[clang] [Clang] Fix error recovery for default arguments (PR #208868)

[Corentin Jabot via cfe-commits]

================
@@ -46,3 +46,14 @@ template <class T> void f();
 template <> void f<int>(int = []{ ; return 0; }()) {} // expected-error{{no function template matches function template specialization 'f'}} \
                                                       // expected-note at -1{{candidate template ignored}}
 }
+
+//PR208868
+struct Y {};
+enum E { e1, e2 };
+
+template<typename T>
+auto foo(E = ({ ; }) ? 0 : 1, E = e2) {  // expected-error {{default argument may not use a GNU statement expression}}
+  return 42;
+}
+
+static_assert(foo<Y>(e1) == 42, ""); // expected-error {{no matching function for call to 'foo'}}
----------------
cor3ntin wrote:

```suggestion
namespace GH208868 {
struct Y {};
enum E { e1, e2 };

template<typename T>
auto foo(E = ({ ; }) ? 0 : 1, E = e2) {  // expected-error {{default argument may not use a GNU statement expression}}
  return 42;
}

static_assert(foo<Y>(e1) == 42, ""); // expected-error {{no matching function for call to 'foo'}}
}
```

https://github.com/llvm/llvm-project/pull/208868