call.go

// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements typechecking of call and selector expressions.

package types

import (
	"go/ast"
	"go/token"
)

func (check *checker) call(x *operand, e *ast.CallExpr) exprKind {
	check.exprOrType(x, e.Fun)

	switch x.mode {
	case invalid:
		check.use(e.Args)
		x.mode = invalid
		x.expr = e
		return statement

	case typexpr:
		// conversion
		T := x.typ
		x.mode = invalid
		switch n := len(e.Args); n {
		case 0:
			check.errorf(e.Rparen, "missing argument in conversion to %s", T)
		case 1:
			check.expr(x, e.Args[0])
			if x.mode != invalid {
				check.conversion(x, T)
				if x.mode != invalid {
					check.conversions[e] = true // for cap/len checking
				}
			}
		default:
			check.errorf(e.Args[n-1].Pos(), "too many arguments in conversion to %s", T)
		}
		x.expr = e
		return conversion

	case builtin:
		id := x.id
		if !check.builtin(x, e, id) {
			x.mode = invalid
		}
		x.expr = e
		return predeclaredFuncs[id].kind

	default:
		// function/method call
		sig, _ := x.typ.Underlying().(*Signature)
		if sig == nil {
			check.invalidOp(x.pos(), "cannot call non-function %s", x)
			x.mode = invalid
			x.expr = e
			return statement
		}

		arg, n, _ := unpack(func(x *operand, i int) { check.expr(x, e.Args[i]) }, len(e.Args), false)
		check.arguments(x, e, sig, arg, n)

		// determine result
		switch len(sig.Results) {
		case 0:
			x.mode = novalue
		case 1:
			x.mode = value
			x.typ = sig.Results[0].Type // unpack tuple
		default:
			x.mode = value
			x.typ = NewTuple(sig.Results...)
		}
		x.expr = e

		return statement
	}
}

// use type-checks each list element.
// Useful to make sure a list of expressions is evaluated
// (and variables are "used") in the presence of other errors.
func (check *checker) use(list []ast.Expr) {
	var x operand
	for _, e := range list {
		check.rawExpr(&x, e, nil)
	}
}

// A getter sets x as the i'th operand, where 0 <= i < n and n is the total
// number of operands (context-specific, and maintained elsewhere). A getter
// type-checks the i'th operand; the details of the actual check are getter-
// specific.
type getter func(x *operand, i int)

// unpack takes a getter get and a number of operands n. If n == 1 and the
// first operand is a function call, or a comma,ok expression and allowCommaOk
// is set, the result is a new getter and operand count providing access to the
// function results, or comma,ok values, respectively. The third result value
// reports if it is indeed the comma,ok case. In all other cases, the incoming
// getter and operand count are returned unchanged, and the third result value
// is false.
//
// In other words, if there's exactly one operand that - after type-checking by
// calling get - stands for multiple operands, the resulting getter provides access
// to those operands instead.
//
// Note that unpack may call get(..., 0); but if the result getter is called
// at most once for a given operand index i (including i == 0), that operand
// is guaranteed to cause only one call of the incoming getter with that i.
//
func unpack(get getter, n int, allowCommaOk bool) (getter, int, bool) {
	if n == 1 {
		// possibly result of an n-valued function call or comma,ok value
		var x0 operand
		get(&x0, 0)
		if x0.mode == invalid {
			return func(x *operand, i int) {
				if i != 0 {
					unreachable()
				}
				x.mode = invalid
			}, 1, false
		}

		if t, ok := x0.typ.(*Tuple); ok {
			// result of an n-valued function call
			return func(x *operand, i int) {
				x.mode = value
				x.expr = x0.expr
				x.typ = (*t)[i].Type
			}, len(*t), false
		}

		if x0.mode == mapindex || x0.mode == commaok {
			// comma-ok value
			if allowCommaOk {
				a := [2]Type{x0.typ, Typ[UntypedBool]}
				return func(x *operand, i int) {
					x.mode = value
					x.expr = x0.expr
					x.typ = a[i]
				}, 2, true
			}
			x0.mode = value
		}

