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[SR-5440] Typealias in constrained extension misinterprets the where clause #48014

jepers opened this issue Jul 12, 2017 · 7 comments


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jepers commented Jul 12, 2017

Previous ID SR-5440
Radar rdar://problem/33268536
Original Reporter @jepers
Type Bug
Status Closed
Resolution Done
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, AcceptsInvalid
Assignee @slavapestov
Priority Medium

md5: 64a011da616f479d8c228dd46791be39

relates to:

  • SR-5392 Inconsistent "invalid redeclaration of typealias"
  • SR-7217 Protocol composition with conflicting typealiases does not diagnose
  • SR-7605 Overly-exuberant associatedtype deduction?
  • SR-3793 Protocol extension cannot implement associated type
  • SR-10466 Accepts invalid because where clause seems to be ignored.
  • SR-7516 Compiler accepts ambiguous and/or invalid use of associated types
  • SR-7976 Unexpected behavior from conditional conformances
  • SR-10158 Associated type inference resolution ignores conditionality of extensions

Issue Description:

The following seemingly invalid program compiles ( ! ), and the result is the same as if the where-clauses had been removed, so it seems like typealiases added in constrained extensions ignores the constraint:

struct S<T> {
    var v: (A, B, C)
extension S where T == Int {
    typealias A = Int
extension S where T == Bool {
    typealias B = Bool
extension S where T == Float {
    typealias C = Float
print(S<String>.A.self) // Int
print(S<String>.B.self) // Bool
print(S<String>.C.self) // Float
print(S<Double>.A.self) // Int
print(S<Double>.B.self) // Bool
print(S<Double>.C.self) // Float

The same is true when using a protocol instead of a struct:

protocol P {
    associatedtype Q
    var v: (D, E, F) { get }
extension P where Q == Int {
    typealias D = Int
extension P where Q == Bool {
    typealias E = Bool
extension P where Q == Float {
    typealias F = Float
struct R<T> : P {
    typealias Q = T
    var v: (D, E, F)
print(R<String>.D.self) // Int
print(R<String>.E.self) // Bool
print(R<String>.F.self) // Float
print(R<Double>.D.self) // Int
print(R<Double>.E.self) // Bool
print(R<Double>.F.self) // Float

As the following program shows, it seems like the where-clause is actually misinterpreted rather than ignored:

// typealias misinterprets where clause of extension

struct S<A> {
    var hmm: (A, B, C)
extension S where A == Bool {
    typealias B = A
    static func printB() { print(B.self) }
extension S where A == Float {
    typealias C = A
    static func printC() { print(C.self) }
let a = S(hmm: ("strange", true, Float(42.1)))
// Will print:
▿ TestXc9b3.S<Swift.String>
  ▿ hmm: (3 elements)
    - .0: "strange"
    - .1: true
    - .2: 42.0999985

Note: I have only used (a single) same-type constraint (eg A == Float) in the where-clause here, but there are similar problems when constraining to protocol types (eg A: FloatingPoint).

Tested in Xcode 9 beta 3, same behavior in default toolchain and dev snapshot 2017-07-12.

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huonw mannequin commented Jul 12, 2017

@swift-ci create

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huonw mannequin commented Jun 13, 2018

The first two cases are fixed by d57330975a39187e4129d91f566024c9f8923530, but the last case is very curly.

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huonw mannequin commented Jun 13, 2018

Ah, and that patch only addressed {{typealias}}s specifically: there's still fundamental issues with nested types. The following compiles without error:

struct Generic<Foo> {}
extension Generic where Foo == Int {
    struct X {}


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huonw mannequin commented Jun 13, 2018

#17168 for that specific example, but still no handling of decls (checking the types used in the decls of v and hmm).

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jepers commented Mar 23, 2019

This issue is still not resolved (in Swift 4.2, haven't tried 5.0) and has been in progress for a very long time.

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jepers commented Apr 12, 2019

The following (afaics invalid) program still compiles with Swift 5 / Xcode 10.2 and the latest dev snapshot (2019-04-10):

struct S<T> {
    var a : A
extension S where T == Never {
    typealias A = Int
let v = S<Int>(a: 123)

even though it required Int == Never.

I filed this as a separate (more specific) bug:

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slavapestov commented Nov 15, 2019


@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
This issue was closed.
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