[clang] [Clang] warn on discarded [[nodiscard]] function results after casting in C (PR #104677)

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk created https://github.com/llvm/llvm-project/pull/104677

Fixes #104391

>From cd7ce740464c9c46ab231cf773fd1cf28e3495eb Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk at outlook.com>
Date: Sat, 17 Aug 2024 19:43:45 +0300
Subject: [PATCH] [Clang] warn on discarded [[nodiscard]] function results
 after casting in C

---
 clang/docs/ReleaseNotes.rst     | 2 ++
 clang/lib/Sema/SemaStmt.cpp     | 3 ++-
 clang/test/Sema/c2x-nodiscard.c | 6 ++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ffdd063ec99037..c8c6a98264b3a2 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -214,6 +214,8 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses the use of ``main`` in an ``extern`` context as invalid according to [basic.start.main] p3. Fixes #GH101512.
 
+- Clang now diagnoses when the result of a [[nodiscard]] function is discarded after being cast in C. Fixes #GH104391.
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index d283eaa511011b..ba681671eb3290 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -281,7 +281,8 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S, unsigned DiagID) {
   E = WarnExpr;
   if (const auto *Cast = dyn_cast<CastExpr>(E))
     if (Cast->getCastKind() == CK_NoOp ||
-        Cast->getCastKind() == CK_ConstructorConversion)
+        Cast->getCastKind() == CK_ConstructorConversion ||
+        Cast->getCastKind() == CK_IntegralCast)
       E = Cast->getSubExpr()->IgnoreImpCasts();
 
   if (const CallExpr *CE = dyn_cast<CallExpr>(E)) {
diff --git a/clang/test/Sema/c2x-nodiscard.c b/clang/test/Sema/c2x-nodiscard.c
index cb33c0c242e7db..8b48fe8118dfe7 100644
--- a/clang/test/Sema/c2x-nodiscard.c
+++ b/clang/test/Sema/c2x-nodiscard.c
@@ -54,3 +54,9 @@ void test_missiles(void) {
   launch_missiles();
 }
 
+[[nodiscard]] int f3();
+
+void f4() {
+#define M (unsigned int) f3()
+  M; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}