Fix issue 21255: Mangle aliased overload set passed to template alias parameter.

[FeepingCreature]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21255	normal	"overload alias ... forward declaration" when overload set of imported template functions is passed to alias template parameter

Testing this PR locally

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

dub run digger -- build "master + dmd#11740"

[UplinkCoder]

Please add in the commit message what the change actually does

…

On Wed, Sep 16, 2020, 10:40 AM Nicholas Wilson ***@***.***> wrote: ***@***.**** approved this pull request. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#11740 (review)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAVWSCFS24E25ACNYPPD5G3SGB2XXANCNFSM4ROO5OFA> .

[UplinkCoder]

Needs a better/longer commit message.

Try to explain the problem you are fixing to someone who only has a passing familiarity with DMD.

[FeepingCreature]

Commit changed to work differently.

The problem is that overload declarations are kind of "fake declarations" without many of the properties DMD expects from ordinary declarations. So when we try to mangle them we run into a cryptic "forward declaration" error arising from DMD thinking they're "default" linkage, because that's LINK.init, and then it errors with "forward declaration" for no reason I can discern, because DMD is an undocumented cryptic mess at the best of days. Anyways, there's already code for mangling an overload set; we just have to avoid crashing until we reach it.

So now the commit just bypasses the bit that checks for special cases in declarations, ie. pragma(mangle) and extern(C), because those have to be checked when the OverDeclaration mangler recurses back into the contained declarations.

tl;dr: OverDeclaration is not a real subtype of Declaration. Why? ¯\_(ツ)_/¯

[UplinkCoder]

@FeepingCreature yes please put that into the commit message.

[FeepingCreature]

(moved explanation to commit message)

[UplinkCoder]

I hope we won't regret this :)

[FeepingCreature]

Shouldn't. Afaict, this only triggers if dmd would crash anyways. OverDeclarations are never mangleable as declarations, because they never have valid LINKage.

In other words, there's no way we'll ever regret it. :)

[UplinkCoder]

@FeepingCreature I hear you. Yeah it's not that big of a deal as long as changes in the future don't break that unstated invariant.