stdlib: Add SIMDMaskScalar.SIMDMaskScalar == SIMDMaskScalar requirement to SIMDScalar

In theory this is a source break if someone had a weird custom
conforming type, but I suspect in practice conformances to
this protocol are never defined.

The reason we want this requirement is that often you will see
code like the following:

  protocol Point {
    associatedtype Scalar: SIMDScalar
    associatedtype Vector: SIMD where Vector.Scalar == Scalar
  }

  extension Point where Vector == SIMD2<Scalar> { ... }

When `Vector` is equated with `SIMD2<Scalar>`, we get an infinite
sequence of implied same-type requirements:

  Vector.MaskStorage == SIMD2<Scalar.MaskScalar>
  Vector.MaskStorage.MaskStorage == SIMD2<Scalar.MaskScalar.MaskScalar>
  ...

The protocol fails to typecheck with an error because the requirement
machine cannot build a rewrite system.

If SIMDScalar requires that MaskScalar.MaskScalar == MaskScalar, then
we instead get

  Vector.MaskStorage == SIMD2<Scalar.MaskScalar>
  Vector.MaskStorage.MaskStorage == SIMD2<Scalar.MaskScalar>

  Vector.MaskStorage.MaskStorage == Vector.MaskStorage
  ...

Which ties off the recursion.

In theory, a more advanced implementation could represent this kind of
infinite recursion in 'closed form', but we don't have that yet, and I
believe adding this same-type requirement makes sense anyway.

Fixes rdar://problem/95075552.

Author: slavapestov

stdlib/public/core/SIMDVector.swift

@@ -62,6 +62,7 @@ extension SIMDStorage {
 /// A type that can be used as an element in a SIMD vector.
 public protocol SIMDScalar {
   associatedtype SIMDMaskScalar: SIMDScalar & FixedWidthInteger & SignedInteger
+    where SIMDMaskScalar.SIMDMaskScalar == SIMDMaskScalar
   associatedtype SIMD2Storage: SIMDStorage where SIMD2Storage.Scalar == Self
   associatedtype SIMD4Storage: SIMDStorage where SIMD4Storage.Scalar == Self
   associatedtype SIMD8Storage: SIMDStorage where SIMD8Storage.Scalar == Self

test/Generics/rdar95075552.swift

@@ -0,0 +1,32 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures 2>&1 | %FileCheck %s
+
+// CHECK-LABEL: .Tree@
+// CHECK-NEXT: Requirement signature: <Self where Self.[Tree]Distance : FloatingPoint, Self.[Tree]Distance : SIMDScalar, Self.[Tree]Distance == Self.[Tree]Point.[SIMDStorage]Scalar, Self.[Tree]Point : SIMD>
+protocol Tree {
+  associatedtype Distance: FloatingPoint & SIMDScalar
+  associatedtype Point: SIMD where Point.Scalar == Distance
+}
+
+// CHECK-LABEL: .QuadTree@
+// CHECK-NEXT: Requirement signature: <Self where Self : Tree, Self.[Tree]Point == SIMD2<Self.[Tree]Distance>>
+protocol QuadTree : Tree where Point == SIMD2<Distance> {}
+
+// CHECK-LABEL: .OctTree@
+// CHECK-NEXT: Requirement signature: <Self where Self : Tree, Self.[Tree]Point == SIMD3<Self.[Tree]Distance>>
+protocol OctTree : Tree where Point == SIMD3<Distance> {}
+
+func sameType<T>(_: T.Type, _: T.Type) {}
+
+extension QuadTree {
+  func foo() {
+    sameType(Point.MaskStorage.self, SIMD2<Distance.SIMDMaskScalar>.self)
+    sameType(Point.MaskStorage.MaskStorage.self, SIMD2<Distance.SIMDMaskScalar>.self)
+  }
+}
+
+extension OctTree {
+  func foo() {
+    sameType(Point.MaskStorage.self, SIMD3<Distance.SIMDMaskScalar>.self)
+    sameType(Point.MaskStorage.MaskStorage.self, SIMD3<Distance.SIMDMaskScalar>.self)
+  }
+}

