swift

504b794f - [AutoDiff] Support '@differentiable(linear)' function conversion. (#27687)

[AutoDiff] Support '@differentiable(linear)' function conversion. (#27687) - Support the following implicit conversions: - Non-differentiable to `@differentiable(linear)` - Sema: Emit `LinearFunctionExpr`. - SILGen: Lower `LinearFunctionExpr` to `linear_function`. - `@differentiable(linear)` to non-differentiable - Sema: Emit `LinearFunctionExtractOriginalExpr`. - SILGen: Lower `LinearFunctionExtractOriginalExpr` to `linear_function_extract [original]`. - Reject the following implicit conversions: - `@differentiable` to `@differentiable(linear)` - This conversion is not rejected because a `@differentiable` function can never come directly from a closure expression or from a declaration/member reference. - `@differentiable(linear)` to `@differentiable` ([TF-908](https://bugs.swift.org/browse/TF-908)) - This is supported by design, but is not yet implemented due to its complexity. This requires thunking `@differentiable(linear)` to a derivative (JVP) function where the derivative function returns the original result and the same linear map with `@nondiff` parameters partial-applied away. - Properly handle `linear_function`, `differentiable_function_extract`, and `linear_function_extract` in `swift::isGuaranteedForwardingValueKind` and `swift::isOwnershipForwardingValueKind`. This fixed a previously uncaught assertion, which was because `differentiable_function_extract` was in `swift::isOwnershipForwardingValueKind` while it should really be in `swift::isGuaranteedForwardingValueKind`. For complete conversion rules among `@differentiable` functions, see [Differentiable Programming Mega-Proposal - Type conversion](https://github.com/dan-zheng/swift/blob/differentiable-programming/docs/DifferentiableProgramming.md#type-conversion). Resolves [TF-900](https://bugs.swift.org/browse/TF-900).