ListFormat.tsv joins values with a literal \t instead of a tab #2627

Package

dio

Version

5.11.1 (also current main)

Operating-System

All (pure Dart)

Adapter

Default Dio (all adapters; the bug is in the shared encoder)

Dart Version

3.12.2

Steps to Reproduce

ListFormat.tsv is documented as "Tab-separated values. e.g. (foo\tbar\tbaz)", but _getSeparatorChar in dio/lib/src/utils.dart returns the raw string r'\t', i.e. a backslash followed by the letter t:

Transformer.urlEncodeQueryMap({
  'a': const ListParam(['foo', 'bar'], ListFormat.tsv),
});

Transformer.urlEncodeMap({
  'a': const ListParam(['foo', 'bar'], ListFormat.tsv),
});

The same separator is used for queryParameters on requests and for FormData.fromMap(..., ListFormat.tsv).

Expected Result

Values separated by a tab character:

• urlEncodeQueryMap: a=foo%09bar
• urlEncodeMap: a=foo%09bar

Actual Result

Values separated by a literal backslash and t:

• urlEncodeQueryMap: a=foo\tbar (a raw backslash in the URL)
• urlEncodeMap: a=foo%5Ctbar

A server splitting the value on tabs receives the single value foo\tbar instead of foo and bar.