[clang] Don't mark lambda non-dependent if nested in a generic lambda. (PR #149121)

[Younan Zhang via cfe-commits]

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@@ -15512,6 +15512,8 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
     DC = DC->getParent();
   if ((getSema().isUnevaluatedContext() ||
        getSema().isConstantEvaluatedContext()) &&
+      !(dyn_cast_or_null<CXXRecordDecl>(DC->getParent()) &&
+        cast<CXXRecordDecl>(DC->getParent())->isGenericLambda()) &&
       (DC->isFileContext() || !DC->getParent()->isDependentContext()))
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zyn0217 wrote:

So if I understand correctly, the inner generic lambda was previously caught by `!DC->getParent()->isDependentContext()` because its parent, despite it being a generic, is living in a non-dependent context such that the parent is never dependent which entails !isDependentContext()?

I am not quite sure if the approach is correct because the semantic of the dependency kind for lambda is sometimes counter-intuitive. For example, we even consider a generic lambda non-dependent if it lives in e.g. a namespace scope.

On the other hand, if the outer lambda (assuming it is generic) gets instantiated, then the 'generic' flag on it is still preserved, see `TransformTemplateParameterList()` in this function below and `CompleteLambdaCallOperator`, so at the time when the nested lambdas are half-transformed, the inner dependency kind might still be unexpected.

@mizvekov has been working on the removal of the DependencyKind and he might have some ideas too.


https://github.com/llvm/llvm-project/pull/149121