Fix error message for @option array without values (#435)

Author: KthKuang

Sources/ArgumentParser/Parsable Properties/Option.swift

@@ -200,7 +200,7 @@ extension Option where Value: ExpressibleByArgument {
 
 /// The strategy to use when parsing a single value from `@Option` arguments.
 ///
-/// - SeeAlso: `ArrayParsingStrategy``
+/// - SeeAlso: ``ArrayParsingStrategy``
 public struct SingleValueParsingStrategy: Hashable {  
   internal var base: ArgumentDefinition.ParsingStrategy
 

Sources/ArgumentParser/Parsing/ArgumentSet.swift

@@ -356,6 +356,14 @@ extension ArgumentSet {
         usedOrigins.formUnion(origin)
         inputArguments.removeAll(in: origin)
 
+        // Fix incorrect error message
+        // for @Option array without values (see issue #434).
+        guard let first = inputArguments.elements.first,
+              first.isValue
+        else {
+          throw ParserError.missingValueForOption(origin, parsed.name)
+        }
+        
         // ...and then consume the arguments until hitting an option
         while let (origin2, value) = inputArguments.popNextElementIfValue() {
           let origins = origin.inserting(origin2)

Tests/ArgumentParserUnitTests/ErrorMessageTests.swift

@@ -210,6 +210,27 @@ extension ErrorMessageTests {
   }
 }
 
+// (see issue #434).
+private struct EmptyArray: ParsableArguments {
+  @Option(parsing: .upToNextOption)
+  var array: [String] = []
+  
+  @Flag(name: [.short, .long])
+  var verbose = false
+}
+
+extension ErrorMessageTests {
+  func testEmptyArrayOption() {
+    AssertErrorMessage(EmptyArray.self, ["--array"], "Missing value for '--array <array>'")
+    
+    AssertErrorMessage(EmptyArray.self, ["--array", "--verbose"], "Missing value for '--array <array>'")
+    AssertErrorMessage(EmptyArray.self, ["-verbose", "--array"], "Missing value for '--array <array>'")
+    
+    AssertErrorMessage(EmptyArray.self, ["--array", "-v"], "Missing value for '--array <array>'")
+    AssertErrorMessage(EmptyArray.self, ["-v", "--array"], "Missing value for '--array <array>'")
+  }
+}
+
 // MARK: -
 
 fileprivate struct Repeat: ParsableArguments {