https://codereview.qt-project.org/c/qt/qtbase/+/582403
From: Sam James <sam@gentoo.org>
Date: Sat, 10 Aug 2024 16:43:05 +0100
Subject: [PATCH] Fix ODR violation for IsFloatType_v

With recent GCC 15 trunk, I started to see:
```
ld: .../kwalletentry.cc.o:(.rodata+0x0): multiple definition of `QtPrivate::IsFloatType_v<_Float16>';
	src/runtime/kwalletd/backend/CMakeFiles/KF6WalletBackend.dir/cbc.cc.o:(.rodata+0x0): first defined here
```

The issue is that constexpr is only implicitly inline for functions or
static data members [0], so the two constexpr IsFloatType_v definitions
here cause an ODR violation.

Explicitly mark them as inline constexpr.

[0] http://eel.is/c++draft/dcl.constexpr#1.sentence-3
--- a/src/corelib/global/qcomparehelpers.h
+++ b/src/corelib/global/qcomparehelpers.h
@@ -348,9 +348,9 @@
 
 template <typename T>
-constexpr bool IsFloatType_v = std::is_floating_point_v<T>;
+inline constexpr bool IsFloatType_v = std::is_floating_point_v<T>;
 
 #if QFLOAT16_IS_NATIVE
 template <>
-constexpr bool IsFloatType_v<QtPrivate::NativeFloat16Type> = true;
+inline constexpr bool IsFloatType_v<QtPrivate::NativeFloat16Type> = true;
 #endif