swiftlang/swift

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

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#50,745 创建于 2018年7月10日

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 (14 评论) (0 反应) (0 负责人)Swift (69,989 star) (10,719 fork)batch import
SILGenbugcompilerdiagnostics qualitygood first issuepattern matchingpatternsstatementsswift 5.9switchswitch casestype checker

描述
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

贡献者指南

技术栈
swift
领域
developer experiencetooling
议题类型
bug
难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。
4
预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。
over 1 week
活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。
stale
清晰度议题是否清楚说明期望改动、验收标准和下一步。
clear
前置要求
Swift language knowledgecompiler internals familiaritypattern matching in Swift
新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。
40
研究方向
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.
[SR-8213] Compiler may indicate "case will never be executed" even if it is unavoidable to execute the case in switch statement. · swiftlang/swift#50745 | Good First Issue