go/parser: better error message for incorrect type switch header

Fixes 11829.

Change-Id: I2e39f61e12953147b0cd6a11d29179c500c94964
Reviewed-on: https://go-review.googlesource.com/14566Reviewed-by: Chris Manghane <cmang@golang.org>

src/go/parser/parser.go

@@ -1910,14 +1910,23 @@ func isTypeSwitchAssert(x ast.Expr) bool {
 	return ok && a.Type == nil
 }
 
-func isTypeSwitchGuard(s ast.Stmt) bool {
+func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
 	switch t := s.(type) {
 	case *ast.ExprStmt:
-		// x.(nil)
+		// x.(type)
 		return isTypeSwitchAssert(t.X)
 	case *ast.AssignStmt:
-		// v := x.(nil)
-		return len(t.Lhs) == 1 && t.Tok == token.DEFINE && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0])
+		// v := x.(type)
+		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
+			switch t.Tok {
+			case token.ASSIGN:
+				// permit v = x.(type) but complain
+				p.error(t.TokPos, "expected ':=', found '='")
+				fallthrough
+			case token.DEFINE:
+				return true
+			}
+		}
 	}
 	return false
 }
@@ -1963,7 +1972,7 @@ func (p *parser) parseSwitchStmt() ast.Stmt {
 		p.exprLev = prevLev
 	}
 
-	typeSwitch := isTypeSwitchGuard(s2)
+	typeSwitch := p.isTypeSwitchGuard(s2)
 	lbrace := p.expect(token.LBRACE)
 	var list []ast.Stmt
 	for p.tok == token.CASE || p.tok == token.DEFAULT {

src/go/parser/short_test.go

@@ -64,7 +64,7 @@ var invalids = []string{
 	`package p; func f() { for _ = range x ; /* ERROR "expected '{'" */ ; {} };`,
 	`package p; func f() { for ; ; _ = range /* ERROR "expected operand" */ x {} };`,
 	`package p; func f() { for ; _ /* ERROR "expected boolean or range expression" */ = range x ; {} };`,
-	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type) {} };`,
+	`package p; func f() { switch t = /* ERROR "expected ':=', found '='" */ t.(type) {} };`,
 	`package p; func f() { switch t /* ERROR "expected switch expression" */ , t = t.(type) {} };`,
 	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type), t {} };`,
 	`package p; var a = [ /* ERROR "expected expression" */ 1]int;`,