[clang-tools-extra] [clang-tidy] Improve bugprone-unused-return-value check (PR #66573)

[Congcong Cai via cfe-commits]

================
@@ -130,26 +130,35 @@ UnusedReturnValueCheck::UnusedReturnValueCheck(llvm::StringRef Name,
                                             "::std::error_condition;"
                                             "::std::errc;"
                                             "::std::expected;"
-                                            "::boost::system::error_code"))) {}
+                                            "::boost::system::error_code"))),
+      AllowCastToVoid(Options.get("AllowCastToVoid", false)) {}
 
 void UnusedReturnValueCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
   Options.store(Opts, "CheckedFunctions", CheckedFunctions);
   Options.store(Opts, "CheckedReturnTypes",
                 utils::options::serializeStringList(CheckedReturnTypes));
+  Options.store(Opts, "AllowCastToVoid", AllowCastToVoid);
 }
 
 void UnusedReturnValueCheck::registerMatchers(MatchFinder *Finder) {
   auto FunVec = utils::options::parseStringList(CheckedFunctions);
 
-  auto MatchedCallExpr = expr(ignoringImplicit(ignoringParenImpCasts(
-      callExpr(callee(functionDecl(
-                   // Don't match void overloads of checked functions.
-                   unless(returns(voidType())),
-                   anyOf(isInstantiatedFrom(hasAnyName(FunVec)),
-                         returns(hasCanonicalType(hasDeclaration(
-                             namedDecl(matchers::matchesAnyListedName(
-                                 CheckedReturnTypes)))))))))
-          .bind("match"))));
+  auto MatchedDirectCallExpr =
+      expr(callExpr(callee(functionDecl(
+                        // Don't match void overloads of checked functions.
+                        unless(returns(voidType())),
+                        anyOf(isInstantiatedFrom(hasAnyName(FunVec)),
+                              returns(hasCanonicalType(hasDeclaration(
+                                  namedDecl(matchers::matchesAnyListedName(
+                                      CheckedReturnTypes)))))))))
+               .bind("match"));
+
+  auto CheckCastToVoid =
+      AllowCastToVoid ? castExpr(unless(hasCastKind(CK_ToVoid))) : castExpr();
+  auto MatchedCallExpr = expr(
+      anyOf(MatchedDirectCallExpr,
+            explicitCastExpr(unless(cxxFunctionalCastExpr()), CheckCastToVoid,
----------------
HerrCai0907 wrote:

I am not pretty sure about this part. If we ignore other part and unfold it, it looks like
```c++
expr(
  explicitCastExpr(
    castExpr(
      unless(hasCastKind(CK_ToVoid))
    )
  )
)
```
Should it be
```c++
  explicitCastExpr(
      unless(hasCastKind(CK_ToVoid))
  )
```

https://github.com/llvm/llvm-project/pull/66573