go/types: make sure we are safe for nil in underIs

This CL is a clean port CL 363658 from types2 to go/types.

Change-Id: Ie2032f85a9cfca62161c2e629c78f1ecd8c6e4c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/364537
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/types/expr.go

@@ -679,6 +679,9 @@ func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, const
 	case *TypeParam:
 		// TODO(gri) review this code - doesn't look quite right
 		ok := u.underIs(func(t Type) bool {
+			if t == nil {
+				return false
+			}
 			target, _, _ := check.implicitTypeAndValue(x, t)
 			return target != nil
 		})

src/go/types/predicates.go

@@ -149,7 +149,9 @@ func hasNil(t Type) bool {
 	case *Slice, *Pointer, *Signature, *Interface, *Map, *Chan:
 		return true
 	case *TypeParam:
-		return u.underIs(hasNil)
+		return u.underIs(func(u Type) bool {
+			return u != nil && hasNil(u)
+		})
 	}
 	return false
 }

src/go/types/type.go

@@ -65,6 +65,9 @@ func match(x, y Type) Type {
 func structuralType(typ Type) Type {
 	var su Type
 	if underIs(typ, func(u Type) bool {
+		if u == nil {
+			return false
+		}
 		if su != nil {
 			u = match(su, u)
 			if u == nil {