[Clang] Ignore invalid base classes

clang/docs/ReleaseNotes.rst

@@ -923,6 +923,7 @@ Bug Fixes to C++ Support
 - Improved handling of variables with ``consteval`` constructors, to
   consistently treat the initializer as manifestly constant-evaluated.
   (#GH135281)
+- Switch `ParseBaseClause` to use `BaseResult::isUsable()` instead of `isInvalid()`, fixing dropped or mis-parsed base specifiers in C++ classes. (#GH147186)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseDeclCXX.cpp

@@ -2252,7 +2252,10 @@ void Parser::ParseBaseClause(Decl *ClassDecl) {
   while (true) {
     // Parse a base-specifier.
     BaseResult Result = ParseBaseSpecifier(ClassDecl);
-    if (Result.isInvalid()) {
+    // Skip any base-specifier we couldn’t actually build into a usable
+    // CXXBaseSpecifier (covers both syntactic invalidity and
+    // other un-usable cases).
+    if (!Result.isUsable()) {
       // Skip the rest of this base specifier, up until the comma or
       // opening brace.
       SkipUntil(tok::comma, tok::l_brace, StopAtSemi | StopBeforeMatch);

clang/test/SemaCXX/invalid-base-inheritance.cpp

@@ -0,0 +1,13 @@
+// Tests that invalid base-specifiers no longer crash the compiler.
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+class X; // expected-note {{forward declaration of 'X'}} expected-note {{forward declaration of 'X'}}
+
+class A : X { // expected-error {{base class has incomplete type}}
+};
+
+class Y : int { // expected-error {{expected class name}}
+};
+
+class Z : X*, virtual int { // expected-error {{base class has incomplete type}} expected-error {{expected class name}}
+};