go/parser: improve error recovery from invalid selector exprs

Before this CL, the parser consumed the next token following an invalid
selector expr no matter what it was. This leads to poor error recovery
when this next token is a closing delimiter or other reasonable element
of a stop set. As a side-effect, x/tools tests broke when parser logic
for type parameters was introduced, as they threw off the parser
synchronization to the point where the x/tools test bailed out.

This CL introduces a targeted fix that allows the x/tools tests to pass.
More general improvement for parser error recovery should be done for
go1.17.

Change-Id: I44d73d34b6063e62d16a23d24ab7cbce6500239d
Reviewed-on: https://go-review.googlesource.com/c/go/+/293792
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/parser/parser.go

@@ -1754,7 +1754,14 @@ func (p *parser) parsePrimaryExpr(lhs bool) (x ast.Expr) {
 			default:
 				pos := p.pos
 				p.errorExpected(pos, "selector or type assertion")
+				// TODO(rFindley) The check for token.RBRACE below is a targeted fix
+				//                to error recovery sufficient to make the x/tools tests to
+				//                pass with the new parsing logic introduced for type
+				//                parameters. Remove this once error recovery has been
+				//                more generally reconsidered.
+				if p.tok != token.RBRACE {
 					p.next() // make progress
+				}
 				sel := &ast.Ident{NamePos: pos, Name: "_"}
 				x = &ast.SelectorExpr{X: x, Sel: sel}
 			}