From 0842b01f0269cdd8099a5fbe849c5c2876a0d744 Mon Sep 17 00:00:00 2001
From: Fabian Kosmale <fabian.kosmale@qt.io>
Date: Thu, 20 Jan 2022 17:01:51 +0100
Subject: QProperty: Work around constexpr issues on MSVC
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

MSVC complains about illegal uses of void, even though the code should
be guarded by an if constexpr statement.
 qproperty.h(85): error C2182: 'val': illegal use of type 'void'
 qproperty.h(92): error C2182: 'abstract declarator': illegal use of type 'void'

Avoid the issue by using a dedicated dummy type instead of void.

Fixes: QTBUG-100072
Task-number: QTQAINFRA-4242
Pick-to: 6.3 6.2
Change-Id: Idc4886c26c3794e709eb762ba836139f720f8133
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
---
 src/corelib/kernel/qpropertyprivate.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

(limited to 'src/corelib/kernel/qpropertyprivate.h')

diff --git a/src/corelib/kernel/qpropertyprivate.h b/src/corelib/kernel/qpropertyprivate.h
index 8fe8cc7ff2..ddb09bb119 100644
--- a/src/corelib/kernel/qpropertyprivate.h
+++ b/src/corelib/kernel/qpropertyprivate.h
@@ -184,6 +184,12 @@ struct QPropertyProxyBindingData
 namespace QtPrivate {
 struct BindingEvaluationState;
 
+/*  used in BindingFunctionVTable::createFor; on all other compilers, void would work, but on
+    MSVC this causes C2182 when compiling in C++20 mode. As we only need to provide some default
+    value which gets ignored, we introduce this dummy type.
+*/
+struct MSVCWorkAround {};
+
 struct BindingFunctionVTable
 {
     using CallFn = bool(*)(QMetaType, QUntypedPropertyData *, void *);
@@ -194,7 +200,7 @@ struct BindingFunctionVTable
     const MoveCtrFn moveConstruct;
     const qsizetype size;
 
-    template<typename Callable, typename PropertyType=void>
+    template<typename Callable, typename PropertyType=MSVCWorkAround>
     static constexpr BindingFunctionVTable createFor()
     {
         static_assert (alignof(Callable) <= alignof(std::max_align_t), "Bindings do not support overaligned functors!");
@@ -205,7 +211,7 @@ struct BindingFunctionVTable
                     static_assert (std::is_invocable_r_v<bool, Callable, QMetaType, QUntypedPropertyData *> );
                     auto untypedEvaluationFunction = static_cast<Callable *>(f);
                     return std::invoke(*untypedEvaluationFunction, metaType, dataPtr);
-                } else if constexpr (!std::is_same_v<PropertyType, void>) { // check for void to workaround MSVC issue
+                } else if constexpr (!std::is_same_v<PropertyType, MSVCWorkAround>) {
                     Q_UNUSED(metaType);
                     QPropertyData<PropertyType> *propertyPtr = static_cast<QPropertyData<PropertyType> *>(dataPtr);
                     // That is allowed by POSIX even if Callable is a function pointer
@@ -232,7 +238,7 @@ struct BindingFunctionVTable
     }
 };
 
-template<typename Callable, typename PropertyType=void>
+template<typename Callable, typename PropertyType=MSVCWorkAround>
 inline constexpr BindingFunctionVTable bindingFunctionVTable = BindingFunctionVTable::createFor<Callable, PropertyType>();
 
 
-- 
cgit v1.2.3