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

Fixes #104391

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -218,6 +218,8 @@ 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
 ----------------------------------
 

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)) {

clang/test/Sema/c2x-nodiscard.c

@@ -54,3 +54,9 @@ void test_missiles(void) {
   launch_missiles();
 }
 
+[[nodiscard]] int f3();
+
+void GH104391() {
+#define M (unsigned int) f3()
+  M; // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}