scala/scala3

Warn when a pattern-bound variable shadows a regular variable

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#10,749 opened on Dec 10, 2020

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 (5 comments) (2 reactions) (1 assignee)Scala (6,247 stars) (1,159 forks)batch import
area:pattern-matchingarea:reportingbackloghelp wanteditype:enhancement

Description

If someone writes:

object Test {
  def foo[T](x: Int, y: Int) = x match {
    case y => y
  }
}

It means they forgot backticks and actually wanted to write case `y` => y, we should warn them about it (same with pattern-bound type variables).

Contributor guide

Tech stack
scala
Domain
developer experience
Issue type
feature
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.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Scala pattern matchingScala 3 compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
The issue requests a warning when a pattern bound variable shadows a regular variable, a common mistake. The Scala 3 compiler's type checker or pattern matching desugaring likely needs modification. The issue is assigned to a maintainer, so first check the assignee's progress and discuss on the issue. Relevant files might include the typer or pattern handling in the compiler source.