[4.2] Add constraints to FixedWidthInteger.Stride and .Magnitude (#17716) #17766
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…#17716) * Add constraints to FixedWidthInteger.Stride and .Magnitude Add the constraint that these associatedtypes themselves conform to FixedWidthInteger and to SignedInteger and UnsignedInteger, respectively. These are effectively part of the semantic requirement of the protocol already, because the requirements on .min and .max effectively force FixedWidthInteger to be twos-complement, which requires Magnitude not be Self for signed types. Also, in practice it's generally necessary to have these two constraints in order to effectively use the FixedWidthInteger protocol anyway; witness that by adding them to the protocol, we eliminate them as constraints from a number of extensions. This also resolves https://bugs.swift.org/browse/SR-8156
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stephentyrone
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lancep
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Jul 5, 2018
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Add the constraint that these associatedtypes themselves conform to FixedWidthInteger and to SignedInteger and UnsignedInteger, respectively. These are effectively part of the semantic requirement of the protocol already, because the requirements on .min and .max effectively force FixedWidthInteger to be twos-complement, which requires Magnitude not be Self for signed types. Also, in practice it's generally necessary to have these two constraints in order to effectively use the FixedWidthInteger protocol anyway; witness that by adding them to the protocol, we eliminate them as constraints from a number of extensions.
This also resolves https://bugs.swift.org/browse/SR-8156
rdar://problem/41789616 [SR-8156]: Difficult to use random integers in generic contexts
Explanation: The primary motivation is to make it possible to use .random(in:) in generic contexts on FixedWidthInteger, without adding a pile of conformances. More generally, it brings the actual protocol more closely in-line with the documented semantics.
Scope: Adds (minor) additional requirements on any FixedWidthInteger type. These are all satisfied by all existing standard library types, and are already required by the documented semantics of FixedWidthInteger, if not explicitly by the protocol.
Issue: rdar://problem/41789616
Risk: Medium. This is an ABI change, but one that is unlikely to effect much (any?) third-party code.
Testing: Standard test suite, source compatibility test, compiler performance test (In #17716)
Reviewer: @lancep, @milseman, @lorentey