[clang] [clang] Fix assertion failure when transforming co_yield in invalid coroutine (PR #176082)

[Aniket Singh via cfe-commits]

https://github.com/Aniketsingh54 created https://github.com/llvm/llvm-project/pull/176082

Clang would previously crash with an assertion failure in `LocalInstantiationScope::findInstantiationOf` when compiling a C++20 coroutine that was missing `promise_type::final_suspend`.

The crash occurred during `TreeTransform::TransformCoyieldExpr` because the compiler attempted to rebuild the coroutine body even after the promise object failed to form correctly. This led to a lookup for a declaration that was never instantiated.

This patch adds a check for `CoroutinePromise` validity in `TransformCoyieldExpr`. If the promise is null or invalid, we now bail out early, preventing the assertion and allowing the compiler to recover gracefully after emitting the error.

Fixes #175720

>From 58fb31621f29f699cf3ca678198096a0ed6f8eaf Mon Sep 17 00:00:00 2001
From: Aniket Singh <amiket.singh.3200.00 at gmail.com>
Date: Thu, 15 Jan 2026 09:32:15 +0530
Subject: [PATCH] [clang] Fix assertion failure when transforming co_yield in
 invalid coroutine

Clang would previously crash with an assertion failure in `LocalInstantiationScope::findInstantiationOf` when compiling a C++20 coroutine that was missing `promise_type::final_suspend`.

The crash occurred during `TreeTransform::TransformCoyieldExpr` because the compiler attempted to rebuild the coroutine body even after the promise object failed to form correctly. This led to a lookup for a declaration that was never instantiated.

This patch adds a check for `CoroutinePromise` validity in `TransformCoyieldExpr`. If the promise is null or invalid, we now bail out early, preventing the assertion and allowing the compiler to recover gracefully after emitting the error.

Fixes #175720
---
 clang/lib/Sema/TreeTransform.h                |  9 ++++++
 .../SemaCXX/coroutine-final-suspend-crash.cpp | 32 +++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 clang/test/SemaCXX/coroutine-final-suspend-crash.cpp

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index a53d578fc35ac..b51780d36ad11 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -8956,6 +8956,15 @@ TreeTransform<Derived>::TransformDependentCoawaitExpr(DependentCoawaitExpr *E) {
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformCoyieldExpr(CoyieldExpr *E) {
+  // FINAL FIX: Strict check.
+  // 1. If we are in a function scope, but the CoroutinePromise is missing
+  // (null),
+  //    it means the coroutine setup failed (e.g. valid promise type not found).
+  // 2. If the promise exists but is invalid, we also fail.
+  if (clang::sema::FunctionScopeInfo *FSI = getSema().getCurFunction()) {
+    if (!FSI->CoroutinePromise || FSI->CoroutinePromise->isInvalidDecl())
+      return ExprError();
+  }
   ExprResult Result = getDerived().TransformInitializer(E->getOperand(),
                                                         /*NotCopyInit*/false);
   if (Result.isInvalid())
diff --git a/clang/test/SemaCXX/coroutine-final-suspend-crash.cpp b/clang/test/SemaCXX/coroutine-final-suspend-crash.cpp
new file mode 100644
index 0000000000000..c0cb388005b1a
--- /dev/null
+++ b/clang/test/SemaCXX/coroutine-final-suspend-crash.cpp
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+#include "Inputs/std-coroutine.h"
+
+struct Tag {};
+struct Y {
+  bool await_ready() const;
+  void await_suspend(std::coroutine_handle<>) const;
+  void await_resume() const;
+};
+
+struct Promise {
+  Tag get_return_object();
+  std::suspend_always initial_suspend();
+  // We intentionally omit final_suspend to trigger the error path
+  void unhandled_exception();
+  Y yield_value(int);
+  void return_void();
+};
+
+template <class... Args> struct std::coroutine_traits<Tag, Args...> {
+  using promise_type = Promise;
+};
+
+template <class T> struct S {
+  // The error is diagnosed at the function declaration, not the yield statement.
+  template <class U> static Tag f() { // expected-error {{no member named 'final_suspend' in 'Promise'}}
+    co_yield 0;
+  }
+};
+
+Tag g() { return S<int>::f<void>(); }
\ No newline at end of file