Allow single, attached value with .upToNextOption (apple#610)

Fixes an issue where a single attached value triggered an error on
`.upToNextOption` options.

Fixes apple#609.

Sources/ArgumentParser/Parsing/ArgumentSet.swift

@@ -327,17 +327,20 @@ struct LenientParser {
 
     case .upToNextOption:
       // Use an attached value if it exists...
+      var foundAttachedValue = false
       if let value = parsed.value {
         // This was `--foo=bar` style:
         try update(origin, parsed.name, value, &result)
         usedOrigins.formUnion(origin)
+        foundAttachedValue = true
       } else if argument.allowsJoinedValue,
           let (origin2, value) = inputArguments.extractJoinedElement(at: originElement) {
         // Found a joined argument
         let origins = origin.inserting(origin2)
         try update(origins, parsed.name, String(value), &result)
         usedOrigins.formUnion(origins)
         inputArguments.removeAll(in: usedOrigins)
+        foundAttachedValue = true
       }
 
       // Clear out the initial origin first, since it can include
@@ -350,7 +353,13 @@ struct LenientParser {
       guard let first = inputArguments.elements.first,
             first.isValue
       else {
-        throw ParserError.missingValueForOption(origin, parsed.name)
+        // No independent values to be found, which is an error if there was
+        // no `--foo=bar`-style value already found.
+        if foundAttachedValue {
+          break
+        } else {
+          throw ParserError.missingValueForOption(origin, parsed.name)
+        }
       }
 
       // ...and then consume the arguments until hitting an option

Tests/ArgumentParserEndToEndTests/RepeatingEndToEndTests.swift

@@ -210,6 +210,24 @@ extension RepeatingEndToEndTests {
       XCTAssertEqual(qux.names, ["one", "two"])
       XCTAssertNil(qux.extra)
     }
+
+    AssertParse(Qux.self, ["--verbose", "--names=one"]) { qux in
+      XCTAssertTrue(qux.verbose)
+      XCTAssertEqual(qux.names, ["one"])
+      XCTAssertNil(qux.extra)
+    }
+
+    AssertParse(Qux.self, ["--verbose", "--names=one", "two"]) { qux in
+      XCTAssertTrue(qux.verbose)
+      XCTAssertEqual(qux.names, ["one", "two"])
+      XCTAssertNil(qux.extra)
+    }
+
+    AssertParse(Qux.self, ["--names=one", "--verbose", "two"]) { qux in
+      XCTAssertTrue(qux.verbose)
+      XCTAssertEqual(qux.names, ["one"])
+      XCTAssertEqual(qux.extra, "two")
+    }
   }
 
   func testParsing_repeatingStringUpToNext_Fails() throws {