[coroutine] Create coroutine body in the correct eval context

clang/docs/ReleaseNotes.rst

@@ -804,6 +804,9 @@ Bug Fixes in This Version
   Objective-C++ property accesses to not be converted to a function call
   to the getter in the placement-args of new-expressions.
   Fixes (`#65053 <https://github.com/llvm/llvm-project/issues/65053>`_)
+- Fix an issue with missing symbol definitions when the first coroutine
+  statement appears in a discarded ``if constexpr`` branch.
+  Fixes (`#78290 <https://github.com/llvm/llvm-project/issues/78290>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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();

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
+namespace GH78290 {
+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{};
+}
+} // namespace GH78290