		// single value
		return func(x *operand, i int) {
			if i != 0 {
				unreachable()
			}
			*x = x0
		}, 1, false
	}

	// zero or multiple values
	return get, n, false
}

// arguments checks argument passing for the call with the given signature.
// The arg function provides the operand for the i'th argument.
func (check *checker) arguments(x *operand, call *ast.CallExpr, sig *Signature, arg getter, n int) {
	passSlice := false
	if call.Ellipsis.IsValid() {
		// last argument is of the form x...
		if sig.IsVariadic {
			passSlice = true
		} else {
			check.errorf(call.Ellipsis, "cannot use ... in call to non-variadic %s", call.Fun)
			// ok to continue
		}
	}

	// evaluate arguments
	for i := 0; i < n; i++ {
		arg(x, i)
		if x.mode != invalid {
			check.argument(sig, i, x, passSlice && i == n-1)
		}
	}

	// check argument count
	if sig.IsVariadic {
		// a variadic function accepts an "empty"
		// last argument: count one extra
		n++
	}
	if n < len(sig.Params) {
		check.errorf(call.Rparen, "too few arguments in call to %s", call.Fun)
		// ok to continue
	}
}

// argument checks passing of argument x to the i'th parameter of the given signature.
// If passSlice is set, the argument is followed by ... in the call.
func (check *checker) argument(sig *Signature, i int, x *operand, passSlice bool) {
	n := len(sig.Params)

	// determine parameter type
	var typ Type
	switch {
	case i < n:
		typ = sig.Params[i].Type
	case sig.IsVariadic:
		typ = sig.Params[n-1].Type
		if debug {
			if _, ok := typ.(*Slice); !ok {
				check.dump("%s: expected unnamed slice type, got %s", sig.Params[n-1].Pos(), typ)
			}
		}
	default:
		check.errorf(x.pos(), "too many arguments")
		return
	}

	if passSlice {
		// argument is of the form x...
		if i != n-1 {
			check.errorf(x.pos(), "can only use ... with matching parameter")
			return
		}
		if _, ok := x.typ.Underlying().(*Slice); !ok {
			check.errorf(x.pos(), "cannot use %s as parameter of type %s", x, typ)
			return
		}
	} else if sig.IsVariadic && i >= n-1 {
		// use the variadic parameter slice's element type
		typ = typ.(*Slice).Elem
	}

	if !check.assignment(x, typ) && x.mode != invalid {
		check.errorf(x.pos(), "cannot pass argument %s to parameter of type %s", x, typ)
	}
}

func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
	// these must be declared before the "goto Error" statements
	var (
		obj      Object
		index    []int
		indirect bool
	)

	sel := e.Sel.Name
	// If the identifier refers to a package, handle everything here
	// so we don't need a "package" mode for operands: package names
	// can only appear in qualified identifiers which are mapped to
	// selector expressions.
	if ident, ok := e.X.(*ast.Ident); ok {
		if pkg, _ := check.topScope.LookupParent(ident.Name).(*PkgName); pkg != nil {
			check.recordObject(ident, pkg)
			pkg.used = true
			exp := pkg.Pkg.scope.Lookup(sel)
			if exp == nil {
				if !pkg.Pkg.fake {
					check.errorf(e.Pos(), "%s not declared by package %s", sel, ident)
				}
				goto Error
			}
			if !exp.IsExported() {
				check.errorf(e.Pos(), "%s not exported by package %s", sel, ident)
				// ok to continue
			}
			check.recordSelection(e, PackageObj, nil, exp, nil, false)
			// Simplified version of the code for *ast.Idents:
			// - imported packages use types.Scope and types.Objects
			// - imported objects are always fully initialized
			switch exp := exp.(type) {
			case *Const:
				assert(exp.Val() != nil)
				x.mode = constant
				x.typ = exp.Type
				x.val = exp.val
			case *TypeName:
				x.mode = typexpr
				x.typ = exp.Type
			case *Var:
				x.mode = variable
				x.typ = exp.Type
			case *Func:
				x.mode = value
				x.typ = exp.Type
			case *Builtin:
				x.mode = builtin
				x.typ = exp.Type
				x.id = exp.id
			default:
				unreachable()
			}
			x.expr = e
			return
		}
	}

