allow cast of type, not only expressions

[dlangBugzillaToGithub]

basile-z reported this on 2019-06-29T10:14:30Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20010

CC List

• Bolpat

Description

People write stuff like : 

    mixin("alias Func = " ~ typeof(&impl).stringof ~ " pure nothrow;");

to set the attributes of a function. We could have something like

    alias Func = cast(pure nothrow) typeof(&impl);

or even using the TypeCtor style

    alias Func = pure(nothrow(typeof(&impl)));

This probably requires a DIP but I open an issue to keep track of the idea.

[dlangBugzillaToGithub]

qs.il.paperinik commented on 2024-06-05T13:16:27Z

I’d be in favor of the cast.

Grammar:
```diff
    CastQual:
-       cast ( TypeCtors? ) UnaryExpression
+       cast ( MemberFunctionAttributes? ) UnaryExpression
+       cast ( ... MemberFunctionAttributes ) UnaryExpression
+       cast ( ! ...[opt] MemberFunctionAttributes ) UnaryExpression
```

Note that `MemberFunctionAttributes` includes `TypeCtors`.

Semantics of `cast()` must remain so that it only removes `TypeCtor`s. To remove other things, either use e.g. `cast(@system)` or fix https://issues.dlang.org/show_bug.cgi?id=24587.

One issue I see, however, is with delegate types: `cast(immutable)` would produce `immutable(R delegate())` and there’s no way to go from `R delegate()` to `R delegate() immutable` with a cast. This is what the second rule does: It applies the member function attribute to the function type even if it’s a type qualifier.

`cast(pure)` is unambiguous, so it doesn’t require `cast(... pure)`, but `cast(... pure)` should be legal. The `...` essentially becomes `typeof(UnaryExpression)`.

As for Issue 24587 (negated forms), that’s the last rule: `cast(! ... const)` removes `const` as a member function attribute.