[clang][CodeComplete] Dont perform fallback completion for incomplete member ref

Summary:
Clang performs expression based completion whenever it can't figure out
base of a member reference expression. It might be quite confusing in cases like
incomplete types. This patch disables that fallback.

Unfortunately `ParsePostfixExpressionSuffix` is quite tangled and this patch
adds more to it.

Reviewers: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77570

clang/lib/Parse/ParseExpr.cpp

@@ -2018,12 +2018,19 @@ Parser::ParsePostfixExpressionSuffix(ExprResult LHS) {
           return ParsePostfixExpressionSuffix(Base);
         }
 
-        LHS = Actions.ActOnStartCXXMemberReference(getCurScope(), Base,
+        LHS = Actions.ActOnStartCXXMemberReference(getCurScope(), Base, OpLoc,
-                                                   OpLoc, OpKind, ObjectType,
+                                                   OpKind, ObjectType,
                                                    MayBePseudoDestructor);
-        if (LHS.isInvalid())
+        if (LHS.isInvalid()) {
+          // Clang will try to perform expression based completion as a
+          // fallback, which is confusing in case of member references. So we
+          // stop here without any completions.
+          if (Tok.is(tok::code_completion)) {
+            cutOffParsing();
+            return ExprError();
+          }
           break;
-
+        }
         ParseOptionalCXXScopeSpecifier(
             SS, ObjectType, LHS.get() && LHS.get()->containsErrors(),
             /*EnteringContext=*/false, &MayBePseudoDestructor);

clang/test/CodeCompletion/incomplete-member.cpp

@@ -0,0 +1,8 @@
+struct IncompleteType;
+
+void foo() {
+  IncompleteType *f;
+  f->x;
+}
+// RUN: not %clang_cc1 -fsyntax-only -code-completion-at=%s:5:6 %s -o - | FileCheck %s -allow-empty
+// CHECK-NOT: COMPLETION: