Mode error in semidet list pattern matching without intermediate variable

[jrfondren]

This is observed on rotd-2019-09-05 and on a Mercury build from Jul 14.

This module:

:- module split2.
:- interface.
:- import_module io.
:- pred main(io::di, io::uo) is det.
:- implementation.
:- import_module list, string.

main(!IO) :-
    Line = "example.com: exa==root==addon==example.example.com==/home/exa==127.0.0.1:80==127.0.0.1:443====0==",
    ( if
        [_Domain, Rest] = split_at_string(": ", Line),
        [User, _Owner, _, _, _Docroot | _] = split_at_string("==", Rest)
        %L = split_at_string("==", Rest),
        %L = [User, _Owner, _, _, _Docroot | _]
    then
        io.print_line(User, !IO)
    else
        io.set_exit_status(1, !IO)
    ).

Fails to compile with these errors:

split2.m:013: In clause for `'__Unify__'(in, (unique(list.'[|]'(free,
split2.m:013:   unique(list.'[|]'(free, unique(list.'[|]'(free,
split2.m:013:   unique(list.'[|]'(free, free)))))))) >>
split2.m:013:   bound(list.'[|]'(ground, bound(list.'[|]'(ground,
split2.m:013:   bound(list.'[|]'(ground, bound(list.'[|]'(ground,
split2.m:013:   ground))))))))), (ground >> bound(list.'[|]'(ground,
split2.m:013:   bound(list.'[|]'(ground, bound(list.'[|]'(ground,
split2.m:013:   bound(list.'[|]'(ground, ground))))))))))':
split2.m:013:   mode error in unification of `HeadVar__1' and `list.[ArgX1 |
split2.m:013:   V_15]'.
split2.m:013:   Variable `HeadVar__1' has instantiatedness
split2.m:013:     unique(
split2.m:013:       '[|]'(
split2.m:013:         free,
split2.m:013:         unique(
split2.m:013:           '[|]'(
split2.m:013:             free,
split2.m:013:             unique(
split2.m:013:               '[|]'(
split2.m:013:                 free,
split2.m:013:                 unique(
split2.m:013:                   '[|]'(free, free)
split2.m:013:                 )
split2.m:013:               )
split2.m:013:             )
split2.m:013:           )
split2.m:013:         )
split2.m:013:       )
split2.m:013:     ),
split2.m:013:   term `list.[ArgX1 | V_15]' has instantiatedness
split2.m:013:   `named inst list.'[|]'(ground, free)'.

If the offending line is commented out and the next two lines are uncommented, it works just fine. I encountered this bug when adapting code that did some very simple parsing of cPanel's 'userdatadomains' file to only retrieve the User field.

[AlaskanEmily]

You can work around this by changing the line to:

split_at_string("==", Rest) = [User, _Owner, _, _, _Docroot | _]