Forbid co_await and co_yield in invalid expr contexts (#130455)

Fix #78426 

C++26 introduced braced initializer lists as template arguments.
However, such contexts should be considered invalid for co_await and
co_yield. This commit explicitly rules out the possibility of using
these exprs in template arguments.

---------

Co-authored-by: cor3ntin <[email protected]>

Author: NewSigma

clang/docs/ReleaseNotes.rst

@@ -295,6 +295,8 @@ Bug Fixes to C++ Support
   direct-list-initialized from an array is corrected to direct-initialization.
 - Clang no longer crashes when a coroutine is declared ``[[noreturn]]``. (#GH127327)
 - Clang now uses the parameter location for abbreviated function templates in ``extern "C"``. (#GH46386)
+- Clang will emit an error instead of crash when use co_await or co_yield in
+  C++26 braced-init-list template parameter initialization. (#GH78426)
 - Fixes matching of nested template template parameters. (#GH130362)
 - Correctly diagnoses template template paramters which have a pack parameter
   not in the last position.

clang/lib/Sema/SemaCoroutine.cpp

@@ -788,7 +788,11 @@ static bool checkSuspensionContext(Sema &S, SourceLocation Loc,
   // First emphasis of [expr.await]p2: must be a potentially evaluated context.
   // That is, 'co_await' and 'co_yield' cannot appear in subexpressions of
   // \c sizeof.
-  if (S.isUnevaluatedContext()) {
+  const auto ExprContext = S.currentEvaluationContext().ExprContext;
+  const bool BadContext =
+      S.isUnevaluatedContext() ||
+      ExprContext != Sema::ExpressionEvaluationContextRecord::EK_Other;
+  if (BadContext) {
     S.Diag(Loc, diag::err_coroutine_unevaluated_context) << Keyword;
     return false;
   }
@@ -798,7 +802,6 @@ static bool checkSuspensionContext(Sema &S, SourceLocation Loc,
     S.Diag(Loc, diag::err_coroutine_within_handler) << Keyword;
     return false;
   }
-
   return true;
 }
 

clang/test/SemaCXX/coroutine-decltype.cpp renamed to clang/test/SemaCXX/coroutine-unevaluate.cpp

@@ -32,3 +32,11 @@ MyTask DoAnotherthing() {
   static_assert(__is_same(void, decltype(co_yield 0))); // expected-error {{'co_yield' cannot be used in an unevaluated context}}
   co_return;
 }
+
+template<class>
+struct Task {};
+
+void BracedInitListCXX26() {
+  []() -> Task<{ co_await 1 }> {}; // expected-error {{'co_await' cannot be used in an unevaluated context}}
+  []() -> Task<{ co_yield 1 }> {}; // expected-error {{'co_yield' cannot be used in an unevaluated context}}
+}