[Clang] Do not skip over RequiresExprBodyDecl when creating lambdas (#147764)

When we create a lambda, we would skip over declaration contexts
representing a require expression body, which would lead to wrong
lookup.

Note that I wasn't able to establish why the code
in `Sema::createLambdaClosureType` was there to begin with (it's not
exactly recent)

The changes to mangling only ensure the status quo is preserved and do
not attempt to address the known issues of
mangling lambdas in require clauses.

In particular the itanium mangling is consistent with Clang before this
patch but differs from GCC's.

Fixes #147650

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -926,6 +926,7 @@ Bug Fixes to C++ Support
 - Fix a bug where private access specifier of overloaded function not respected. (#GH107629)
 - Correctly handle allocations in the condition of a ``if constexpr``.(#GH120197) (#GH134820)
 - Fixed a crash when handling invalid member using-declaration in C++20+ mode. (#GH63254)
+- Fix name lookup in lambda appearing in the body of a requires expression. (#GH147650)
 - Fix a crash when trying to instantiate an ambiguous specialization. (#GH51866)
 - Improved handling of variables with ``consteval`` constructors, to
   consistently treat the initializer as manifestly constant-evaluated.

clang/lib/AST/ItaniumMangle.cpp

@@ -1088,6 +1088,9 @@ void CXXNameMangler::mangleNameWithAbiTags(GlobalDecl GD,
     return;
   }
 
+  while (DC->isRequiresExprBody())
+    DC = DC->getParent();
+
   if (DC->isTranslationUnit() || isStdNamespace(DC)) {
     // Check if we have a template.
     const TemplateArgumentList *TemplateArgs = nullptr;

clang/lib/Sema/SemaLambda.cpp

@@ -249,8 +249,6 @@ Sema::createLambdaClosureType(SourceRange IntroducerRange, TypeSourceInfo *Info,
                               unsigned LambdaDependencyKind,
                               LambdaCaptureDefault CaptureDefault) {
   DeclContext *DC = CurContext;
-  while (!(DC->isFunctionOrMethod() || DC->isRecord() || DC->isFileContext()))
-    DC = DC->getParent();
 
   bool IsGenericLambda =
       Info && getGenericLambdaTemplateParameterList(getCurLambda(), *this);

clang/test/CodeGenCXX/mangle-requires.cpp

@@ -32,3 +32,16 @@ template <typename T> void g(int n) requires requires (T m) {
 } {}
 // CHECK: define {{.*}}@_Z1gIiEviQrQT__XplfL0p_fp_E(
 template void g<int>(int);
+
+
+namespace GH147650 {
+
+template <int> int b;
+template <int b>
+void f()
+    requires requires { [] { (void)b; }; } {}
+void test() {
+    f<42>();
+}
+// CHECK-LABEL:define {{.*}} void @"_ZN8GH1476501fILi42EEEvvQrqXLNS_3$_0EEE"()
+}

clang/test/CodeGenCXX/ms-mangle-requires.cpp

@@ -57,3 +57,15 @@ void m_fn2() {
 } // namespace Regression2
 
 }
+
+namespace GH147650 {
+
+template <int> int b;
+template <int b>
+void f()
+    requires requires { [] { (void)b; }; } {}
+void test() {
+    f<42>();
+}
+// CHECK-LABEL:define {{.*}} void @"??$f@$0CK@@GH147650@@YAXXZ"()
+}

clang/test/SemaTemplate/concepts-lambda.cpp

@@ -340,3 +340,13 @@ void foo() {
 }
 
 }
+
+namespace GH147650 {
+template <int> int b;
+template <int b>
+void f()
+    requires requires { [] { (void)b; static_assert(b == 42); }; } {}
+void test() {
+    f<42>();
+}
+}