Fix issue 20065: support empty AliasSeq tuples in compiletime array literals

[FeepingCreature]

Fixes:

import std.meta;

// Let me ask a question.
void main() {
    // This works.
    enum string[] array1 = [AliasSeq!("foo")]; 
    // So why doesn't this?
    enum string[] array2 = [AliasSeq!()];
}

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20065	enhancement	Empty AliasSeq can't be used to form compiletime array literal

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + dmd#10197"

[thewilsonator]

This is a pretty simple fix, might as well target stable.

[WalterBright]

I agree with @Basile-z

[FeepingCreature]

Rebased on stable, std.meta import removed.

phobos dependency removed.

[Geod24]

Looks like you also improved an error message in the process :)

[thewilsonator]

Indeed

Test fail_compilation/fail8262.d failed: 
expected:
----
fail_compilation/fail8262.d(32): Error: cannot interpret `Tuple8262!1` at compile time
fail_compilation/fail8262.d(27): Error: template instance `fail8262.T8262!(Tuple8262!1)` error instantiating
fail_compilation/fail8262.d(19): Error: cannot implicitly convert expression `S(0)` of type `S` to `int`
----
actual:
----
fail_compilation/fail8262.d(32): Error: initializer must be an expression, not `Tuple8262!1`
fail_compilation/fail8262.d(27): Error: template instance `fail8262.T8262!(Tuple8262!1)` error instantiating
fail

[thewilsonator]

core.exception.AssertError@dmd/dinterpret.d(5074): Assertion failure

[FeepingCreature]

Seems to be some places where CTFE is too defensive in what types of expressions it'll accept. Maybe fixed now? Let's run the tests.

[thewilsonator]

Seems to like it, you still need to correct the error message above.

[FeepingCreature]

Now that this needed one or two more changes before going green, should this still go on stable or should I move back to master?

[thewilsonator]

Stable is fine.

[stefan-koch-sociomantic]

dinterpret.d is not supposed to handle type-exps.
Nor' is the rest of the compiler really.
Prepare for unforeseen consequences.

[FeepingCreature]

Ah damn, that sounds worrying. Noting that I don't really know the implications, and relied on the testsuite, afaics the problem happened originally because in enum string[] foo = [AliasSeq!()], the () is a type, not a value... the changes are just to allow the () to propagate to whatever part of the compiler notices this case ordinarily.

All the tests were green though, so...

Maybe I should have nailed things down so that only this specific case was allowed.