go/parser: leave checking of LHS in short var decls to type checker

Instead of checking at parse-time that the LHS of a short variable
declaration contains only identifiers, leave the check to the the
type checker which tests this already.

This removes a duplicate error and matches the behavior of the
syntax package.

For #54511.

Change-Id: I4c68f2bd8a0e015133685f9308beb98e714a83fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/426476
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -2487,8 +2487,6 @@ func (p *parser) commClause() *CommClause {
 		//
 		// All these (and more) are recognized by simpleStmt and invalid
 		// syntax trees are flagged later, during type checking.
-		// TODO(gri) eventually may want to restrict valid syntax trees
-		// here.
 
 	case _Default:
 		p.next()

src/go/parser/parser.go

@@ -1867,11 +1867,7 @@ func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
 		} else {
 			y = p.parseList(true)
 		}
-		as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
-		if tok == token.DEFINE {
-			p.checkAssignStmt(as)
-		}
-		return as, isRange
+		return &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}, isRange
 	}
 
 	if len(x) > 1 {
@@ -1918,14 +1914,6 @@ func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
 	return &ast.ExprStmt{X: x[0]}, false
 }
 
-func (p *parser) checkAssignStmt(as *ast.AssignStmt) {
-	for _, x := range as.Lhs {
-		if _, isIdent := x.(*ast.Ident); !isIdent {
-			p.errorExpected(x.Pos(), "identifier on left side of :=")
-		}
-	}
-}
-
 func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
 	x := p.parseRhs() // could be a conversion: (some type)(x)
 	if t := unparen(x); t != x {
@@ -2245,11 +2233,7 @@ func (p *parser) parseCommClause() *ast.CommClause {
 				pos := p.pos
 				p.next()
 				rhs := p.parseRhs()
-				as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
-				if tok == token.DEFINE {
-					p.checkAssignStmt(as)
-				}
-				comm = as
+				comm = &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
 			} else {
 				// lhs must be single receive operation
 				if len(lhs) > 1 {

src/go/parser/short_test.go

@@ -143,7 +143,6 @@ var invalids = []string{
 	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type), t {} };`,
 	`package p; func f() { _ = (<-<- /* ERROR "expected 'chan'" */ chan int)(nil) };`,
 	`package p; func f() { _ = (<-chan<-chan<-chan<-chan<-chan<- /* ERROR "expected channel type" */ int)(nil) };`,
-	`package p; func f() { var t []int; t /* ERROR "expected identifier on left side of :=" */ [0] := 0 };`,
 	`package p; func f() { if x := g(); x /* ERROR "expected boolean expression" */ = 0 {}};`,
 	`package p; func f() { _ = x = /* ERROR "expected '=='" */ 0 {}};`,
 	`package p; func f() { _ = 1 == func()int { var x bool; x = x = /* ERROR "expected '=='" */ true; return x }() };`,

src/go/types/testdata/fixedbugs/issue43087.go

@@ -24,7 +24,7 @@ func _() {
 
 func _() {
 	var a []int
-	a /* ERROR expected identifier */ /* ERROR non-name .* on left side of := */ [0], b := 1, 2
+	a /* ERROR non-name .* on left side of := */ [0], b := 1, 2
 	_ = a
 	_ = b
 }