
   643  			asig := arg.typ.(*Signature)
   644  			k := j + asig.TypeParams().Len()
   645  			// targs[j:k] are the inferred type arguments for asig
   646  			arg.typ = check.instantiateSignature(call.Pos(), arg.expr, asig, targs[j:k], nil) // TODO(gri) provide xlist if possible (partial instantiations)
   647  			check.record(arg)                                                                 // record here because we didn't use the usual expr evaluators
   648  			j = k
   649  		}
   650  	}
   651  
   652  	// check arguments
   653  	if len(args) > 0 {
   654  		context := check.sprintf("argument to %s", call.Fun)
   655  		for i, a := range args {
   656  			check.assignment(a, sigParams.vars[i].typ, context)
   657  		}
   658  	}
   659  
   660  	return
   661  }
   662  
   663  var cgoPrefixes = [...]string{
   664  	"_Ciconst_",
   665  	"_Cfconst_",
   666  	"_Csconst_",
   667  	"_Ctype_",
   668  	"_Cvar_", // actually a pointer to the var
   669  	"_Cfpvar_fp_",
   670  	"_Cfunc_",
   671  	"_Cmacro_", // function to evaluate the expanded expression
   672  }
   673  
   674  func (check *Checker) selector(x *operand, e *ast.SelectorExpr, wantType bool) {
   675  	// these must be declared before the "goto Error" statements
   676  	var (
   677  		obj      Object
   678  		index    []int
   679  		indirect bool
   680  	)
   681  
   682  	sel := e.Sel.Name
   683  	// If the identifier refers to a package, handle everything here
   684  	// so we don't need a "package" mode for operands: package names
   685  	// can only appear in qualified identifiers which are mapped to
   686  	// selector expressions.
   687  	if ident, ok := e.X.(*ast.Ident); ok {
   688  		obj := check.lookup(ident.Name)
   689  		if pname, _ := obj.(*PkgName); pname != nil {
   690  			assert(pname.pkg == check.pkg)
   691  			check.recordUse(ident, pname)
   692  			check.usedPkgNames[pname] = true
   693  			pkg := pname.imported
   694  
   695  			var exp Object
   696  			funcMode := value
   697  			if pkg.cgo {
   698  				// cgo special cases C.malloc: it's
   699  				// rewritten to _CMalloc and does not
   700  				// support two-result calls.
   701  				if sel == "malloc" {
   702  					sel = "_CMalloc"
   703  				} else {
   704  					funcMode = cgofunc
   705  				}
   706  				for _, prefix := range cgoPrefixes {
   707  					// cgo objects are part of the current package (in file
   708  					// _cgo_gotypes.go). Use regular lookup.
   709  					exp = check.lookup(prefix + sel)
   710  					if exp != nil {
   711  						break
   712  					}
   713  				}
   714  				if exp == nil {
   715  					if isValidName(sel) {
   716  						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e)) // cast to ast.Expr to silence vet
   717  					}
   718  					goto Error
   719  				}
   720  				check.objDecl(exp)
   721  			} else {
   722  				exp = pkg.scope.Lookup(sel)
   723  				if exp == nil {
   724  					if !pkg.fake && isValidName(sel) {
   725  						// Try to give a better error message when selector matches an object name ignoring case.
   726  						exps := pkg.scope.lookupIgnoringCase(sel, true)
   727  						if len(exps) >= 1 {
   728  							// report just the first one
   729  							check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s (but have %s)", ast.Expr(e), exps[0].Name())
   730  						} else {
   731  							check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e))
   732  						}
   733  					}
   734  					goto Error
   735  				}
   736  				if !exp.Exported() {
   737  					check.errorf(e.Sel, UnexportedName, "name %s not exported by package %s", sel, pkg.name)
   738  					// ok to continue
   739  				}
   740  			}
   741  			check.recordUse(e.Sel, exp)
   742  
   743  			// Simplified version of the code for *ast.Idents:
   744  			// - imported objects are always fully initialized
   745  			switch exp := exp.(type) {
   746  			case *Const:
   747  				assert(exp.Val() != nil)
   748  				x.mode = constant_
   749  				x.typ = exp.typ
   750  				x.val = exp.val
   751  			case *TypeName:
   752  				x.mode = typexpr
   753  				x.typ = exp.typ
   754  			case *Var:
   755  				x.mode = variable
   756  				x.typ = exp.typ
   757  				if pkg.cgo && strings.HasPrefix(exp.name, "_Cvar_") {
   758  					x.typ = x.typ.(*Pointer).base
   759  				}
   760  			case *Func:
   761  				x.mode = funcMode
   762  				x.typ = exp.typ
   763  				if pkg.cgo && strings.HasPrefix(exp.name, "_Cmacro_") {
   764  					x.mode = value
   765  					x.typ = x.typ.(*Signature).results.vars[0].typ
   766  				}
   767  			case *Builtin:
   768  				x.mode = builtin
   769  				x.typ = exp.typ
   770  				x.id = exp.id
   771  			default:
   772  				check.dump("%v: unexpected object %v", e.Sel.Pos(), exp)
   773  				panic("unreachable")
   774  			}
   775  			x.expr = e
   776  			return
   777  		}
   778  	}
   779  
   780  	check.exprOrType(x, e.X, false)
   781  	switch x.mode {
   782  	case builtin:
   783  		// types2 uses the position of '.' for the error
   784  		check.errorf(e.Sel, UncalledBuiltin, "invalid use of %s in selector expression", x)
   785  		goto Error
   786  	case invalid:
   787  		goto Error
   788  	}
   789  
   790  	// Avoid crashing when checking an invalid selector in a method declaration
   791  	// (i.e., where def is not set):
   792  	//
   793  	//   type S[T any] struct{}
   794  	//   type V = S[any]
   795  	//   func (fs *S[T]) M(x V.M) {}
   796  	//
   797  	// All codepaths below return a non-type expression. If we get here while
   798  	// expecting a type expression, it is an error.
