[stdlib] Fix Int(_:radix:) accepting unintentional cases (SR-187)

Author: PatrickPijnappel

stdlib/public/core/IntegerParsing.swift.gyb

@@ -70,18 +70,17 @@ internal func _parseUnsignedAsciiAsUIntMax(
 /// - Note: For text matching the regular expression "-0+", the result
 ///   is `0`, not `nil`.
 internal func _parseAsciiAsUIntMax(
-  u16: String.UTF16View, _ radix: Int, _ maximum: UIntMax
+  utf16: String.UTF16View, _ radix: Int, _ maximum: UIntMax
 ) -> UIntMax? {
-  if u16.isEmpty { return nil }
-  let c = u16.first
-  if _fastPath(c != _ascii16("-")) {
-    let unsignedText
-      = c == _ascii16("+") ? u16.dropFirst() : u16
-    return _parseUnsignedAsciiAsUIntMax(unsignedText, radix, maximum)
-  }
-  else {
-    return _parseAsciiAsIntMax(u16, radix, 0) == 0 ? 0 : nil
-  }
+  if utf16.isEmpty { return nil }
+  // Parse (optional) sign
+  let (digitsUTF16, hasMinus) = _parseOptionalAsciiSign(utf16)
+  // Parse digits
+  guard let result = _parseUnsignedAsciiAsUIntMax(digitsUTF16, radix, maximum) else { return nil }
+  // Disallow < 0
+  if hasMinus && result != 0 { return nil }
+  // Return
+  return result
 }
 
 /// If text is an ASCII representation in the given `radix` of a
@@ -91,23 +90,28 @@ internal func _parseAsciiAsUIntMax(
 /// - Note: For text matching the regular expression "-0+", the result
 ///   is `0`, not `nil`.
 internal func _parseAsciiAsIntMax(
-  u16: String.UTF16View, _ radix: Int, _ maximum: IntMax
+  utf16: String.UTF16View, _ radix: Int, _ maximum: IntMax
 ) -> IntMax? {
   _sanityCheck(maximum >= 0, "maximum should be non-negative")
+  if utf16.isEmpty { return nil }
+  // Parse (optional) sign
+  let (digitsUTF16, hasMinus) = _parseOptionalAsciiSign(utf16)
+  // Parse digits
+  let absValueMax = UIntMax(bitPattern: maximum) + (hasMinus ? 1 : 0)
+  guard let absValue = _parseUnsignedAsciiAsUIntMax(digitsUTF16, radix, absValueMax) else { return nil }
+  // Return signed result
+  return IntMax(bitPattern: hasMinus ? 0 &- absValue : absValue)
+}
 
-  if u16.isEmpty { return nil }
-
-  // Drop any leading "-"
-  let negative = u16.first == _ascii16("-")
-  let absResultText = negative ? u16.dropFirst() : u16
-
-  let absResultMax = UIntMax(bitPattern: maximum) + (negative ? 1 : 0)
-
-  // Parse the result as unsigned
-  if let absResult = _parseAsciiAsUIntMax(absResultText, radix, absResultMax) {
-    return IntMax(bitPattern: negative ? 0 &- absResult : absResult)
+/// Strip an optional single leading ASCII plus/minus sign from `utf16`.
+private func _parseOptionalAsciiSign(
+  utf16: String.UTF16View
+) -> (digitsUTF16: String.UTF16View, isMinus: Bool) {
+  switch utf16.first {
+  case _ascii16("-")?: return (utf16.dropFirst(), true)
+  case _ascii16("+")?: return (utf16.dropFirst(), false)
+  default: return (utf16, false)
   }
-  return nil
 }
 
 //===--- Loop over all integer types --------------------------------------===//