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lazy type alias: type parameters are not considered constrained in impls #116100

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fmease opened this issue Sep 23, 2023 · 0 comments · Fixed by #120780
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lazy type alias: type parameters are not considered constrained in impls #116100

fmease opened this issue Sep 23, 2023 · 0 comments · Fixed by #120780
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C-bug Category: This is a bug. F-lazy_type_alias `#![feature(lazy_type_alias)]` requires-incomplete-features This issue requires the use of incomplete features. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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fmease commented Sep 23, 2023 •
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Uplifted from #112792 (comment).

#![feature(lazy_type_alias)]
#![allow(incomplete_features)]

type Faa<A> = Foo<A>;
struct Foo<A>(A);

impl<A> Faa<A> {}
error[E0207]: the type parameter `A` is not constrained by the impl trait, self type, or predicates
 --> src/lib.rs:11:6
  |
7 | impl<A> Faa<A> {}
  |      ^ unconstrained type parameter
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Sep 23, 2023
@fmease fmease added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. C-bug Category: This is a bug. F-lazy_type_alias `#![feature(lazy_type_alias)]` and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Sep 23, 2023
@fmease fmease mentioned this issue Sep 23, 2023
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@fmease fmease added the requires-incomplete-features This issue requires the use of incomplete features. label Sep 23, 2023
@fmease fmease self-assigned this Oct 5, 2023
@fmease fmease mentioned this issue Feb 8, 2024
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bors added a commit to rust-lang-ci/rust that referenced this issue Feb 17, 2024
@bors
Auto merge of rust-lang#120780 - fmease:lta-in-impls, r=oli-obk
4993747 
Properly deal with weak alias types as self types of impls

Fixes rust-lang#114216.
Fixes rust-lang#116100.

Not super happy about the two ad hoc “normalization” implementations for weak alias types:

1. In `inherent_impls`: The “peeling”, normalization to [“WHNF”][whnf]: Semantically that's exactly what we want (neither proper normalization nor shallow normalization would be correct here). Basically a weak alias type is “nominal” (well...^^) if the WHNF is nominal. [rust-lang#97974](rust-lang#97974) followed the same approach.
2. In `constrained_generic_params`: Generic parameters are constrained by a weak alias type if the corresp. “normalized” type constrains them (where we only normalize *weak* alias types not arbitrary ones). Weak alias types are injective if the corresp. “normalized” type is injective.

Both have ad hoc overflow detection mechanisms.

**Coherence** is handled in rust-lang#117164.

r? `@oli-obk` or types

[whnf]: https://en.wikipedia.org/wiki/Lambda_calculus_definition#Weak_head_normal_form
@bors bors closed this as completed in d3df8ff Feb 18, 2024
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C-bug Category: This is a bug. F-lazy_type_alias `#![feature(lazy_type_alias)]` requires-incomplete-features This issue requires the use of incomplete features. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Properly deal with weak alias types as self types of impls fmease/rust
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