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[SR-7217] Protocol composition with conflicting typealiases does not diagnose #49765

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itaiferber opened this issue Mar 16, 2018 · 1 comment

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@itaiferber
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itaiferber commented Mar 16, 2018

Previous ID SR-7217
Radar None
Original Reporter @itaiferber
Type Bug
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, TypeChecker
Assignee None
Priority Medium

md5: c5f27b0f94d34e042696e3afc9cf3681

relates to:

  • SR-7516 Compiler accepts ambiguous and/or invalid use of associated types
  • SR-5392 Inconsistent "invalid redeclaration of typealias"
  • SR-5440 Typealias in constrained extension misinterprets the where clause

Issue Description:

From discussion in https://forums.swift.org/t/decode-a-json-object-of-unknown-format-into-a-dictionary-with-decodable-in-swift-4/6138/38, the following typealias conflict produces no diagnostics in Swift 4.1:

struct Foo {}
struct Bar {}

protocol P1 { typealias T = Foo }
protocol P2 { typealias T = Bar }

typealias P3 = P1 & P2
print(P3.T.self) // => Bar()

typealias P4 = P2 & P1
print(P4.T.self) // => Bar()

Code completion shows two overloads for P3.T and P4.T but there are no warnings or complaints otherwise. The ordering of the composition makes no difference on which type "wins out".

@belkadan
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belkadan commented Mar 16, 2018

cc @rudkx, @DougGregor

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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