swiftlang/swift

[SR-8797] Misleading compiler errors

Open

#51,305 opened on Sep 19, 2018

View on GitHub
 (7 comments) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
bugcompilerdiagnostics qualitygood first issuetype checker

Description
Previous ID SR-8797
Radar rdar://problem/44616337
Original Reporter @jeremyabannister
Type Bug
Votes 0
Component/s Compiler
Labels Bug, DiagnosticsQoI, StarterBug, TypeChecker
Assignee None
Priority Medium

md5: f2dc6c9ca2ac0b74c004d15fc50e5a7b

Issue Description:

typealias Executable <Input> = (Input)->()
let test: Executable<Void>? = { }

The code above leads to the error:

Cannot convert value of type '() -> ()' to specified type '((Void) -> ())?'

Whereas this code compiles successfully:

typealias Executable <Input> = (Input)->()
let test: Executable<Void>? = { _ in }

Contributor guide

Tech stack
swift
Domain
developer experiencetooling
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Swift syntaxType inference understandingClosure basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Investigate the type checker's handling of closures assigned to optional type aliases. The error message suggests a mismatch in types, but the actual issue might be in how type aliases with unapplied generic parameters are resolved. Look at `lib/Sema/TypeCheckConstraints.cpp` and related diagnostic files. Search for similar issues with 'Executable' or closure coercion. The fix may involve adjusting the diagnostic to provide a clearer explanation or uncovering a deeper type inference bug.