[clang] Surface error for plain return statement in coroutine earlier (PR #100985)

[Ilya Biryukov via cfe-commits]

================
@@ -291,6 +318,50 @@ void mixed_coreturn_template2(bool b, T) {
     return; // expected-error {{not allowed in coroutine}}
 }
 
+struct promise_handle;
+
+struct Handle : std::coroutine_handle<promise_handle> { // expected-note 4{{not viable}}
+    // expected-note at -1 4{{not viable}}
+    using promise_type = promise_handle;
+};
+
+struct promise_handle {
+    Handle get_return_object() noexcept {
+      { return Handle(std::coroutine_handle<Handle::promise_type>::from_promise(*this)); }
+    }
+    suspend_never initial_suspend() const noexcept { return {}; }
+    suspend_never final_suspend() const noexcept { return {}; }
+    void return_void() const noexcept {}
+    void unhandled_exception() const noexcept {}
+};
+
+Handle mixed_return_value() {
+  co_await a; // expected-note {{function is a coroutine due to use of 'co_await' here}}
+  return 0; // expected-error {{return statement not allowed in coroutine}}
+  // expected-error at -1 {{no viable conversion from returned value of type}}
+  // CHECK-NOT: error: no viable conversion from returned value of type
+  // CHECK: error: return statement not allowed in coroutine
+  // CHECK: error: no viable conversion from returned value of type
+}
+
+Handle mixed_return_value_return_first(bool b) {
+   if (b) {
+        return 0; // expected-error {{no viable conversion from returned value of type}}
+        // expected-error at -1 {{return statement not allowed in coroutine}}
----------------
ilya-biryukov wrote:

Could you run `clang-format` over these lines? (E.g. via `git-clang-format` or "Format Lines" action from VSCode with Clangd installed). The `// expected-error at -1` should get nicely aligned with the previous lines.

https://github.com/llvm/llvm-project/pull/100985