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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help wantedhigh priority

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

Tech stack
rust
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Gleam type system basicstype inference conceptsRust basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the type checker's unification logic in the Gleam compiler (likely in src/type .rs or similar). The issue shows a recursive type where `Test(a)` is both the parameter and the variant. The error indicates a failure to unify identical types, possibly due to a missing normalization step. Compare with similar issue #1893 for context. Look for where type variables are replaced or where cyclic types are handled.