test/api-digester/Outputs/stability-stdlib-source-x86_64.swift.expected

@@ -26,3 +26,5 @@ Func Unicode.UTF32.Parser.parseScalar(from:) has generic signature change from <
 Func Unicode.UTF32.decode(_:) has generic signature change from <I where I : Swift.IteratorProtocol, I.Element == Swift.Unicode.UTF32.CodeUnit> to <I where I : Swift.IteratorProtocol, I.Element == Swift.UInt32>
 Func Unicode.UTF8.decode(_:) has generic signature change from <I where I : Swift.IteratorProtocol, I.Element == Swift.Unicode.UTF8.CodeUnit> to <I where I : Swift.IteratorProtocol, I.Element == Swift.UInt8>
 Constructor Mirror.init(_:children:displayStyle:ancestorRepresentation:) has generic signature change from <Subject, C where C : Swift.Collection, C.Element == Swift.Mirror.Child> to <Subject, C where C : Swift.Collection, C.Element == (label: Swift.String?, value: Any)>
+
+Protocol SIMDScalar has generic signature change from <Self == Self.SIMD16Storage.Scalar, Self.SIMD16Storage : Swift.SIMDStorage, Self.SIMD2Storage : Swift.SIMDStorage, Self.SIMD32Storage : Swift.SIMDStorage, Self.SIMD4Storage : Swift.SIMDStorage, Self.SIMD64Storage : Swift.SIMDStorage, Self.SIMD8Storage : Swift.SIMDStorage, Self.SIMDMaskScalar : Swift.FixedWidthInteger, Self.SIMDMaskScalar : Swift.SIMDScalar, Self.SIMDMaskScalar : Swift.SignedInteger, Self.SIMD16Storage.Scalar == Self.SIMD2Storage.Scalar, Self.SIMD2Storage.Scalar == Self.SIMD32Storage.Scalar, Self.SIMD32Storage.Scalar == Self.SIMD4Storage.Scalar, Self.SIMD4Storage.Scalar == Self.SIMD64Storage.Scalar, Self.SIMD64Storage.Scalar == Self.SIMD8Storage.Scalar> to <Self == Self.SIMD16Storage.Scalar, Self.SIMD16Storage : Swift.SIMDStorage, Self.SIMD2Storage : Swift.SIMDStorage, Self.SIMD32Storage : Swift.SIMDStorage, Self.SIMD4Storage : Swift.SIMDStorage, Self.SIMD64Storage : Swift.SIMDStorage, Self.SIMD8Storage : Swift.SIMDStorage, Self.SIMDMaskScalar : Swift.FixedWidthInteger, Self.SIMDMaskScalar : Swift.SIMDScalar, Self.SIMDMaskScalar : Swift.SignedInteger, Self.SIMDMaskScalar == Self.SIMDMaskScalar.SIMDMaskScalar, Self.SIMD16Storage.Scalar == Self.SIMD2Storage.Scalar, Self.SIMD2Storage.Scalar == Self.SIMD32Storage.Scalar, Self.SIMD32Storage.Scalar == Self.SIMD4Storage.Scalar, Self.SIMD4Storage.Scalar == Self.SIMD64Storage.Scalar, Self.SIMD64Storage.Scalar == Self.SIMD8Storage.Scalar>

test/api-digester/stability-stdlib-abi-without-asserts.test

@@ -80,6 +80,8 @@ Protocol Collection has generic signature change from <Self : Swift.Sequence, Se
 
