This prevents an ICE with GCC trying to declare e.g. Variant<String&>.
Using a concept is a bit overkill here, but clang otherwise trips over
the friendship declaration to other Variant types:
template<typename... NewTs>
friend struct Variant;
Without using a concept, clang believes this is re-declaring the Variant
type with differing requirements ("error: requires clause differs in
template redeclaration").
This is an issue on systems that don't have the empty base class
optimisation (such as windows), and we normally don't need to care -
however static_cast is technically the right thing to use, so let's use
that instead.
Co-Authored-By: Daniel Bertalan <dani@danielbertalan.dev>
We shouldn't let copy/move ctors or assignments be instantiated if the
assignee type does not have a copy/move constructor (even if they're not
used anywhere).
This makes the following code behave as expected:
Variant<int, String> x { some_string() };
x.visit(
[](String const&) {}, // Expectation is for this to be called
[](auto&) {});
This is an interface to downcast(), which degrades errors into runtime
errors, and allows seemingly-correct-but-not-quite constructs like the
following to appear to compile, but fail at runtime:
Variant<NonnullRefPtr<T>, U> foo = ...;
Variant<RefPtr<T>, U> bar = foo;
The expectation here is that `foo` is converted to a RefPtr<T> if it
contains one, and remains a U otherwise, but in reality, the
NonnullRefPtr<T> variant is simply dropped on the floor, and the
resulting variant becomes invalid, failing the assertion in downcast().
This commit adds a Variant<Ts...>(Variant<NewTs...>) constructor that
ensures that no alternative can be left out at compiletime, for the
users that were using this interface for merely increasing the number of
alternatives (for instance, LibSQL's Value class).
Either not returning *this, or in the case of Variant, not checking for
self assignment. In AK::Atomic, we can't return *this due to the wrapper
semantics Atomic implements.
c27abaabc4 moved this out of the global
namespace, but did not qualify its users.
While this seems to be fine (sometimes, somehow), let's qualify it to
avoid random breakage.
This commit makes use of the conditionally trivial special member
functions introduced in C++20. Basically, `Optional` and `Variant`
inherits whether its wrapped type is trivially copy constructible,
trivially copy assignable or trivially destructible. This lets the
compiler optimize optimize a large number of their use cases.
The constraints have been applied to `Optional`'s converting
constructors too in order to make the API more explicit.
This feature is not supported by Clang yet, so we use conditional
compilation so that Lagom can be built on macOS. Once Clang has P0848R3
support, these can be removed.
Checking for this (and get()'ing it) is always invalid, so let's just
disallow it.
This also finds two bugs where the code is checking for types that can
never actually be in the variant (which was actually a refactor
artifact).
This changes Variant::visit() to forward the value returned by the
selected visitor invocation. By perfectly forwarding the returned value,
this allows for the visitor to return by value or reference.
Note that all provided visitors must return the same type - the compiler
will otherwise fail with the message: "inconsistent deduction for auto
return type".
This allows the construction of `Variant<int, int, int>`.
While this might not seem useful, it is very useful for making variants
that contain a series of member function pointers, which I plan to use
in LibGL for glGenLists() and co.
typeid() and RTTI was a nice clutch to implement this, but let's move
away from the horrible slowness and implement variants using type
indices for faster variants.
Also adds an AK::Empty struct, because 'empty' variants are useful, but
this implementation leaves that to the user (i.e. a variant cannot
actually be empty, but it can contain an instance of Empty - i.e. a
byte).
Note that this is more of a constrained Any type, but they basically do
the same things anyway :^)
