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fix subtype match of generic object types #24430

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merged 1 commit into from
Nov 12, 2024

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@metagn metagn commented Nov 12, 2024

split from #24425

Matching tyGenericBody performs a match on the last child of the generic body, in this case the uninstantied tyObject type. If the object contains no generic fields, this ends up being the same type as all instantiated ones, but if it does, a new type is created. This fails the sameObjectTypes check that subtype matching for object types uses. To fix this, also consider that the pattern type could be the generic uninstantiated object type of the matched type in subtype matching.

@Araq Araq merged commit 511ab72 into nim-lang:devel Nov 12, 2024
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Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 511ab72

Hint: mm: orc; opt: speed; options: -d:release
177442 lines; 8.840s; 652.32MiB peakmem

Araq pushed a commit that referenced this pull request Nov 16, 2024
fixes #22479, fixes #24374, depends on #24429 and #24430

When instantiating generic types which directly have nominal types
(object, distinct, ref/ptr object but not enums[^1]) as their values,
the nominal type is now copied (in the case of ref objects, its child as
well) so that it receives a fresh ID and `typeInst` field. Previously
this only happened if it contained any generic types in its structure,
as is the case for all other types.

This solves #22479 and #24374 by virtue of the IDs being unique, which
is what destructors check for. Technically types containing generic
param fields work for the same reason. There is also the benefit that
the `typeInst` field is correct. However issues like #22445 aren't
solved because the compiler still uses structural object equality checks
for inheritance etc. which could be removed in a later PR.

Also fixes a pre-existing issue where destructors bound to object types
with generic fields would not error when attempting to define a user
destructor after the fact, but the error message doesn't show where the
implicit destructor was created now since it was only created for
another instance. To do this, a type flag is used that marks the generic
type symbol when a generic instance has a destructor created. Reusing
`tfCheckedForDestructor` for this doesn't work.

Maybe there is a nicer design that isn't an overreliance on the ID
mechanism, but the shortcomings of `tyGenericInst` are too ingrained in
the compiler to use for this. I thought about maybe adding something
like `tyNominalGenericInst`, but it's really much easier if the nominal
type itself directly contains the information of its generic parameters,
or at least its "symbol", which the design is heading towards.

[^1]: See [this
test](https://github.com/nim-lang/Nim/blob/21420d8b0976dc034feb90ab2878ae0dd63121ae/lib/std/enumutils.nim#L102)
in enumutils. The field symbols `b0`/`b1` always have the uninstantiated
type `B` because enum fields don't expect to be generic, so no generic
instance of `B` matches its own symbols. Wouldn't expect anyone to use
generic enums but maybe someone does.
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