[clang] fix infinite recursion (llvm#143244)

fix llvm#141789

The direct cause of infinite recursion is that `T` is changing from
`struct X` and `S<X>` infinitely, this pr add a check that if `T`
visited before then return false directly.

```plaintext
/home/backlight/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:7196] FD->getType().getAsString()=struct X, T.getAsString()=S<X>, FD->getType().getCanonicalType().getUnqualifiedType().getAsString()=struct X, CanUnqualT.getAsString()=struct S<struct X>, 
/home/backlight/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:7196] FD->getType().getAsString()=S<X>, T.getAsString()=struct X, FD->getType().getCanonicalType().getUnqualifiedType().getAsString()=struct S<struct X>, CanUnqualT.getAsString()=struct X,
```

llvm#104829 fix similar infinite
recursion, but I think it is no longer needed so I kind of revert it.

Author: Backl1ght

clang/docs/ReleaseNotes.rst

@@ -703,6 +703,7 @@ Bug Fixes in This Version
   the second clause of a C-style ``for`` loop. (#GH139818)
 - Fixed a bug with constexpr evaluation for structs containing unions in case of C++ modules. (#GH143168)
 - Fixed incorrect token location when emitting diagnostics for tokens expanded from macros. (#GH143216)
+- Fixed an infinite recursion when checking constexpr destructors. (#GH141789)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDeclCXX.cpp

@@ -7159,7 +7159,10 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
     // "effectively constexpr" for better compatibility.
     // See https://github.com/llvm/llvm-project/issues/102293 for more info.
     if (isa<CXXDestructorDecl>(M)) {
-      auto Check = [](QualType T, auto &&Check) -> bool {
+      llvm::SmallDenseSet<QualType> Visited;
+      auto Check = [&Visited](QualType T, auto &&Check) -> bool {
+        if (!Visited.insert(T->getCanonicalTypeUnqualified()).second)
+          return false;
         const CXXRecordDecl *RD =
             T->getBaseElementTypeUnsafe()->getAsCXXRecordDecl();
         if (!RD || !RD->isCompleteDefinition())
@@ -7168,16 +7171,11 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
         if (!RD->hasConstexprDestructor())
           return false;
 
-        QualType CanUnqualT = T.getCanonicalType().getUnqualifiedType();
         for (const CXXBaseSpecifier &B : RD->bases())
-          if (B.getType().getCanonicalType().getUnqualifiedType() !=
-                  CanUnqualT &&
-              !Check(B.getType(), Check))
+          if (!Check(B.getType(), Check))
             return false;
         for (const FieldDecl *FD : RD->fields())
-          if (FD->getType().getCanonicalType().getUnqualifiedType() !=
-                  CanUnqualT &&
-              !Check(FD->getType(), Check))
+          if (!Check(FD->getType(), Check))
             return false;
         return true;
       };

clang/test/SemaCXX/gh102293.cpp

@@ -45,3 +45,20 @@ class quux : quux { // expected-error {{base class has incomplete type}} \
   virtual int c();
 };
 }
+
+// Ensure we don't get infinite recursion from the check, however. See GH141789
+namespace GH141789 {
+template <typename Ty>
+struct S {
+  Ty t; // expected-error {{field has incomplete type 'GH141789::X'}}
+};
+
+struct T {
+  ~T();
+};
+
+struct X { // expected-note {{definition of 'GH141789::X' is not complete until the closing '}'}}
+  S<X> next; // expected-note {{in instantiation of template class 'GH141789::S<GH141789::X>' requested here}}
+  T m;
+};
+}