[clang] fix crash on generic lambda with lambda in decltype

[v1nh1shungry]

Summary

Relevant issue: https://github.com/llvm/llvm-project/issues/59771

During the instantiation of a generic lambda, a non-generic lambda in
the trailing decltype is a DeclContext but not a dependent context,
so we shouldn't call PerformDependentDiagnostics on it.

[aaron.ballman]

This fixes #57170 (59771 was closed as a dupe of that one). I think this LG, but I'd appreciate a second set of eyes on it as there have been plenty of changes in this area recently.

[erichkeane]

I think that makes sense to me, LGTM.

[v1nh1shungry]

Thanks for reviewing!

[shafik]

Makes sense to me as well.

clang/lib/Sema/SemaTemplateInstantiate.cpp

void transformedLocalDecl(Decl *Old, ArrayRef<Decl *> NewDecls) {
           NewMD->setInstantiationOfMemberFunction(OldMD,
                                                   TSK_ImplicitInstantiation);
       }
 
       SemaRef.CurrentInstantiationScope->InstantiatedLocal(Old, New);
 
       // We recreated a local declaration, but not by instantiating it. There
       // may be pending dependent diagnostics to produce.
-      if (auto *DC = dyn_cast<DeclContext>(Old))
+      if (auto *DC = dyn_cast<DeclContext>(Old); DC && DC->isDependentContext())
         SemaRef.PerformDependentDiagnostics(DC, TemplateArgs);
     }
 
     /// Transform the definition of the given declaration by
     /// instantiating it.
     Decl *TransformDefinition(SourceLocation Loc, Decl *D);
 
     /// Transform the first qualifier within a scope by instantiating the

clang/test/SemaCXX/lambda-unevaluated.cpp

 void f()  noexcept(noexcept([] static { return 0; }()));
 
 using a = decltype([] static { return 0; });
 using b = decltype([] static { return 0; }());
 using c = decltype([]() static noexcept(noexcept([] { return 0; }())) { return 0; });
 using d = decltype(sizeof([] static { return 0; }));
 
 }
+
+namespace lambda_in_trailing_decltype {
+auto x = ([](auto) -> decltype([] {}()) {}(0), 2);
+}