Final switch on default error template

[somzzz]

Compiler lowers final switch default case to this (which is a runtime error).
Old implementation is in core/exception.d

Compiler changes: somzzz/dmd@d1bbf1e

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[PetarKirov]

This would be a breaking change. A better solution is to change onSwitchError like so:

@safe pure nothrow @nogc
void onSwitchError(string file = __FILE__, size_t line = __LINE__)
{
    throw staticError!SwitchError(file, line, null);
}

That way we keep compatibility with code that catches Error(s) (e.g. test runners, event loop / thread schedulers, etc.) and we make the implementation @nogc as well.
Also, exceptions provide better error messages.

[somzzz]

@ZombineDev I didn't notice staticError, thanks for mentioning it!
It's not @nogc though.

src/core/exception.d(586): Error: @nogc function 'core.exception.__switch_errorT!().__switch_errorT' cannot call non-@nogc function 'core.exception.staticError!(SwitchError, string, ulong, typeof(null)).staticError'

Or after adding the @nogc tag to staticError:

src/core/exception.d(703): Error: @nogc function 'core.exception.staticError!(SwitchError, string, ulong, typeof(null)).staticError' cannot call non-@nogc constructor 'core.exception.SwitchError.this'

With the current solution, an AssertError is still thrown and dmd unittests pass. Still, it looks like onAssertError isn't @nogc either. https://github.com/dlang/druntime/blob/master/src/core/exception.d#L437

It shouldn't have been possible to mark code using final switch as @nogc in the first place.

[somzzz]

Made SwitchError constructor @nogc and the rest was inferred. Similarly, __switch_errorT needs to be @trusted to call staticError.

[PetarKirov]

Thanks for the update, now everything LGTM.

[PetarKirov]

Keep in mind, that we may need to add a if (__ctfe) assert (0, "informative message...") branch, as I'm not sure if staticError would work in CTFE.
I prefer to leave the PR as it is for now, and cross the bridges when we come to them.

[jpf91]

We already have that compile time check for final switch and normal switch always requires a default branch.

I guess this runtime check is only used to verify that a value is not out-of-range, probably caused by an invalid cast. Ping @andralex

[andralex]

@jpf91 take a look at https://dpaste.dzfl.pl/a33b4b74d774

[jpf91]

Thanks. I didn't know final switch also works on non-enumeral values.