[clang] fix(85447): clang 18.1.0 crashes in clang::ASTContext::getTypeInfoImpl (#89850)

Fixes #85447

--- 

This PR resolves a crash in `ActOnUninitializedDecl` due to an oversight
in updating the `isInvalidDecl` state before invocation. The crash
occurs due to a missing invocation of `setInvalidDecl()` for an invalid
`Anon` declaration. To address this issue, the `setInvalidDecl()` method
is now properly invoked to mark the `Anon` declaration as invalid before
running `ActOnUninitializedDecl()`.

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -649,6 +649,7 @@ Bug Fixes to C++ Support
 - Fix a bug on template partial specialization with issue on deduction of nontype template parameter
   whose type is `decltype(auto)`. Fixes (#GH68885).
 - Clang now correctly treats the noexcept-specifier of a friend function to be a complete-class context.
+- Fix an assertion failure when parsing an invalid members of an anonymous class. (#GH85447)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -5790,6 +5790,9 @@ Decl *Sema::BuildAnonymousStructOrUnion(Scope *S, DeclSpec &DS,
     Anon = VarDecl::Create(Context, Owner, DS.getBeginLoc(),
                            Record->getLocation(), /*IdentifierInfo=*/nullptr,
                            Context.getTypeDeclType(Record), TInfo, SC);
+    if (Invalid)
+      Anon->setInvalidDecl();
+
     ProcessDeclAttributes(S, Anon, Dc);
 
     // Default-initialize the implicit variable. This initialization will be

clang/test/Sema/incomplete-struct-decl.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -x c++ -fsyntax-only -verify=cxx,expected %s
+
+template <class a> using __impl_of = a; // expected-note {{'__impl_of' declared here}} \
+                                           expected-note {{template is declared here}}
+struct {                                // expected-error {{anonymous structs and classes must be class members}} \
+                                           expected-note {{to match this '{'}}
+  __impl_;                              // expected-error {{no template named '__impl_'; did you mean '__impl_of'?}} \
+                                           expected-error {{cannot specify deduction guide for alias template '__impl_of'}} \
+                                           expected-error {{expected ';' after struct}}
+                                        // expected-error {{expected '}'}}