[stdlib] Add exponentiation precedence and operator

Author: xwu

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -1243,6 +1243,30 @@ extension ${Self} {
   }
 }
 
+//===----------------------------------------------------------------------===//
+// Placeholder Operators
+//===----------------------------------------------------------------------===//
+
+//  TODO: Remove these operators once protocol-based math functions are
+//  incorporated from Swift Numerics into the standard library.
+
+${Availability(bits)}
+extension ${Self} {
+  @_alwaysEmitIntoClient
+  @available(*, unavailable,
+    message: "Import C standard library or Foundation to use 'pow' instead")
+  public static func ** (_ lhs: Self, _ rhs: Self) -> Self {
+    fatalError()
+  }
+  
+  @_alwaysEmitIntoClient
+  @available(*, unavailable,
+    message: "Import C standard library or Foundation to use 'pow' instead")
+  public static func **= (_ lhs: inout Self, _ rhs: Self) {
+    fatalError()
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // Strideable Conformance
 //===----------------------------------------------------------------------===//
@@ -1346,10 +1370,6 @@ internal struct _${Self}AnyHashableBox: _AnyHashableBox {
 ${Availability(bits)}
 extension ${Self} : ConcurrentValue { }
 
-//===----------------------------------------------------------------------===//
-// Deprecated operators
-//===----------------------------------------------------------------------===//
-
 % if bits in [16,80]:
 #else
 
@@ -1374,6 +1394,10 @@ public struct ${Self} {
 % end
 % end # for bits in all_floating_point_types
 
+//===----------------------------------------------------------------------===//
+// Deprecated Operators
+//===----------------------------------------------------------------------===//
+
 @_transparent
 @available(*, unavailable,
   message: "For floating point numbers use truncatingRemainder instead")

stdlib/public/core/Policy.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -368,7 +368,10 @@ precedencegroup MultiplicationPrecedence {
 precedencegroup BitwiseShiftPrecedence {
   higherThan: MultiplicationPrecedence
 }
-
+precedencegroup ExponentiationPrecedence {
+  associativity: right
+  higherThan: MultiplicationPrecedence
+}
 
 //===----------------------------------------------------------------------===//
 // Standard operators
@@ -397,6 +400,7 @@ prefix operator ..<: Comparable
 // Standard infix operators.
 
 // "Exponentiative"
+infix operator  **: ExponentiationPrecedence
 
 infix operator  <<: BitwiseShiftPrecedence, BinaryInteger
 infix operator &<<: BitwiseShiftPrecedence, FixedWidthInteger
@@ -463,6 +467,7 @@ infix operator ||: LogicalDisjunctionPrecedence, Bool
 
 // Compound
 
+infix operator  **=: AssignmentPrecedence
 infix operator   *=: AssignmentPrecedence, Numeric
 infix operator  &*=: AssignmentPrecedence, FixedWidthInteger
 infix operator   /=: AssignmentPrecedence, BinaryInteger