FStarLang/FStar

Failure of type inference with repeated variables

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#1,007 opened on Apr 21, 2017

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area/proof-assistantcomponent/typecheckercomponent/unifiergood first issuekind/enhancementmilestone/everest-v1

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Description

Lax-typechecking the following snippet

let f (x y : int) (w:(b:bool & (if b then equals x y else equals y x))) =
  match w with
  | (| b, Refl |) -> ()

fails with the error

(Error) (equals (?26524 x y w b) (?26524 x y w b)) is not a subtype of the expected type (match b with
	| true  -> (equals x y)
	|_  -> (equals y x))

Contributor guide

Research direction
Investigate the type checker's handling of repeated variables in pattern matching, specifically the inference of equalities in dependent pattern matches.
Tech stack
None
Domain
compilers
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.
3-5 days
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
Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40