Fix SystemChar isLetter logic

- Fixes range expression in isLetter logic and adds test coverage.

Author: rauhul

Sources/System/SystemString.swift

@@ -53,8 +53,8 @@ extension SystemChar {
   internal var isLetter: Bool {
     guard isASCII else { return false }
     let asciiRaw: UInt8 = numericCast(rawValue)
-    return (UInt8(ascii: "a") ..< UInt8(ascii: "z")).contains(asciiRaw) ||
-           (UInt8(ascii: "A") ..< UInt8(ascii: "Z")).contains(asciiRaw)
+    return (UInt8(ascii: "a") ... UInt8(ascii: "z")).contains(asciiRaw) ||
+           (UInt8(ascii: "A") ... UInt8(ascii: "Z")).contains(asciiRaw)
   }
 }
 

Tests/SystemTests/SystemCharTest.swift

@@ -0,0 +1,39 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2020 - 2021 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+*/
+
+import XCTest
+
+#if SYSTEM_PACKAGE
+@testable import SystemPackage
+#else
+@testable import System
+#endif
+
+final class SystemCharTest: XCTestCase {
+  func testIsLetter() {
+    let valid = SystemString(
+      "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
+    for char in valid {
+      XCTAssertTrue(char.isLetter)
+    }
+
+    // non printable
+    for value in 0..<(UInt8(ascii: " ")) {
+      XCTAssertFalse(SystemChar(codeUnit: value).isLetter)
+    }
+    XCTAssertFalse(SystemChar(codeUnit: 0x7F).isLetter) // DEL
+
+    // misc other
+    let invalid = SystemString(
+      ##" !"#$%&'()*+,-./0123456789:;<=>?@[\]^_`{|}~"##)
+    for char in invalid {
+      XCTAssertFalse(char.isLetter)
+    }
+  }
+}

Tests/SystemTests/XCTestManifests.swift

@@ -94,6 +94,15 @@ extension MockingTest {
     ]
 }
 
+extension SystemCharTest {
+    // DO NOT MODIFY: This is autogenerated, use:
+    //   `swift test --generate-linuxmain`
+    // to regenerate.
+    static let __allTests__SystemCharTest = [
+        ("testIsLetter", testIsLetter),
+    ]
+}
+
 extension SystemStringTest {
     // DO NOT MODIFY: This is autogenerated, use:
     //   `swift test --generate-linuxmain`
@@ -115,6 +124,7 @@ public func __allTests() -> [XCTestCaseEntry] {
         testCase(FilePathTest.__allTests__FilePathTest),
         testCase(FilePermissionsTest.__allTests__FilePermissionsTest),
         testCase(MockingTest.__allTests__MockingTest),
+        testCase(SystemCharTest.__allTests__SystemCharTest),
         testCase(SystemStringTest.__allTests__SystemStringTest),
     ]
 }