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

[Utkarsh Saxena via cfe-commits]

https://github.com/usx95 created https://github.com/llvm/llvm-project/pull/78589

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`.

>From 07c6ad92fda63e69489933b5cededf8f03a68ef3 Mon Sep 17 00:00:00 2001
From: Utkarsh Saxena <usx at google.com>
Date: Thu, 18 Jan 2024 14:38:22 +0000
Subject: [PATCH] [coroutine] Create coroutine body in the correct eval context

---
 clang/lib/Sema/SemaCoroutine.cpp              |  4 +++
 clang/test/SemaCXX/coroutine-promise-ctor.cpp | 34 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 clang/test/SemaCXX/coroutine-promise-ctor.cpp

diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index 0e0f8f67dcd73e1..4e600fd29ee7395 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 000000000000000..c8a976e838241f0
--- /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