[Development] QVariant and types with throwing dtors

[Fabian Kosmale]

Hi,

Qt's container classes (at least those listed in 
https://doc.qt.io/qt-6/containers.html + QCache) reject types with 
throwing destructorss via static_assert[1].

There is however one "container" which doesn't reject such types: 
QVariant. Even though it's own destructor is noexcept, it doesn't reject 
types whose destructor is potentially throwing. My question is: Do we 
want to change this, and if so, how? There are at least three alternatives:

1. Don't do anything; that's the behavior we have since at least Qt 5. 
If the dtor doesn't actually throw, everything is fine; if it does 
throw, we're calling std::terminate. Might lead to unexpected results, 
but doing nothing obviously doesn't break any existing code. It is 
however inconsistent with other types.
2. Warn, but still accept such types [2].  I've implemented that in 
https://codereview.qt-project.org/c/qt/qtbase/+/580560. Anyone one 
stores a type with a throwing dtor would get a warning, but everything 
still compiles. The warning can however be considered spurious if the 
author knows that given their usage, the type would never throw.
3. Reject such types at compile time. That would be consistent with 
other types, but might break existing code, even code that works 
perfectly fine because it can never actually throw.

I'm leaning towards either 1 (nobody ever reported a bug about QVariant 
calling std::terminate so far), or 2 ( more consistent with our policy 
for containers, highlights that such usage is potentially dangerous).

I'd welcome any comments on concerns.

Kind regards,
Fabian

[1] There are however still some methods which are marked as 
conditionally noexept, like QCache::take, even though that can never be 
the case given the static_assert.
[2] At least in cases where we can detect it at compile time; the API 
using QMetaType would require storing the necessary information in 
QMetaType, and a runtime check.

-- 
Fabian Kosmale
Manager R&D

The Qt Company GmbH
Erich-Thilo-Str. 10
D-12489 Berlin
fabian.kosmale at qt.io
+49 1638686070