Test::More plan() requires explicit `scalar`? Behaviour difference wrt Test-Simple-1.302136

[cxw42]

First-time cperl user, here courtesy of the cpantesters report on my first CPAN distribution, Text::PerlPP. All tests pass on p5p perl 5.10.1 (cygwin x86) and 5.26.1 (cygwin x64) (both perlbrew; CPAN Test::More 1.302136). However, the plan call fails on cperl 5.26.2c (cygwin x64) (perlbrew; Test::More 1.401015c).

My test code has an array of testcases. I then plan tests => count_tests(\@testcases, 1, 2); (here) using a function to count the tests (some list entries have multiple tests). On cperl, plan gives me the error Got an undefined number of tests.

I have the following MCVE. Save as foo.t and run as perl foo.t for the failure or perl foo.t 1 for success.

use 5.010001; use strict; use warnings; use Test::More;

sub count_tests { return scalar @{+shift}; }

my @testcases=qw(foo);

plan tests => count_tests(\@testcases) unless @ARGV;        # fails
plan tests => scalar count_tests(\@testcases) if @ARGV;     # succeeds

# Workaround:
#my $ntests = count_tests(\@testcases);
#plan tests => $ntests;                          # succeeds
#plan tests => scalar $ntests;                   # succeeds

ok(!!1, 'yep');

Why do I need to specify scalar on the plan line, even though count_tests() is returning a scalar?

I do not know if this is intended behaviour or a bug. Obviously, I have a workaround :) , but I would still like to know. This probably won't be my last CPAN release, and I would like to write cperl-compatible test cases.

I searched the issues and didn't find this one. I did see discussion of strict typing in #153, and I wonder if that might be similar. Also, #255 says TODO Test::Script Have_Plan oddity in Test::Simple, but doesn't explain what oddity that was. Was it indeed this one?

Thanks very much!

[rurban]

This is a FAQ entry. See https://perl11.org/cperl/STATUS.html under FAQ.
tl;dr: The plan argument is typed, it can only be a number.

count_tests returns scalar (type :Number), but the type-optimizer is/was not so good yet, to infer that up to the caller. I think it is now, but to write compat. code, the plan and skip args need to be typed. type :Any is illegal.