gleam-lang/gleam

Type inference issue expected Test(a) but found Test(a)

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#2,550 opened on Jan 17, 2024

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Description

type Test(a) {
  Test(a)
}
fn it(value: Test(a)) {
  it2(value)
}
fn it2(value: Test(a)) -> Test(a) {
  it(value)
}

Gives this error on the test2 call

Type mismatch

The type of this returned value doesn't match the return type
annotation of this function.

Expected type:

    Test(a)

Found type:

    Test(a)

Similar issue #1893

Contributor guide

Research direction
Investigate the type unification logic in the Gleam compiler. Focus on how type variables are compared and why identical generic types like Test(a) are reported as mismatched. Look at the handling of recursive type definitions and the equality check for generic parameters.
Tech stack
rust
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
RustGleam
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50