[clang] [coroutine] Create coroutine body in the correct eval context (PR #78589)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-coroutines

Author: Utkarsh Saxena (usx95)

<details>
<summary>Changes</summary>

Fixes: https://github.com/llvm/llvm-project/issues/78290

See the bug for more context.
```
Gen ACoroutine() {
  if constexpr (0) // remove it make clang compile.
    co_return;
  co_await Gen{};
}
```
We miss symbol of ctor of promise_type if the first coroutine statement happens to be inside the disabled branch of `if constexpr`.

This happens because the promise object is built when we see the first coroutine statement which is present in `ExpressionEvaluationContext::DiscardedStatement` context due to `if constexpr (0)`. This makes clang believe that the promise constructor is only odr-used and not really "used".

The expr evaluation context for the coroutine body should not be related to the context in which the first coroutine statement appears. We override the context to `PotentiallyEvaluated`.

---
Full diff: https://github.com/llvm/llvm-project/pull/78589.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaCoroutine.cpp (+4) 
- (added) clang/test/SemaCXX/coroutine-promise-ctor.cpp (+34) 


``````````diff
diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index 0e0f8f67dcd73e..4e600fd29ee739 100644
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -21,6 +21,7 @@
 #include "clang/AST/StmtCXX.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Lex/Preprocessor.h"
+#include "clang/Sema/EnterExpressionEvaluationContext.h"
 #include "clang/Sema/Initialization.h"
 #include "clang/Sema/Overload.h"
 #include "clang/Sema/ScopeInfo.h"
@@ -773,6 +774,9 @@ bool Sema::checkFinalSuspendNoThrow(const Stmt *FinalSuspend) {
 
 bool Sema::ActOnCoroutineBodyStart(Scope *SC, SourceLocation KWLoc,
                                    StringRef Keyword) {
+  // Ignore previous expr evaluation contexts.
+  EnterExpressionEvaluationContext PotentiallyEvaluated(
+      *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
   if (!checkCoroutineContext(*this, KWLoc, Keyword))
     return false;
   auto *ScopeInfo = getCurFunction();
diff --git a/clang/test/SemaCXX/coroutine-promise-ctor.cpp b/clang/test/SemaCXX/coroutine-promise-ctor.cpp
new file mode 100644
index 00000000000000..c8a976e838241f
--- /dev/null
+++ b/clang/test/SemaCXX/coroutine-promise-ctor.cpp
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++20 -ast-dump %s | FileCheck %s
+#include "Inputs/std-coroutine.h"
+
+// Github issue: https://github.com/llvm/llvm-project/issues/78290
+class Gen {
+   public:
+    class promise_type {
+       public:
+        template<typename... Args>
+        explicit promise_type(Args...) {}
+        // CHECK:       CXXConstructorDecl {{.*}} used promise_type 'void ()' {{.*}}
+        // CHECK-NEXT:     TemplateArgument pack
+        // CHECK-NEXT:     CompoundStmt {{.*}}
+        Gen get_return_object() { return {}; }
+
+        void unhandled_exception() {}
+        void return_void() {}
+        std::suspend_always await_transform(Gen gen) { return {}; }
+
+        std::suspend_always initial_suspend() { return {}; }
+        // CHECK: CXXMethodDecl {{.*}} used initial_suspend {{.*}}
+        std::suspend_always final_suspend() noexcept { return {}; }
+        // CHECK: CXXMethodDecl {{.*}} used final_suspend {{.*}}
+    };
+};
+
+Gen CoroutineBody() {
+    if constexpr (0) {
+        co_await Gen{};
+    }
+    co_await Gen{};
+}
+
+int main() { return 0; }
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/78589