 Protocol StringProtocol has generic signature change from <Self : Swift.BidirectionalCollection, Self : Swift.Comparable, Self : Swift.ExpressibleByStringInterpolation, Self : Swift.Hashable, Self : Swift.LosslessStringConvertible, Self : Swift.TextOutputStream, Self : Swift.TextOutputStreamable, Self.Element == Swift.Character, Self.Index == Swift.String.Index, Self.StringInterpolation == Swift.DefaultStringInterpolation, Self.SubSequence : Swift.StringProtocol, Self.UTF16View : Swift.BidirectionalCollection, Self.UTF8View : Swift.Collection, Self.UnicodeScalarView : Swift.BidirectionalCollection, Self.UTF16View.Element == Swift.UInt16, Self.UTF16View.Index == Swift.String.Index, Self.UTF8View.Element == Swift.UInt8, Self.UTF8View.Index == Swift.String.Index, Self.UnicodeScalarView.Element == Swift.Unicode.Scalar, Self.UnicodeScalarView.Index == Swift.String.Index, Self.SubSequence.UTF16View.Index == Swift.String.Index, Self.SubSequence.UTF8View.Index == Swift.String.Index, Self.SubSequence.UnicodeScalarView.Index == Swift.String.Index> to <Self : Swift.BidirectionalCollection, Self : Swift.Comparable, Self : Swift.ExpressibleByStringInterpolation, Self : Swift.Hashable, Self : Swift.LosslessStringConvertible, Self : Swift.TextOutputStream, Self : Swift.TextOutputStreamable, Self.Element == Swift.Character, Self.Index == Swift.String.Index, Self.StringInterpolation == Swift.DefaultStringInterpolation, Self.SubSequence : Swift.StringProtocol, Self.UTF16View : Swift.BidirectionalCollection, Self.UTF8View : Swift.Collection, Self.UnicodeScalarView : Swift.BidirectionalCollection, Self.UTF16View.Element == Swift.UInt16, Self.UTF16View.Index == Swift.String.Index, Self.UTF8View.Element == Swift.UInt8, Self.UTF8View.Index == Swift.String.Index, Self.UnicodeScalarView.Element == Swift.Unicode.Scalar, Self.UnicodeScalarView.Index == Swift.String.Index>
 
+Protocol SIMDScalar has generic signature change from <Self == Self.SIMD16Storage.Scalar, Self.SIMD16Storage : Swift.SIMDStorage, Self.SIMD2Storage : Swift.SIMDStorage, Self.SIMD32Storage : Swift.SIMDStorage, Self.SIMD4Storage : Swift.SIMDStorage, Self.SIMD64Storage : Swift.SIMDStorage, Self.SIMD8Storage : Swift.SIMDStorage, Self.SIMDMaskScalar : Swift.FixedWidthInteger, Self.SIMDMaskScalar : Swift.SIMDScalar, Self.SIMDMaskScalar : Swift.SignedInteger, Self.SIMD16Storage.Scalar == Self.SIMD2Storage.Scalar, Self.SIMD2Storage.Scalar == Self.SIMD32Storage.Scalar, Self.SIMD32Storage.Scalar == Self.SIMD4Storage.Scalar, Self.SIMD4Storage.Scalar == Self.SIMD64Storage.Scalar, Self.SIMD64Storage.Scalar == Self.SIMD8Storage.Scalar> to <Self == Self.SIMD16Storage.Scalar, Self.SIMD16Storage : Swift.SIMDStorage, Self.SIMD2Storage : Swift.SIMDStorage, Self.SIMD32Storage : Swift.SIMDStorage, Self.SIMD4Storage : Swift.SIMDStorage, Self.SIMD64Storage : Swift.SIMDStorage, Self.SIMD8Storage : Swift.SIMDStorage, Self.SIMDMaskScalar : Swift.FixedWidthInteger, Self.SIMDMaskScalar : Swift.SIMDScalar, Self.SIMDMaskScalar : Swift.SignedInteger, Self.SIMDMaskScalar == Self.SIMDMaskScalar.SIMDMaskScalar, Self.SIMD16Storage.Scalar == Self.SIMD2Storage.Scalar, Self.SIMD2Storage.Scalar == Self.SIMD32Storage.Scalar, Self.SIMD32Storage.Scalar == Self.SIMD4Storage.Scalar, Self.SIMD4Storage.Scalar == Self.SIMD64Storage.Scalar, Self.SIMD64Storage.Scalar == Self.SIMD8Storage.Scalar>
+
 // These haven't actually been removed; they were replaced with @_alwaysEmitIntoClient
 // alternatives. The old silgen-name entry point was preserved under an alternative
 // source-level name. This causes a false positive with the ABI checker.