[Concepts] Add Decls from the outer scope of the current lambda for conversion function constraints (#83420)

This fixes the case shown by
#64808 (comment).

Similar to
f9caa12,
we have some calls to constraint checking for a lambda's conversion
function while determining the conversion sequence.

This patch addresses the problem where the requires-expression within
such a lambda references to a Decl outside of the lambda by adding these
Decls to the current instantiation scope.

I'm abusing the flag `ForOverloadResolution` of
CheckFunctionConstraints, which is actually meant to consider the Decls
from parent DeclContexts.

---------

Co-authored-by: cor3ntin <[email protected]>

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -387,6 +387,8 @@ Bug Fixes to C++ Support
   Fixes (#GH80997)
 - Fix an issue where missing set friend declaration in template class instantiation.
   Fixes (#GH84368).
+- Fixed a crash while checking constraints of a trailing requires-expression of a lambda, that the
+  expression references to an entity declared outside of the lambda. (#GH64808)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaConcept.cpp

@@ -692,11 +692,15 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
   // A lambda conversion operator has the same constraints as the call operator
   // and constraints checking relies on whether we are in a lambda call operator
   // (and may refer to its parameters), so check the call operator instead.
+  // Note that the declarations outside of the lambda should also be
+  // considered. Turning on the 'ForOverloadResolution' flag results in the
+  // LocalInstantiationScope not looking into its parents, but we can still
+  // access Decls from the parents while building a lambda RAII scope later.
   if (const auto *MD = dyn_cast<CXXConversionDecl>(FD);
       MD && isLambdaConversionOperator(const_cast<CXXConversionDecl *>(MD)))
     return CheckFunctionConstraints(MD->getParent()->getLambdaCallOperator(),
                                     Satisfaction, UsageLoc,
-                                    ForOverloadResolution);
+                                    /*ShouldAddDeclsFromParentScope=*/true);
 
   DeclContext *CtxToSave = const_cast<FunctionDecl *>(FD);
 

clang/test/SemaTemplate/concepts.cpp

@@ -1085,3 +1085,32 @@ template void Struct<void>::bar<>();
 template int Struct<void>::field<1, 2>;
 
 }
+
+namespace GH64808 {
+
+template <class T> struct basic_sender {
+  T func;
+  basic_sender(T) : func(T()) {}
+};
+
+auto a = basic_sender{[](auto... __captures) {
+  return []() // #note-a-1
+    requires((__captures, ...), false) // #note-a-2
+  {};
+}()};
+
+auto b = basic_sender{[](auto... __captures) {
+  return []()
+    requires([](int, double) { return true; }(decltype(__captures)()...))
+  {};
+}(1, 2.33)};
+
+void foo() {
+  a.func();
+  // expected-error@-1{{no matching function for call}}
+  // expected-note@#note-a-1{{constraints not satisfied}}
+  // expected-note@#note-a-2{{evaluated to false}}
+  b.func();
+}
+
+} // namespace GH64808