[clang] [Clang] prevent assertion failure by avoiding always-dependent lambdas in constraint-related nested scopes (PR #173776)

[Younan Zhang via cfe-commits]

================
@@ -282,3 +281,11 @@ static_assert(__is_same_as(int, helper<int>));
 
 
 } // namespace GH138018
+
+namespace GH172814 {
+auto t() {
+  int x = 0;
+  return [](auto w=[&] { x += w(); }); // expected-error {{lambda expression in default argument cannot capture any entity}} \
+                                       // expected-error {{expected body of lambda expression}}
----------------
zyn0217 wrote:

Can we test a valid example?

```cpp
auto f() {
  int a = 0;
  return [](auto w = [&] { return a; }) {};
};
```

https://github.com/llvm/llvm-project/pull/173776