[clang] [Clang][Sema] Fix the lambda call expression inside of a type alias declaration (PR #82310)

[via cfe-commits]

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@@ -313,9 +313,75 @@ Response HandleRecordDecl(const CXXRecordDecl *Rec,
 
   // This is to make sure we pick up the VarTemplateSpecializationDecl that this
   // lambda is defined inside of.
-  if (Rec->isLambda())
+  if (Rec->isLambda()) {
     if (const Decl *LCD = Rec->getLambdaContextDecl())
       return Response::ChangeDecl(LCD);
+    // Attempt to retrieve the template arguments for a using alias declaration.
+    // This is necessary for constraint checking, since we always keep
+    // constraints relative to the primary template.
+    if (ForConstraintInstantiation && !SemaRef.CodeSynthesisContexts.empty()) {
+      for (auto &CSC : llvm::reverse(SemaRef.CodeSynthesisContexts)) {
+        if (CSC.Kind != Sema::CodeSynthesisContext::SynthesisKind::
+                            TypeAliasTemplateInstantiation)
+          continue;
+        auto *TATD = cast<TypeAliasTemplateDecl>(CSC.Entity),
+             *CurrentTATD = TATD;
+        FunctionDecl *LambdaCallOperator = Rec->getLambdaCallOperator();
+        // Retrieve the 'primary' template for a lambda call operator. It's
+        // unfortunate that we only have the mappings of call operators rather
+        // than lambda classes.
+        while (true) {
+          auto *FTD = dyn_cast_if_present<FunctionTemplateDecl>(
+              LambdaCallOperator->getDescribedTemplate());
+          if (FTD && FTD->getInstantiatedFromMemberTemplate()) {
+            LambdaCallOperator =
+                FTD->getInstantiatedFromMemberTemplate()->getTemplatedDecl();
+          } else if (auto *Prev = cast<CXXMethodDecl>(LambdaCallOperator)
+                                      ->getInstantiatedFromMemberFunction())
+            LambdaCallOperator = Prev;
+          else
+            break;
+        }
+        // Same applies for type alias Decl. We perform this to obtain the
+        // "canonical" template parameter depths.
+        while (TATD->getInstantiatedFromMemberTemplate())
+          TATD = TATD->getInstantiatedFromMemberTemplate();
+        // Tell if we're currently inside of a lambda expression that is
+        // surrounded by a using alias declaration. e.g.
+        //   template <class> using type = decltype([](auto) { ^ }());
+        // By checking if:
+        //  1. The lambda expression and the using alias declaration share the
+        //  same declaration context.
+        //  2. They have the same template depth.
+        // Then we assume the template arguments from the using alias
+        // declaration are essential for constraint instantiation. We have to do
+        // so since a TypeAliasTemplateDecl (or a TypeAliasDecl) is never a
+        // DeclContext, nor does it have an associated specialization Decl from
+        // which we could collect these template arguments.
+        if (cast<CXXRecordDecl>(LambdaCallOperator->getDeclContext())
+                    ->getTemplateDepth() == TATD->getTemplateDepth() &&
+            getLambdaAwareParentOfDeclContext(LambdaCallOperator) ==
+                TATD->getDeclContext()) {
+          Result.addOuterTemplateArguments(CurrentTATD,
+                                           CSC.template_arguments(),
+                                           /*Final=*/false);
+          // Visit the parent of the current type alias declaration rather than
+          // the lambda thereof. We have the following case:
+          // struct S {
+          //  template <class> using T = decltype([]<Concept> {} ());
+          // };
+          // void foo() {
+          //   S::T var;
+          // }
+          // The instantiated lambda expression (which we're visiting at 'var')
+          // has a function DeclContext 'foo' rather than the Record DeclContext
+          // S. This seems to be an oversight that we may want to set a Sema
+          // Context from the CXXScopeSpec before substituting into T to me.
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cor3ntin wrote:

@erichkeane Opinion?

https://github.com/llvm/llvm-project/pull/82310