[clang] [Clang] Preserve coroutine parameter referenced state (PR #70973)

[Yuxuan Chen via cfe-commits]

https://github.com/yuxuanchen1997 created https://github.com/llvm/llvm-project/pull/70973

This PR is proposing a fix for https://github.com/llvm/llvm-project/issues/65971.

Previously, given a coroutine like this
```
task foo(int a) {
  co_return;
}
```
Parameter `a` is never used. However, because C++ coroutines move constructs the variable to a heap allocated coroutine activation frame, we considered all parameters referenced. When diagnosing unused parameters, we cannot distinguish if the variable reference was due to coroutine parameter moves. 

Compiler Explorer shows that GCC warns against this case correctly, but clang does not: https://godbolt.org/z/Wo7dfqeaf

This patch addresses this issue by preserving the original `ParmVarDecl`'s `Referenced` state.

>From 54255c560c01a1fd017c52898b8688e24c4397e5 Mon Sep 17 00:00:00 2001
From: Yuxuan Chen <ych at meta.com>
Date: Wed, 1 Nov 2023 11:59:57 -0700
Subject: [PATCH] [Clang] Preserve coroutine parameter referenced state

---
 clang/lib/Sema/SemaCoroutine.cpp              |  6 ++++
 .../warn-unused-parameters-coroutine.cpp      | 28 +++++++++++++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 clang/test/SemaCXX/warn-unused-parameters-coroutine.cpp

diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
index 38ac406b14adadf..281b7d68d361fcc 100644
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -1965,9 +1965,15 @@ bool Sema::buildCoroutineParameterMoves(SourceLocation Loc) {
     if (PD->getType()->isDependentType())
       continue;
 
+    // Preserve the referenced state of the parameters for diagnosis purposes.
+    bool DeclReferenced = PD->isReferenced();
+
     ExprResult PDRefExpr =
         BuildDeclRefExpr(PD, PD->getType().getNonReferenceType(),
                          ExprValueKind::VK_LValue, Loc); // FIXME: scope?
+
+    PD->setReferenced(DeclReferenced);
+
     if (PDRefExpr.isInvalid())
       return false;
 
diff --git a/clang/test/SemaCXX/warn-unused-parameters-coroutine.cpp b/clang/test/SemaCXX/warn-unused-parameters-coroutine.cpp
new file mode 100644
index 000000000000000..0fd26ea4a21be79
--- /dev/null
+++ b/clang/test/SemaCXX/warn-unused-parameters-coroutine.cpp
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -fsyntax-only -Wunused-parameter -verify -std=c++20 %s
+
+#include "Inputs/std-coroutine.h"
+
+struct awaitable {
+  bool await_ready() noexcept;
+  void await_resume() noexcept;
+  void await_suspend(std::coroutine_handle<>) noexcept;
+};
+
+struct task : awaitable {
+  struct promise_type {
+    task get_return_object() noexcept;
+    awaitable initial_suspend() noexcept;
+    awaitable final_suspend() noexcept;
+    void unhandled_exception() noexcept;
+    void return_void() noexcept;
+  };
+};
+
+task foo(int a) { // expected-warning{{unused parameter 'a'}}
+  co_return;
+}
+
+task bar(int a, int b) { // expected-warning{{unused parameter 'b'}}
+  a = a + 1;
+  co_return;
+}