isScalar needs to reject invalid ASCII

0x80 - 0xFF are invalid ascii, and thus, need to not return true always.

Author: AZero13

stdlib/public/core/ASCII.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 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
@@ -30,7 +30,7 @@ extension Unicode.ASCII: Unicode.Encoding {
   @inline(__always)
   @inlinable
   public static func _isScalar(_ x: CodeUnit) -> Bool {
-    return true
+    return isASCII(x)
   }
 
   @inline(__always)

test/stdlib/Unicode.swift

@@ -110,4 +110,12 @@ UnicodeAPIs.test("UTF-8 and UTF-16 queries") {
   }
 }
 
+
+UnicodeAPIs.test("ASCII._isScalar") {
+  expectTrue(ASCII._isScalar(0))
+  expectTrue(ASCII._isScalar(0x7F))
+  expectFalse(ASCII._isScalar(0x80))
+  expectFalse(ASCII._isScalar(0xFF))
+}
+
 runAllTests()