[C++23] Fix infinite recursion (Clang 19.x regression) (#104829)

d469794 was fixing an issue with
triggering vtable instantiations, but it accidentally introduced
infinite recursion when the type to be checked is the same as the type
used in a base specifier or field declaration.

Fixes #104802

(cherry picked from commit 435cb0d)

Author: AaronBallman

clang/lib/Sema/SemaDeclCXX.cpp

@@ -7056,11 +7056,16 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
         if (!RD->hasConstexprDestructor())
           return false;
 
+        QualType CanUnqualT = T.getCanonicalType().getUnqualifiedType();
         for (const CXXBaseSpecifier &B : RD->bases())
-          if (!Check(B.getType(), Check))
+          if (B.getType().getCanonicalType().getUnqualifiedType() !=
+                  CanUnqualT &&
+              !Check(B.getType(), Check))
             return false;
         for (const FieldDecl *FD : RD->fields())
-          if (!Check(FD->getType(), Check))
+          if (FD->getType().getCanonicalType().getUnqualifiedType() !=
+                  CanUnqualT &&
+              !Check(FD->getType(), Check))
             return false;
         return true;
       };

clang/test/SemaCXX/gh102293.cpp

@@ -1,5 +1,4 @@
 // RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
-// expected-no-diagnostics
 
 template <typename T> static void destroy() {
     T t;
@@ -20,3 +19,29 @@ struct S : HasVT {
   HasD<> v;
 };
 
+// Ensure we don't get infinite recursion from the check, however. See GH104802
+namespace GH104802 {
+class foo {       // expected-note {{definition of 'GH104802::foo' is not complete until the closing '}'}}
+  foo a;          // expected-error {{field has incomplete type 'foo'}}
+
+  virtual int c();
+};
+
+class bar {       // expected-note {{definition of 'GH104802::bar' is not complete until the closing '}'}}
+  const bar a;    // expected-error {{field has incomplete type 'const bar'}}
+
+  virtual int c();
+};
+
+class baz {       // expected-note {{definition of 'GH104802::baz' is not complete until the closing '}'}}
+  typedef class baz blech;
+  blech a;        // expected-error {{field has incomplete type 'blech' (aka 'GH104802::baz')}}
+
+  virtual int c();
+};
+
+class quux : quux { // expected-error {{base class has incomplete type}} \
+                     expected-note {{definition of 'GH104802::quux' is not complete until the closing '}'}}
+  virtual int c();
+};
+}