go/printer: check whether !isTypeElem, instead of combinesWithName wh…

…en ParenExpr

See discussion in CL 610115 and CL 610758.

For #69206

Change-Id: I16f394cb3440106650fb64a466f2723a4dba3871
GitHub-Last-Rev: 37993b5
GitHub-Pull-Request: #69309
Reviewed-on: https://go-review.googlesource.com/c/go/+/611355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/go/printer/nodes.go

@@ -380,7 +380,7 @@ func (p *printer) parameters(fields *ast.FieldList, mode paramMode) {
 		if closing := p.lineFor(fields.Closing); 0 < prevLine && prevLine < closing {
 			p.print(token.COMMA)
 			p.linebreak(closing, 0, ignore, true)
-		} else if mode == typeTParam && fields.NumFields() == 1 && combinesWithName(fields.List[0].Type) {
+		} else if mode == typeTParam && fields.NumFields() == 1 && combinesWithName(stripParensAlways(fields.List[0].Type)) {
 			// A type parameter list [P T] where the name P and the type expression T syntactically
 			// combine to another valid (value) expression requires a trailing comma, as in [P *T,]
 			// (or an enclosing interface as in [P interface(*T)]), so that the type parameter list
@@ -411,7 +411,7 @@ func combinesWithName(x ast.Expr) bool {
 	case *ast.BinaryExpr:
 		return combinesWithName(x.X) && !isTypeElem(x.Y)
 	case *ast.ParenExpr:
-		return combinesWithName(x.X)
+		return !isTypeElem(x.X)
 	}
 	return false
 }