	check.exprOrType(x, e.X)
	if x.mode == invalid {
		goto Error
	}

	obj, index, indirect = LookupFieldOrMethod(x.typ, check.pkg, sel)
	if obj == nil {
		if index != nil {
			// TODO(gri) should provide actual type where the conflict happens
			check.invalidOp(e.Pos(), "ambiguous selector %s", sel)
		} else {
			check.invalidOp(e.Pos(), "%s has no field or method %s", x, sel)
		}
		goto Error
	}

	if x.mode == typexpr {
		// method expression
		m, _ := obj.(*Func)
		if m == nil {
			check.invalidOp(e.Pos(), "%s has no method %s", x, sel)
			goto Error
		}

		// verify that m is in the method set of x.typ
		if !indirect && ptrRecv(m) {
			check.invalidOp(e.Pos(), "%s is not in method set of %s", sel, x.typ)
			goto Error
		}

		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)

		// the receiver type becomes the type of the first function
		// argument of the method expression's function type
		var params []*Var
		sig := m.Type.(*Signature)
		if sig.Params != nil {
			params = sig.Params
		}
		x.mode = value
		x.typ = &Signature{
			Params:     append([]*Var{NewVar(token.NoPos, check.pkg, "", x.typ)}, params...),
			Results:    sig.Results,
			IsVariadic: sig.IsVariadic,
		}

		check.addDeclDep(m)

	} else {
		// regular selector
		switch obj := obj.(type) {
		case *Var:
			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
			if x.mode == variable || indirect {
				x.mode = variable
			} else {
				x.mode = value
			}
			x.typ = obj.Type

		case *Func:
			// TODO(gri) This code appears elsewhere, too. Factor!
			// verify that obj is in the method set of x.typ (or &(x.typ) if x is addressable)
			//
			// spec: "A method call x.m() is valid if the method set of (the type of) x
			//        contains m and the argument list can be assigned to the parameter
			//        list of m. If x is addressable and &x's method set contains m, x.m()
			//        is shorthand for (&x).m()".
			if !indirect && x.mode != variable && ptrRecv(obj) {
				check.invalidOp(e.Pos(), "%s is not in method set of %s", sel, x)
				goto Error
			}

			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)

			if debug {
				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
				typ := x.typ
				if x.mode == variable {
					// If typ is not an (unnamed) pointer or an interface,
					// use *typ instead, because the method set of *typ
					// includes the methods of typ.
					// Variables are addressable, so we can always take their
					// address.
					if _, ok := typ.(*Pointer); !ok && !isInterface(typ) {
						typ = &Pointer{Elem: typ}
					}
				}
				// If we created a synthetic pointer type above, we will throw
				// away the method set computed here after use.
				// TODO(gri) Method set computation should probably always compute
				// both, the value and the pointer receiver method set and represent
				// them in a single structure.
				// TODO(gri) Consider also using a method set cache for the lifetime
				// of checker once we rely on MethodSet lookup instead of individual
				// lookup.
				mset := typ.MethodSet()
				if m := mset.Lookup(check.pkg, sel); m == nil || m.Obj != obj {
					check.dump("%s: (%s).%v -> %s", e.Pos(), typ, obj.Name, m)
					check.dump("%s\n", mset)
					panic("method sets and lookup don't agree")
				}
			}

			x.mode = value

			// remove receiver
			sig := *obj.Type.(*Signature)
			sig.Recv = nil
			x.typ = &sig

		default:
			unreachable()
		}
	}

	// everything went well
	x.expr = e
	return

Error:
	x.mode = invalid
	x.expr = e
}