From 892448b6e027236228b2c6429a6453c038b5c403 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kai=20K=C3=B6hne?= <kai.koehne@qt.io>
Date: Tue, 11 Jul 2023 16:14:07 +0200
Subject: [PATCH] Make Qt headers work with MSVC /W4

This requires explicitly marking constexpr if conditions to fix C4127
issues:

  qtbase/src/corelib/text/qstringbuilder.h(112):
  error C2220: the following warning is treated as an error
  qtbase/src/corelib/text/qstringbuilder.h(112):
  warning C4127: conditional expression is constant
  qtbase/src/corelib/text/qstringbuilder.h(112):
  note: consider using 'if constexpr' statement instead

Change-Id: I9787fb37099f811c52f93c94c9edb4da8aafdfe5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 cmake/QtHeadersClean.cmake        |  2 +-
 src/corelib/kernel/qiterable.h    | 36 +++++++++++++++++--------------
 src/corelib/text/qstringbuilder.h | 11 +++++++---
 3 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/cmake/QtHeadersClean.cmake b/cmake/QtHeadersClean.cmake
index 955f325e3f..5f1cc9fd0c 100644
--- a/cmake/QtHeadersClean.cmake
+++ b/cmake/QtHeadersClean.cmake
@@ -186,7 +186,7 @@ function(qt_internal_add_headersclean_target module_target module_headers)
         # Note we can't enable -Za, as it does not support certain key Microsoft SDK header files
         # we use. Microsoft suggests to use /permissive- instead, which is implicity set by
         # -std:c++latest.
-        set(hcleanFLAGS -std:c++latest -Zc:__cplusplus -WX -W3 -EHsc)
+        set(hcleanFLAGS -std:c++latest -Zc:__cplusplus -WX -W4 -EHsc)
 
         # Because we now add `-DNOMINMAX` to `PlatformCommonInternal`.
         set(hcleanUDEFS -UNOMINMAX)
diff --git a/src/corelib/kernel/qiterable.h b/src/corelib/kernel/qiterable.h
index 1178c5e8a3..4adcdfd76f 100644
--- a/src/corelib/kernel/qiterable.h
+++ b/src/corelib/kernel/qiterable.h
@@ -71,28 +71,32 @@ public:
     QTaggedIterator(Iterator &&it) : Iterator(std::move(it))
     {
         const QMetaContainer metaContainer = this->metaContainer();
-        if (std::is_base_of_v<std::random_access_iterator_tag, IteratorCategory>
-                && !metaContainer.hasRandomAccessIterator()) {
-            qFatal("You cannot use this iterator as a random access iterator");
-            this->clearIterator();
+        if constexpr (std::is_base_of_v<std::random_access_iterator_tag, IteratorCategory>) {
+            if (!metaContainer.hasRandomAccessIterator()) {
+                qFatal("You cannot use this iterator as a random access iterator");
+                this->clearIterator();
+            }
         }
 
-        if (std::is_base_of_v<std::bidirectional_iterator_tag, IteratorCategory>
-                && !metaContainer.hasBidirectionalIterator()) {
-            qFatal("You cannot use this iterator as a bidirectional iterator");
-            this->clearIterator();
+        if constexpr (std::is_base_of_v<std::bidirectional_iterator_tag, IteratorCategory>) {
+            if (!metaContainer.hasBidirectionalIterator()) {
+                qFatal("You cannot use this iterator as a bidirectional iterator");
+                this->clearIterator();
+            }
         }
 
-        if (std::is_base_of_v<std::forward_iterator_tag, IteratorCategory>
-                && !metaContainer.hasForwardIterator()) {
-            qFatal("You cannot use this iterator as a forward iterator");
-            this->clearIterator();
+        if constexpr (std::is_base_of_v<std::forward_iterator_tag, IteratorCategory>) {
+            if (!metaContainer.hasForwardIterator()) {
+                qFatal("You cannot use this iterator as a forward iterator");
+                this->clearIterator();
+            }
         }
 
-        if (std::is_base_of_v<std::input_iterator_tag, IteratorCategory>
-                && !metaContainer.hasInputIterator()) {
-            qFatal("You cannot use this iterator as an input iterator");
-            this->clearIterator();
+        if constexpr (std::is_base_of_v<std::input_iterator_tag, IteratorCategory>) {
+            if (!metaContainer.hasInputIterator()) {
+                qFatal("You cannot use this iterator as an input iterator");
+                this->clearIterator();
+            }
         }
     }
 
diff --git a/src/corelib/text/qstringbuilder.h b/src/corelib/text/qstringbuilder.h
index fb44a56633..53f8865edf 100644
--- a/src/corelib/text/qstringbuilder.h
+++ b/src/corelib/text/qstringbuilder.h
@@ -106,10 +106,15 @@ private:
         // we abuse const_cast / constData here because we know we've just
         // allocated the data and we're the only reference count
         typename T::iterator d = const_cast<typename T::iterator>(s.constData());
-        typename T::const_iterator const start = d;
-        QConcatenable< QStringBuilder<A, B> >::appendTo(*this, d);
 
-        if (!QConcatenable< QStringBuilder<A, B> >::ExactSize && len != d - start) {
+        if constexpr (QConcatenable<QStringBuilder<A, B>>::ExactSize) {
+            QConcatenable<QStringBuilder<A, B>>::appendTo(*this, d);
+            return s;
+        }
+
+        typename T::const_iterator const start = d;
+        QConcatenable<QStringBuilder<A, B>>::appendTo(*this, d);
+        if (len != d - start) {
             // this resize is necessary since we allocate a bit too much
             // when dealing with variable sized 8-bit encodings
             s.resize(d - start);