fix: values that look like flags would not be parsed correctly

Specifically, when parsing into slices or maps.

Fixes #290

mapper.go

@@ -262,8 +262,7 @@ func (r *Registry) RegisterDefaults() *Registry {
 		RegisterKind(reflect.Float32, floatDecoder(32)).
 		RegisterKind(reflect.Float64, floatDecoder(64)).
 		RegisterKind(reflect.String, MapperFunc(func(ctx *DecodeContext, target reflect.Value) error {
-			err := ctx.Scan.PopValueInto("string", target.Addr().Interface())
-			return err
+			return ctx.Scan.PopValueInto("string", target.Addr().Interface())
 		})).
 		RegisterKind(reflect.Bool, boolMapper{}).
 		RegisterKind(reflect.Slice, sliceDecoder(r)).
@@ -450,7 +449,7 @@ func mapDecoder(r *Registry) MapperFunc {
 			}
 			switch v := t.Value.(type) {
 			case string:
-				childScanner = Scan(SplitEscaped(v, sep)...)
+				childScanner = ScanAsType(t.Type, SplitEscaped(v, sep)...)
 
 			case []map[string]interface{}:
 				for _, m := range v {
@@ -492,14 +491,14 @@ func mapDecoder(r *Registry) MapperFunc {
 				keyTypeName, valueTypeName = parts[0], parts[1]
 			}
 
-			keyScanner := Scan(key)
+			keyScanner := ScanAsType(FlagValueToken, key)
 			keyDecoder := r.ForNamedType(keyTypeName, el.Key())
 			keyValue := reflect.New(el.Key()).Elem()
 			if err := keyDecoder.Decode(ctx.WithScanner(keyScanner), keyValue); err != nil {
 				return fmt.Errorf("invalid map key %q", key)
 			}
 
-			valueScanner := Scan(value)
+			valueScanner := ScanAsType(FlagValueToken, value)
 			valueDecoder := r.ForNamedType(valueTypeName, el.Elem())
 			valueValue := reflect.New(el.Elem()).Elem()
 			if err := valueDecoder.Decode(ctx.WithScanner(valueScanner), valueValue); err != nil {
@@ -525,7 +524,7 @@ func sliceDecoder(r *Registry) MapperFunc {
 			}
 			switch v := t.Value.(type) {
 			case string:
-				childScanner = Scan(SplitEscaped(v, sep)...)
+				childScanner = ScanAsType(t.Type, SplitEscaped(v, sep)...)
 
 			case []interface{}:
 				return jsonTranscode(v, target.Addr().Interface())

mapper_test.go

@@ -495,3 +495,21 @@ func (t testMapperVarsContributor) Decode(ctx *kong.DecodeContext, target reflec
 	target.SetString("hi")
 	return nil
 }
+
+func TestValuesThatLookLikeFlags(t *testing.T) {
+	var cli struct {
+		Slice []string
+		Map   map[string]string
+	}
+	k := mustNew(t, &cli)
+	_, err := k.Parse([]string{"--slice", "-foo"})
+	require.Error(t, err)
+	_, err = k.Parse([]string{"--map", "-foo=-bar"})
+	require.Error(t, err)
+	_, err = k.Parse([]string{"--slice=-foo", "--slice=-bar"})
+	require.NoError(t, err)
+	require.Equal(t, []string{"-foo", "-bar"}, cli.Slice)
+	_, err = k.Parse([]string{"--map=-foo=-bar"})
+	require.NoError(t, err)
+	require.Equal(t, map[string]string{"-foo": "-bar"}, cli.Map)
+}

scanner.go

@@ -114,15 +114,20 @@ type Scanner struct {
 	args []Token
 }
 
-// Scan creates a new Scanner from args with untyped tokens.
-func Scan(args ...string) *Scanner {
+// ScanAsType creates a new Scanner from args with the given type.
+func ScanAsType(ttype TokenType, args ...string) *Scanner {
 	s := &Scanner{}
 	for _, arg := range args {
-		s.args = append(s.args, Token{Value: arg})
+		s.args = append(s.args, Token{Value: arg, Type: ttype})
 	}
 	return s
 }
 
+// Scan creates a new Scanner from args with untyped tokens.
+func Scan(args ...string) *Scanner {
+	return ScanAsType(UntypedToken, args...)
+}
+
 // ScanFromTokens creates a new Scanner from a slice of tokens.
 func ScanFromTokens(tokens ...Token) *Scanner {
 	return &Scanner{args: tokens}