From 9ebc45c754ed842e75f928aae5aab39790bd2ef3 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Mon, 26 Aug 2024 11:16:27 +0200
Subject: [PATCH] Give some TLC to FunctorCallBase::call_internal()

The function takes `fn` by universal reference, so it should be
`std::forward`ing `fn` before calling it. This includes the
unevaluated contexts, so use `std::is_nothrow_invocable_v` and
`std::invoke_result_t` instead of `noexcept(fn())` and
`decltype(fn())`. The std traits take the value category into account
automatically. Linters like to warn about universal/forwarding
references without std::forward(), so this should be fixed even without
and actual overloaded op() &/op() && present.

The function passed as `fn` may also be marked as `[[nodiscard]]`, so
make sure that we never ignore the return value (unless it's void) to
avoid throwing warnings.

As a drive-by, replace Q_UNUSED with [[maybe_unused]] and inline the
SlotRet type alias into its only user.

Amends f894f04c9d03878116de61c11d4591da67c10378.

Change-Id: I5e045fc735f461a82244d7e80f81c2a9b1e69046
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 832e47ca44d3af1b7a3f722bad2a1d0c6fd56b5f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/corelib/kernel/qobjectdefs_impl.h | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/src/corelib/kernel/qobjectdefs_impl.h b/src/corelib/kernel/qobjectdefs_impl.h
index 1e953f29b6c..8bb2c7c3f01 100644
--- a/src/corelib/kernel/qobjectdefs_impl.h
+++ b/src/corelib/kernel/qobjectdefs_impl.h
@@ -58,18 +58,17 @@ namespace QtPrivate {
     struct FunctorCallBase
     {
         template <typename R, typename Lambda>
-        static void call_internal(void **args, Lambda &&fn) noexcept(noexcept(fn()))
+        static void call_internal([[maybe_unused]] void **args, Lambda &&fn)
+            noexcept(std::is_nothrow_invocable_v<Lambda>)
         {
-            using SlotRet = decltype(fn());
-            if constexpr (std::is_void_v<R> || std::is_void_v<SlotRet>) {
-                Q_UNUSED(args);
+            if constexpr (std::is_void_v<R> || std::is_void_v<std::invoke_result_t<Lambda>>) {
+                std::forward<Lambda>(fn)();
             } else {
-                if (args[0]) {
-                    *reinterpret_cast<R *>(args[0]) = fn();
-                    return;
-                }
+                if (args[0])
+                    *reinterpret_cast<R *>(args[0]) = std::forward<Lambda>(fn)();
+                else
+                    [[maybe_unused]] auto r = std::forward<Lambda>(fn)();
             }
-            fn();
         }
     };