[stdlib][SR-4818] Add overflow assignment operators

stdlib/public/core/Integers.swift.gyb

@@ -588,6 +588,63 @@ def assignmentOperatorComment(operator, fixedWidth):
   /// - Parameters:
   ///   - lhs: The value to divide.
   ///   - rhs: The value to divide `lhs` by. `rhs` must not be zero.
+""",
+        '&+': """\
+  /// Stores the sum of the two given values in the left-hand-size variable, discarding any overflow.
+  ///
+  /// The masking addition operator (`&+`) silently discards any overflow that
+  /// occurs during the operation. In the following example, the sum of `100`
+  /// and `121` is greater than the maximum representable `Int8` value, so the
+  /// result is the overflowed value:
+  ///
+  ///     var x: Int8 = 10
+  ///     x &+= 21
+  ///     // x == 31
+  ///     var y: Int8 = 100
+  ///     y &+= 121
+  ///     // y == -35 (after overflow)
+  ///
+  /// - Parameters:
+  ///   - lhs: The first value to add.
+  ///   - rhs: The second value to add.
+""",
+        '&-': """\
+  /// Stores the difference of the two given values in the left-hand-size variable, discarding any overflow.
+  ///
+  /// The masking subtraction operator (`&-`) silently discards any overflow
+  /// that occurs during the operation. In the following example, the
+  /// difference of `10` and `21` is less than zero, the minimum representable
+  /// `UInt` value, so the result is the overflowed value:
+  ///
+  ///     var x: Int8 = 21l
+  ///     x &-= 10
+  ///     // x == 11
+  ///     var y: UInt8 = 10
+  ///     y &-= 21
+  ///     // y == 245 (after overflow)
+  ///
+  /// - Parameters:
+  ///   - lhs: A numeric value.
+  ///   - rhs: The value to subtract from `lhs`.
+""",
+        '&*': """\
+  /// Stores the product of the two given values in the left-hand-size variable, discarding any overflow.
+  ///
+  /// The masking multiplication operator (`&*`) silently discards any overflow
+  /// that occurs during the operation. In the following example, the product
+  /// of `10` and `50` is greater than the maximum representable `Int8` value,
+  /// so the result is the overflowed value:
+  ///
+  ///     var x: Int8 = 10
+  ///     x &*= 5
+  ///     // x == 50
+  ///     var y: Int8 = 10
+  ///     y &*= 50
+  ///     // y == -12 (after overflow)
+  ///
+  /// - Parameters:
+  ///   - lhs: The first value to multiply.
+  ///   - rhs: The second value to multiply.
 """,
         '&': """\
   /// Stores the result of performing a bitwise AND operation on the two given
@@ -2740,8 +2797,18 @@ ${unsafeOperationComment(x.operator)}
 ${operatorComment('&' + x.operator, True)}
   @_inlineable // FIXME(sil-serialize-all)
   @_transparent
-  public static func &${x.operator} (lhs: Self, rhs: Self) -> Self {
-    return lhs.${x.name}ReportingOverflow(${callLabel}rhs).partialValue
+  public static func &${x.operator}(lhs: Self, rhs: Self) -> Self {
+    var lhs = lhs
+    lhs &${x.operator}= rhs
+    return lhs
+  }
+
+${assignmentOperatorComment('&' + x.operator, True)}
+  @_inlineable // FIXME(sil-serialize-all)
+  @_transparent
+  public static func &${x.operator}=(lhs: inout Self, rhs: Self) {
+    let result = lhs.${x.name}ReportingOverflow(${callLabel}rhs).partialValue
+    lhs = result
   }
 %   end
 % end

stdlib/public/core/Policy.swift

@@ -698,10 +698,13 @@ infix operator || : LogicalDisjunctionPrecedence
 // Compound
 
 infix operator   *= : AssignmentPrecedence
+infix operator  &*= : AssignmentPrecedence
 infix operator   /= : AssignmentPrecedence
 infix operator   %= : AssignmentPrecedence
 infix operator   += : AssignmentPrecedence
+infix operator  &+= : AssignmentPrecedence
 infix operator   -= : AssignmentPrecedence
+infix operator  &-= : AssignmentPrecedence
 infix operator  <<= : AssignmentPrecedence
 infix operator &<<= : AssignmentPrecedence
 infix operator  >>= : AssignmentPrecedence

utils/SwiftIntTypes.py

@@ -122,7 +122,7 @@ def all_integer_or_real_binary_operator_names():
 
 
 def all_integer_assignment_operator_names():
-    return ['%=', '<<=', '>>=', '&=', '^=', '|=']
+    return ['%=', '<<=', '>>=', '&*=', '&=', '&+=', '&-=', '^=', '|=']
 
 
 def all_integer_or_real_assignment_operator_names():