feat: treat '-<number>' tokens as values to allow negative numeric arguments (Fixes #31)

- Update SplitArguments to classify '-5' and '-3.14' as values, not options
- Add unit tests for negative Int and Double positional arguments

Author: cnaples79

Sources/ArgumentParser/Parsing/SplitArguments.swift

@@ -592,7 +592,14 @@ func parseIndividualArg(_ arg: String, at position: Int) throws
     case 0:
       return [.value(arg, index: index)]
     case 1:
-      // Long option:
+      // If the remainder is a numeric value (e.g. "-5", "-3.14"), treat it as a value
+      // to allow passing negative numbers to positional arguments.
+      // This preserves typical short/long option parsing while enabling
+      // negative numeric literals to be consumed as values.
+      if remainder.allSatisfy({ $0.isNumber }) || isDecimalNumber(remainder) {
+        return [.value(arg, index: index)]
+      }
+      // Otherwise, treat as an option (short or long-with-single-dash)
       let parsed = try ParsedArgument(longArgWithSingleDashRemainder: remainder)
 
       // Short options:
@@ -636,6 +643,22 @@ func parseIndividualArg(_ arg: String, at position: Int) throws
   }
 }
 
+/// Detects a simple decimal number like "123" or "3.14" (no sign).
+private func isDecimalNumber(_ s: Substring) -> Bool {
+  // must contain exactly one dot or none; all other chars are digits
+  var dotCount = 0
+  for ch in s {
+    if ch == "." {
+      dotCount += 1
+      if dotCount > 1 { return false }
+    } else if !ch.isNumber {
+      return false
+    }
+  }
+  // not just a dot
+  return !s.isEmpty && !(s.count == 1 && s.first == ".")
+}
+
 extension SplitArguments {
   /// Parses the given input into an array of `Element`.
   ///

Tests/ArgumentParserUnitTests/NegativeNumberArgumentTests.swift

@@ -0,0 +1,34 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Argument Parser open source project
+//
+// Copyright (c) 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
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import ArgumentParser
+
+final class NegativeNumberArgumentTests: XCTestCase {
+  struct Absolute: ParsableCommand {
+    @Argument var number: Int
+  }
+
+  func testParsesNegativeIntegerAsArgument() throws {
+    let cmd = try Absolute.parse(["-5"]) // should be treated as value, not option
+    XCTAssertEqual(cmd.number, -5)
+  }
+
+  struct FloatArg: ParsableCommand {
+    @Argument var value: Double
+  }
+
+  func testParsesNegativeDoubleAsArgument() throws {
+    let cmd = try FloatArg.parse(["-3.14"]) // negative decimal
+    XCTAssertEqual(cmd.value, -3.14, accuracy: 1e-9)
+  }
+}
+