feat: handle custom parser nested array for multiple flag

src/interfaces/parser.ts

@@ -175,7 +175,7 @@ export type OptionFlagProps = FlagProps & {
   multiple?: boolean;
 }
 
-export type FlagParser<T, I, P = any> = (input: I, context: any, opts: P & OptionFlag<T>) => Promise<T>
+export type FlagParser<T, I, P = any> = (input: I, context: any, opts: P & OptionFlag<T>) => Promise<T | T[]>
 
 export type FlagBase<T, I, P = any> = FlagProps & {
   parse: FlagParser<T, I, P>;

src/parser/parse.ts

@@ -201,7 +201,7 @@ export class Parser<T extends ParserInput, TFlags extends OutputFlags<T['flags']
         const value = flag.parse ? await flag.parse(input, this.context, flag) : input
         if (flag.multiple) {
           flags[token.flag] = flags[token.flag] || []
-          flags[token.flag].push(value)
+          flags[token.flag].push(...(Array.isArray(value) ? value : [value]))
         } else {
           flags[token.flag] = value
         }

test/parser/parse.test.ts

@@ -634,6 +634,22 @@ See more help with --help`)
       })
       expect(out.args).to.deep.include({num: '15'})
     })
+    it('flag multiple with arguments, custom parser', async () => {
+      const out = await parse(
+        ['--foo', './a.txt,./b.txt', '--foo', './c.txt', '--', '15'],
+        {
+          args: [{name: 'num'}],
+          flags: {foo: flags.string({
+            multiple: true,
+            parse: async input => input.split(',').map(i => i.trim()),
+          })},
+        },
+      )
+      expect(out.flags).to.deep.include({
+        foo: ['./a.txt', './b.txt', './c.txt'],
+      })
+      expect(out.args).to.deep.include({num: '15'})
+    })
   })
 
   describe('defaults', () => {