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[SR-13153] @derivative attribute ambiguous original declaration errors #54265

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dan-zheng opened this issue Nov 26, 2019 · 1 comment
Closed

[SR-13153] @derivative attribute ambiguous original declaration errors #54265

dan-zheng opened this issue Nov 26, 2019 · 1 comment
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@dan-zheng
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AutoDiff compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation

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@dan-zheng
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Previous ID SR-13153
Radar None
Original Reporter @dan-zheng
Type Sub-task
Status Closed
Resolution Done
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Sub-task, AutoDiff, DiagnosticsQoI
Assignee @dan-zheng
Priority Medium

md5: 7ea5fa61fb651909a76ff92dae522234

Parent-Task:

  • SR-13149 @derivative and @transpose type-checking diagnostic improvements

blocks:

relates to:

  • TF-1058 [AD] Make @derivative attribute support qualified original declaration names

Issue Description:

@derivative(of🙂 is quite not at feature parity with @differentiable(jvp:vjp🙂 due to @derivative(of🙂 original function lookup ambiguity errors.

protocol P: Differentiable {}
extension Float: P {}

func max<T: P>(_ lhs: T, _ rhs: T) -> T { lhs }

@derivative(of: max(_:_:))
func vjpMax<T: P>(_ lhs: T, _ rhs: T) -> (value: T, pullback: (T) -> (T, T))
where T == T.TangentVector {
  // Type-checking is able to disambiguate the `max(_:_:)` call below.
  // But not the `max(_:_:)` reference above.
  (max(lhs, rhs), { v in (v, v) })
}
$ swiftc tf-1001.swift
tf-1001.swift:6:18: error: type 'T' does not conform to protocol 'Comparable'
@derivative(of: max(_:_:))
                 ^

The current workaround is to use @differentiable(jvp:vjp🙂 on the original function for derivative registration:

protocol P: Differentiable {}
extension Float: P {}

// No ambiguity here.
@differentiable(vjp: vjpMax where T == T.TangentVector)
func max<T: P>(_ lhs: T, _ rhs: T) -> T { lhs }

func vjpMax<T: P>(_ lhs: T, _ rhs: T) -> (value: T, pullback: (T) -> (T, T))
where T == T.TangentVector {
  // Type-checking is able to disambiguate the `max(_:_:)` call below.
  // But not the `max(_:_:)` reference above.
  (max(lhs, rhs), { v in (v, v) })
}

An eventual solution may be to allow qualified original function references in @derivative attribute, e.g. @derivative(of: ModuleName.max).

This issue has been encountered in practical use cases, e.g. ewconnell/swiftrt#4
@dan-zheng
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Done in #28892 by enabling @derivative attribute with qualified original declaration name.

func foo(_ x: Float) -> Float { x }

// 1. Module qualification.
// Immediately useful for derivative registration use cases: TF-1002.
// May be replaced by proper module qualification/`DeclNameRef` work:
// - https://forums.swift.org/t/pitch-fully-qualified-name-syntax/28482
// - https://forums.swift.org/t/what-the-eff-is-declnameref/31594
@derivative(of: MyModule.foo)
func vjpFoo(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
  (foo(x), { $0 })
}

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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AutoDiff compiler The Swift compiler in itself diagnostics QoI Bug: Diagnostics Quality of Implementation
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