[clang] [Clang][Concepts] Correct the CurContext for friend declarations (PR #106890)

[via cfe-commits]

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@@ -1012,7 +1012,14 @@ static const Expr *SubstituteConstraintExpressionWithoutSatisfaction(
   // possible that e.g. constraints involving C<Class<T>> and C<Class> are
   // perceived identical.
   std::optional<Sema::ContextRAII> ContextScope;
-  if (auto *RD = dyn_cast<CXXRecordDecl>(DeclInfo.getDeclContext())) {
+  const DeclContext *DC = [&] {
+    if (!DeclInfo.getDecl())
+      return DeclInfo.getDeclContext();
+    return DeclInfo.getDecl()->getFriendObjectKind()
+               ? DeclInfo.getLexicalDeclContext()
+               : DeclInfo.getDeclContext();
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cor3ntin wrote:

I wonder if we should have a function that handles that logic of getting a lexical context for friends - we do it in a few places

https://github.com/llvm/llvm-project/pull/106890