[AST] Use recovery-expr to preserve incomplete-type-member-access expression.

clang/lib/Sema/SemaExprCXX.cpp

ExprResult Sema::ActOnStartCXXMemberReference(Scope *S, Expr *Base,
 
   // The object type must be complete (or dependent), or
   // C++11 [expr.prim.general]p3:
   //   Unlike the object expression in other contexts, *this is not required to
   //   be of complete type for purposes of class member access (5.2.5) outside
   //   the member function body.
   if (!BaseType->isDependentType() &&
       !isThisOutsideMemberFunctionBody(BaseType) &&
-      RequireCompleteType(OpLoc, BaseType, diag::err_incomplete_member_access))
-    return ExprError();
+      RequireCompleteType(OpLoc, BaseType,
+                          diag::err_incomplete_member_access)) {
+    return CreateRecoveryExpr(Base->getBeginLoc(), Base->getEndLoc(), {Base});
+  }
 
   // C++ [basic.lookup.classref]p2:
   //   If the id-expression in a class member access (5.2.5) is an
   //   unqualified-id, and the type of the object expression is of a class
   //   type C (or of pointer to a class type C), the unqualified-id is looked
   //   up in the scope of class C. [...]
   ObjectType = ParsedType::make(BaseType);
   return Base;

clang/test/AST/ast-dump-recovery.cpp

 // CHECK-NEXT:  | `-DeclRefExpr {{.*}} 'foo'
 // CHECK-NEXT:  `-DeclRefExpr {{.*}} 'x'
 struct Foo {} foo;
 void test(int x) {
   foo.abc;
   foo->func(x);
 }
 
+void AccessIncompleteClass() {
+  struct Forward;
+  Forward* ptr;
+  // CHECK:      CallExpr {{.*}} '<dependent type>'
+  // CHECK-NEXT: `-CXXDependentScopeMemberExpr {{.*}} '<dependent type>'
+  // CHECK-NEXT:   `-RecoveryExpr {{.*}} '<dependent type>' contains-errors
+  // CHECK-NEXT:     `-DeclRefExpr {{.*}} 'Forward *'
+  ptr->method();
+}
+
 struct Foo2 {
   double func();
   class ForwardClass;
   ForwardClass createFwd();
 
   int overload();
   int overload(int, int);
 };

clang/test/OpenMP/declare_reduction_messages.cpp

struct S
   int s;
   // expected-note@+1 {{'foo' declared here}}
   void foo(S &x) {};
   // expected-error@+1 {{too many arguments to function call, expected single argument 'x', have 2 arguments}}
   #pragma omp declare reduction (foo : U, S : omp_out.foo(omp_in, false))
   #pragma omp declare reduction (xxx : U, S : bar(omp_in)) // expected-error {{non-const lvalue reference to type 'S<1>' cannot bind to a value of unrelated type 'U'}}
   static void bar(S &x); // expected-note {{passing argument to parameter 'x' here}}
 };
-// expected-warning@+2 {{extra tokens at the end of '#pragma omp declare reduction' are ignored}}
 // expected-note@+1 {{in instantiation of template class 'S<1>' requested here}}
 #pragma omp declare reduction (bar : S<1> : omp_out.foo(omp_in))
 
 #endif