   799  	//
   800  	// See go.dev/issue/57522 for more details.
   801  	//
   802  	// TODO(rfindley): We should do better by refusing to check selectors in all cases where
   803  	// x.typ is incomplete.
   804  	if wantType {
   805  		check.errorf(e.Sel, NotAType, "%s is not a type", ast.Expr(e))
   806  		goto Error
   807  	}
   808  
   809  	obj, index, indirect = lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel, false)
   810  	if obj == nil {
   811  		// Don't report another error if the underlying type was invalid (go.dev/issue/49541).
   812  		if !isValid(x.typ.Underlying()) {
   813  			goto Error
   814  		}
   815  
   816  		if index != nil {
   817  			// TODO(gri) should provide actual type where the conflict happens
   818  			check.errorf(e.Sel, AmbiguousSelector, "ambiguous selector %s.%s", x.expr, sel)
   819  			goto Error
   820  		}
   821  
   822  		if indirect {
   823  			if x.mode == typexpr {
   824  				check.errorf(e.Sel, InvalidMethodExpr, "invalid method expression %s.%s (needs pointer receiver (*%s).%s)", x.typ, sel, x.typ, sel)
   825  			} else {
   826  				check.errorf(e.Sel, InvalidMethodExpr, "cannot call pointer method %s on %s", sel, x.typ)
   827  			}
   828  			goto Error
   829  		}
   830  
   831  		var why string
   832  		if isInterfacePtr(x.typ) {
   833  			why = check.interfacePtrError(x.typ)
   834  		} else {
   835  			alt, _, _ := lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel, true)
   836  			why = check.lookupError(x.typ, sel, alt, false)
   837  		}
   838  		check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (%s)", x.expr, sel, why)
   839  		goto Error
   840  	}
   841  
   842  	// methods may not have a fully set up signature yet
   843  	if m, _ := obj.(*Func); m != nil {
   844  		check.objDecl(m)
   845  	}
   846  
   847  	if x.mode == typexpr {
   848  		// method expression
   849  		m, _ := obj.(*Func)
   850  		if m == nil {
   851  			check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
   852  			goto Error
   853  		}
   854  
   855  		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
   856  
   857  		sig := m.typ.(*Signature)
   858  		if sig.recv == nil {
   859  			check.error(e, InvalidDeclCycle, "illegal cycle in method declaration")
   860  			goto Error
   861  		}
   862  
   863  		// the receiver type becomes the type of the first function
   864  		// argument of the method expression's function type
   865  		var params []*Var
   866  		if sig.params != nil {
   867  			params = sig.params.vars
   868  		}
   869  		// Be consistent about named/unnamed parameters. This is not needed
   870  		// for type-checking, but the newly constructed signature may appear
   871  		// in an error message and then have mixed named/unnamed parameters.
   872  		// (An alternative would be to not print parameter names in errors,
   873  		// but it's useful to see them; this is cheap and method expressions
   874  		// are rare.)
   875  		name := ""
   876  		if len(params) > 0 && params[0].name != "" {
   877  			// name needed
   878  			name = sig.recv.name
   879  			if name == "" {
   880  				name = "_"
   881  			}
   882  		}
   883  		params = append([]*Var{NewParam(sig.recv.pos, sig.recv.pkg, name, x.typ)}, params...)
   884  		x.mode = value
   885  		x.typ = &Signature{
   886  			tparams:  sig.tparams,
   887  			params:   NewTuple(params...),
   888  			results:  sig.results,
   889  			variadic: sig.variadic,
   890  		}
   891  
   892  		check.addDeclDep(m)
   893  
   894  	} else {
   895  		// regular selector
   896  		switch obj := obj.(type) {
   897  		case *Var:
   898  			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
   899  			if x.mode == variable || indirect {
   900  				x.mode = variable
   901  			} else {
   902  				x.mode = value
   903  			}
   904  			x.typ = obj.typ
   905  
   906  		case *Func:
   907  			// TODO(gri) If we needed to take into account the receiver's
   908  			// addressability, should we report the type &(x.typ) instead?
   909  			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
   910  
   911  			// TODO(gri) The verification pass below is disabled for now because
   912  			//           method sets don't match method lookup in some cases.
   913  			//           For instance, if we made a copy above when creating a
   914  			//           custom method for a parameterized received type, the
   915  			//           method set method doesn't match (no copy there). There
   916  			///          may be other situations.
   917  			disabled := true
   918  			if !disabled && debug {
   919  				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
   920  				// TODO(gri) This only works because we call LookupFieldOrMethod
   921  				// _before_ calling NewMethodSet: LookupFieldOrMethod completes
   922  				// any incomplete interfaces so they are available to NewMethodSet
   923  				// (which assumes that interfaces have been completed already).
   924  				typ := x.typ
   925  				if x.mode == variable {
   926  					// If typ is not an (unnamed) pointer or an interface,
   927  					// use *typ instead, because the method set of *typ
   928  					// includes the methods of typ.
   929  					// Variables are addressable, so we can always take their
   930  					// address.
   931  					if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
   932  						typ = &Pointer{base: typ}
   933  					}
   934  				}
   935  				// If we created a synthetic pointer type above, we will throw
   936  				// away the method set computed here after use.
   937  				// TODO(gri) Method set computation should probably always compute
   938  				// both, the value and the pointer receiver method set and represent
   939  				// them in a single structure.
   940  				// TODO(gri) Consider also using a method set cache for the lifetime
   941  				// of checker once we rely on MethodSet lookup instead of individual
   942  				// lookup.
   943  				mset := NewMethodSet(typ)
   944  				if m := mset.Lookup(check.pkg, sel); m == nil || m.obj != obj {
   945  					check.dump("%v: (%s).%v -> %s", e.Pos(), typ, obj.name, m)
   946  					check.dump("%s\n", mset)
   947  					// Caution: MethodSets are supposed to be used externally
   948  					// only (after all interface types were completed). It's
   949  					// now possible that we get here incorrectly. Not urgent
   950  					// to fix since we only run this code in debug mode.
   951  					// TODO(gri) fix this eventually.
   952  					panic("method sets and lookup don't agree")
   953  				}
   954  			}
   955  
   956  			x.mode = value
   957  
   958  			// remove receiver
   959  			sig := *obj.typ.(*Signature)
   960  			sig.recv = nil
   961  			x.typ = &sig
   962  
   963  			check.addDeclDep(obj)
   964  
   965  		default:
   966  			panic("unreachable")
   967  		}
   968  	}
   969  
   970  	// everything went well
   971  	x.expr = e
   972  	return
   973  
   974  Error:
   975  	x.mode = invalid
   976  	x.typ = Typ[Invalid]
   977  	x.expr = e
   978  }
   979  
   980  // use type-checks each argument.
   981  // Useful to make sure expressions are evaluated
   982  // (and variables are "used") in the presence of
   983  // other errors. Arguments may be nil.
   984  // Reports if all arguments evaluated without error.
   985  func (check *Checker) use(args ...ast.Expr) bool { return check.useN(args, false) }
   986  
   987  // useLHS is like use, but doesn't "use" top-level identifiers.
   988  // It should be called instead of use if the arguments are
   989  // expressions on the lhs of an assignment.
   990  func (check *Checker) useLHS(args ...ast.Expr) bool { return check.useN(args, true) }
   991  
   992  func (check *Checker) useN(args []ast.Expr, lhs bool) bool {
   993  	ok := true
   994  	for _, e := range args {
   995  		if !check.use1(e, lhs) {
   996  			ok = false
   997  		}
   998  	}
   999  	return ok
  1000  }
  1001  
  1002  func (check *Checker) use1(e ast.Expr, lhs bool) bool {
  1003  	var x operand
  1004  	x.mode = value // anything but invalid
  1005  	switch n := ast.Unparen(e).(type) {
  1006  	case nil:
  1007  		// nothing to do
  1008  	case *ast.Ident:
  1009  		// don't report an error evaluating blank
  1010  		if n.Name == "_" {
  1011  			break
  1012  		}
  1013  		// If the lhs is an identifier denoting a variable v, this assignment
  1014  		// is not a 'use' of v. Remember current value of v.used and restore
  1015  		// after evaluating the lhs via check.rawExpr.
  1016  		var v *Var
  1017  		var v_used bool
  1018  		if lhs {
  1019  			if obj := check.lookup(n.Name); obj != nil {
  1020  				// It's ok to mark non-local variables, but ignore variables
  1021  				// from other packages to avoid potential race conditions with
  1022  				// dot-imported variables.
  1023  				if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
  1024  					v = w
  1025  					v_used = check.usedVars[v]
  1026  				}
  1027  			}
  1028  		}
  1029  		check.exprOrType(&x, n, true)
  1030  		if v != nil {
  1031  			check.usedVars[v] = v_used // restore v.used
  1032  		}
  1033  	default:
  1034  		check.rawExpr(nil, &x, e, nil, true)
  1035  	}
  1036  	return x.mode != invalid
  1037  }
  1038
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