[clang] fix lambda dependency issue with late parse attributes (llvm#161765)

This fixes a regression introduced in llvm#147835

When parsing a lambda where the call operator has a late parsed
attribute, we would try to build a 'this' type for the lambda, but in a
lambda 'this' never refers to the lambda class itself.

This late parsed attribute can be added implicitly by the
-ftrapping-math flag.

This patch patch makes it so CXXThisScopeRAII ignores lambdas.

This became observable in llvm#147835 because that made clang lazily create
tag types, and it removed a workaround for a lambda dependency bug where
any previously created tag type for the lambda is discarded after its
dependency is recalculated.

But the 'this' scope created above would defeat this laziness and create
the lambda type too soon, before its dependency was updated.

Since this regression was never released, there are no release notes.

Fixes llvm#161657

Author: mizvekov

clang/lib/Sema/SemaExprCXX.cpp

@@ -1251,6 +1251,10 @@ Sema::CXXThisScopeRAII::CXXThisScopeRAII(Sema &S,
   else
     Record = cast<CXXRecordDecl>(ContextDecl);
 
+  // 'this' never refers to the lambda class itself.
+  if (Record->isLambda())
+    return;
+
   QualType T = S.Context.getCanonicalTagType(Record);
   T = S.getASTContext().getQualifiedType(T, CXXThisTypeQuals);
 

clang/test/SemaTemplate/GH161657.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -ffp-exception-behavior=strict -verify %s
+// expected-no-diagnostics
+
+template <class T> struct S {
+  template <class U> using type1 = decltype([] { return U{}; });
+};
+
+void foo() {
+  using T1 = S<int>::type1<int>;
+  int x = T1()();
+}