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Can't access generic parameters in a generic alias that don't appear in aliased type #24791

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ire4ever1190 opened this issue Mar 20, 2025 · 1 comment · May be fixed by #24822
Open

Can't access generic parameters in a generic alias that don't appear in aliased type #24791

ire4ever1190 opened this issue Mar 20, 2025 · 1 comment · May be fixed by #24822
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Generics Language Design

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@ire4ever1190
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Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-02-28
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: 7e8a650
active boot switches: -d:release

Description

When accessing a generic field from a generic alias it only allows accessing what it aliases.

import std/options

type
  Foo[D: static int; T] = T
  Bar[D] = Option[D]
  Bizz = Foo[9, string]
  Buzz = Foo[9, Option[string]]

echo Bar[string].D # undeclared field: 'D'
echo Bizz.D # 9
echo Buzz.D # undeclared field: 'D'

The Foo example is a bit more useful since it allows you to attach metadata to types without needing to resort to macros.

Current Output

test.nim(9, 17) Error: undeclared field: 'D'
test.nim(11, 10) Error: undeclared field: 'D'

Expected Output

string
9
9

Known Workarounds

No response

Additional Information

I've tracked the issue down to when it calls readTypeParameter it just skips past the alias if both are generics and so the fields on the alias cant be accessed (unless they share the same name)
@ire4ever1190 ire4ever1190 linked a pull request Mar 29, 2025 that will close this issue
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@metagn
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metagn commented Apr 11, 2025 •
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Edit: Forgot there is only ever 1 depth of generic aliases, what I wrote wouldn't work very well. Only the final generic alias or the original generic type can be considered, i.e.:

type
  Foo[A: static int] = object
  Bar[B: static int] = Foo[B]
  Baz[C: static int] = Bar[C]

var x: Baz[123]
echo x.B

B is inaccessible here because the Bar[123] type is removed in handleGenericInvocation, the final type of x is Baz[123] = Foo[123]. We can still make it check both Baz and Foo, probably Foo first for compatibility, but this would be a weird rule.

You might want to use a distinct type like Foo[D: static int; T] = distinct Option[D] instead. Generic aliases tend to break in edge cases like #10220.

Maybe we can recursively check each generic alias before skipping, i.e.:

type
  Foo[A, B: static int] = object
  Bar[A: static int] = Foo[A, A * 2]
  Baz = Bar[123]

var x: Baz
echo x.A
echo x.B

Baz is an alias type, so it's skipped, then it checks if Bar has a param named A or B and uses it if it does, otherwise it skips again to Foo to check if it has the param, so on so forth. This might break code though so instead we could also check from the bottom up.

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Don't skip generic aliases when reading a generic parameter ire4ever1190/Nim
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@metagn @ire4ever1190