[Sema] Use lexical DC for friend functions when getting constraint instantiation args (llvm#77552)

Fixes a crash where the template argument depth computed in the semantic
context for a friend FunctionDecl with a constrained parameter is
compared against arguments in the lexical context for the purpose of
checking if the constraint depends on enclosing template parameters.

Since getTemplateInstantiationArgs in this case follows the semantic DC
for friend FunctionDecls, the resulting depth is incorrect and trips an
assertion.

Fixes llvm#75426

Author: antangelo

clang/docs/ReleaseNotes.rst

@@ -723,6 +723,10 @@ Bug Fixes in This Version
 - Clang now emits correct source location for code-coverage regions in `if constexpr`
   and `if consteval` branches.
   Fixes (`#54419 <https://github.com/llvm/llvm-project/issues/54419>`_)
+- Fix assertion failure when declaring a template friend function with
+  a constrained parameter in a template class that declares a class method
+  or lambda at different depth.
+  Fixes (`#75426 <https://github.com/llvm/llvm-project/issues/75426>`_)
 - Fix an issue where clang cannot find conversion function with template
   parameter when instantiation of template class.
   Fixes (`#77583 <https://github.com/llvm/llvm-project/issues/77583>`_)

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -223,6 +223,9 @@ Response HandleFunction(const FunctionDecl *Function,
       (!Pattern || !Pattern->getLexicalDeclContext()->isFileContext())) {
     return Response::ChangeDecl(Function->getLexicalDeclContext());
   }
+
+  if (ForConstraintInstantiation && Function->getFriendObjectKind())
+    return Response::ChangeDecl(Function->getLexicalDeclContext());
   return Response::UseNextDecl(Function);
 }
 

clang/test/SemaTemplate/GH75426.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+template<typename T> concept C = true;
+
+struct A {
+    template<C T> void f();
+};
+
+auto L = []<C T>{};
+
+template<typename X>
+class Friends {
+    template<C T> friend void A::f();
+    template<C T> friend void decltype(L)::operator()();
+};