swiftlang/swift

[SR-8213] Compiler may indicate "case will never be executed" even if it is unavoidable to execute the case in switch statement.

Open

#50,745 opened on Jul 10, 2018

View on GitHub
 (14 comments) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
SILGenbugcompilerdiagnostics qualitygood first issuepattern matchingpatternsstatementsswift 5.9switchswitch casestype checker

Description
Previous ID SR-8213
Radar None
Original Reporter @YOCKOW
Type Bug

  • OS: macOS, Ubuntu 16.04

  • Swift: 4.1.2, 4.2-DEVELOPMENT-SNAPSHOT-2018-07-08
Votes 0
Component/s Compiler
Labels Bug, DiagnosticsQoI, StarterBug
Assignee Gabriel Igliozzi (JIRA)
Priority Medium

md5: df5f44ab5b8a1d3c8ee6ed4a803d858c

is duplicated by:

  • SR-11318 Incorrect "Case will never be executed" warning

Issue Description:

(As I commented on SR-8208,) The compiler shows a wrong warning, when the following code is compiled:

enum E {
  case a // only one case
}

func f(_ e0:E, _ e1:E) {
  switch (e0, e1) {
  case (.a, _): print("e0 is .a") // -> No warning.
  }
  
  switch (e0, e1) {
  case (.a, _), (_, .a): print("e0 or e1 is .a") // -> warning: case will never be executed
  }
}

f(.a, .a)

Consideration

  • The warning should be
    • case will be always executed
    • (_, .a) will never match
    • ... and so on

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.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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 language knowledgecompiler internals familiaritypattern matching in Swift
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate the Swift compiler's pattern matching diagnostics in the Sema module. Focus on the logic that determines case reachability in switch statements. The issue provides a clear reproduction case. Look for the warning generation in files like lib/Sema/TypeCheckPattern.cpp or lib/Sema/TypeCheckSwitchStmt.cpp. The duplicate issue SR 11318 may contain additional analysis.