[Sema] Add tests for handling of decls hidden by invalid decls

These tests check that invalid declarations don't hide any other
declarations, but valid declarations do hide invalid declarations.

Author: john-brawn-arm

clang/test/SemaCXX/invalid-decl-hiding.cpp

@@ -0,0 +1,59 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// Member Test1 hides class Test1
+class Test1 {
+  static int Test1; // expected-error {{member 'Test1' has the same name as its class}}
+                    // expected-note@-1 {{class 'Test1' is hidden by a non-type declaration of 'Test1' here}}
+  void fn1() {
+    Test1 x; // expected-error {{must use 'class' tag to refer to type 'Test1' in this scope}}
+  }
+  int fn2() {
+    return Test1;
+  }
+};
+
+// Member Test2 doesn't hide class Test2 as its declaration is invalid
+class Test2 { // expected-note {{declared here}}
+  static NoSuchType Test2; // expected-error {{unknown type name 'NoSuchType'}}
+                           // expected-error@-1 {{member 'Test2' has the same name as its class}}
+  void fn1() {
+    Test2 x;
+  }
+  int fn2() {
+    return Test2; // expected-error {{'Test2' does not refer to a value}}
+  }
+};
+
+// Test3a::x doesn't hide Test3b::x as its declaration is invalid
+namespace Test3a {
+  NoSuchType x() { return 0; } // expected-error {{unknown type name 'NoSuchType'}}
+}
+namespace Test3b {
+  class x; // expected-note {{declared here}}
+}
+using Test3a::x;
+using Test3b::x;
+int test3_fn() {
+  return x; // expected-error {{'x' does not refer to a value}}
+}
+
+// Function Test4 hides class Test4, whose declaration is invalid
+class Test4 : public NoSuchType { // expected-error {{expected class name}}
+
+};
+int Test4() { return 0; }
+
+int test4_fn() {
+  return Test4();
+}
+
+// Function Test5 doesn't hide class Test5 when both are invalid
+class Test5 : public NoSuchType { // expected-error {{expected class name}}
+
+};
+NoSuchType Test5() { return 0; } // expected-error {{unknown type name 'NoSuchType'}}
+
+Test5 test5_fn() {
+  Test5 x;
+  return x;
+}