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=head1 NAME

App::Test::Generator::Mutation::ReturnUndef - Replace return expressions
with undef to expose missing undef-return checks in the test suite

=head1 VERSION

Version 0.41

=head1 METHODS

=head2 applies_to

Return true if the given document contains at least one return
statement this mutation strategy could mutate. Used by
L<App::Test::Generator::Mutator> to pre-filter strategies before
calling C<mutate>, so a document with nothing to mutate skips the
walk entirely.

    my $applies = $mutation->applies_to($doc);

=head3 Arguments

=over 4

=item * C<$doc>

A L<PPI::Document> object to inspect.

=back

=head3 Returns

True if the document contains a C<return> statement (PPI::Statement::Break
whose first token is C<return>), false otherwise.

=head3 API specification

=head4 input

    {
        self => { type => OBJECT, isa => 'App::Test::Generator::Mutation::ReturnUndef' },
        doc  => { type => OBJECT, isa => 'PPI::Document' },
    }

=head4 output

    { type => SCALAR }

=cut

=head2 mutate

Walk a PPI document and generate one mutant for each non-bare return
statement, replacing its expression with C<undef>. For example,
C<return $result> becomes C<return undef>.

Bare C<return;> statements are skipped because they already return
undef - mutating them would produce a redundant mutant that can never
be killed.

    my $mutation = App::Test::Generator::Mutation::ReturnUndef->new;
    my $doc      = PPI::Document->new(\$source);
    my @mutants  = $mutation->mutate($doc);

    for my $m (@mutants) {
        print $m->id, ': ', $m->description, "\n";
    }

=head3 Arguments

=over 4

=item * C<$self>

An instance of C<App::Test::Generator::Mutation::ReturnUndef>.

=item * C<$doc>

A L<PPI::Document> object representing the parsed source to mutate.
The document is not modified by this method.

=back

=head3 Returns

A list of L<App::Test::Generator::Mutant> objects, one per qualifying
return statement found in the document. Returns an empty list if no
non-bare return statements are found.

Each mutant carries a C<transform> closure that when called with a
fresh L<PPI::Document> copy will replace the targeted return expression
with the literal C<undef>.

=head3 Notes

Mutant IDs include both line and column number to ensure uniqueness
when multiple return statements appear in the same source file.

Only return statements with an expression child are mutated - bare
C<return;> statements are skipped as they already return undef.

Each mutant's optional C<context> field is set to C<conditional> if
the return statement sits inside (or is itself the keyword of) an
C<if>/C<unless>/C<while>/C<until> compound statement, or C<statement>
otherwise; its C<line_content> field holds the raw source text of the
mutated line. Both are consumed by
L<App::Test::Generator::Mutator>'s fast-mode dedup.

=head3 API specification

=head4 input

    {
        self => {
            type => OBJECT,
            isa  => 'App::Test::Generator::Mutation::ReturnUndef',
        },
        doc => {
            type => OBJECT,
            isa  => 'PPI::Document',
        },
    }

=head4 output

    {
        type     => ARRAYREF,
        elements => {
            type => OBJECT,
            isa  => 'App::Test::Generator::Mutant',
        },
    }

=cut

=head1 AUTHOR

Nigel Horne, C<< <njh at nigelhorne.com> >>

=head1 LICENCE AND COPYRIGHT

Copyright 2026 Nigel Horne.

Usage is subject to the terms of GPL2.
If you use it,
please let me know.

=cut