From @zoffixznet

The slip (|) before a range has higher precedence and it interprets the starting point of the range as a single-item list. This leads to two types of bugs, and both situations should likely be error messages​:

1) Infinite loop when range contains letters, as the range starts from 1 and tries to reach the second letter by increasing a number​:

<ZoffixW> m​: .say for |"g".."z";
<camelia> rakudo-moar 3259ba​: OUTPUT«(timeout)1␤2␤3␤4␤5␤6␤7␤8␤9␤10␤11␤12␤13␤14␤15␤16␤17␤18␤19␤20␤21␤22␤23␤24␤25␤26␤27␤28␤29␤30␤31␤32␤33␤34␤35␤36␤37␤38␤39␤40␤41␤42␤43␤44␤45␤46␤47␤48␤49␤50␤51␤5…»

2) Incorrect range is produced when numbers are used for the range. It'll always start at 1 and proceed until the end number​:
<ZoffixW> m​: .say for |10..20
<camelia> rakudo-moar 3259ba​: OUTPUT«1␤2␤3␤4␤5␤6␤7␤8␤9␤10␤11␤12␤13␤14␤15␤16␤17␤18␤19␤20␤»
<ZoffixW> m​: .say for |10..5
<camelia> rakudo-moar 3259ba​: OUTPUT«1␤2␤3␤4␤5␤»

Relevant IRC conversation​: http://irclog.perlgeek.de/perl6/2016-01-15#i_11894289

From @skids

On Fri, 15 Jan 2016 10​:24​:16 -0800, cpan@​zoffix.com wrote​:

The slip (|) before a range has higher precedence and it interprets
the starting point of the range as a single-item list. This leads to
two types of bugs, and both situations should likely be error
messages​:

1) Infinite loop when range contains letters, as the range starts from
1 and tries to reach the second letter by increasing a number​:

<ZoffixW> m​: .say for |"g".."z";
<camelia> rakudo-moar 3259ba​:
OUTPUT«(timeout)1␤2␤3␤4␤5␤6␤7␤8␤9␤10␤11␤12␤13␤14␤15␤16␤17␤18␤19␤20␤21␤22␤23␤24␤25␤26␤27␤28␤29␤30␤31␤32␤33␤34␤35␤36␤37␤38␤39␤40␤41␤42␤43␤44␤45␤46␤47␤48␤49␤50␤51␤5…»

2) Incorrect range is produced when numbers are used for the range.
It'll always start at 1 and proceed until the end number​:
<ZoffixW> m​: .say for |10..20
<camelia> rakudo-moar 3259ba​:
OUTPUT«1␤2␤3␤4␤5␤6␤7␤8␤9␤10␤11␤12␤13␤14␤15␤16␤17␤18␤19␤20␤»
<ZoffixW> m​: .say for |10..5
<camelia> rakudo-moar 3259ba​: OUTPUT«1␤2␤3␤4␤5␤»

Relevant IRC conversation​: http://irclog.perlgeek.de/perl6/2016-01-
15#i_11894289

I submitted​:

rakudo/rakudo#1182

...which warns on these​:

$ perl6 -e 'say |4..5'
Potential difficulties​:
  To apply a Slip flattener to a range, parenthesize the whole range.
  (Or parenthesize the whole endpoint expression, if you meant that.)
  at -e​:1
  ------> say ⏏|4..5
1..5
$ perl6 -e 'say 4..5'
Potential difficulties​:
  To stringify a range, parenthesize the whole range.
  (Or parenthesize the whole endpoint expression, if you meant that.)
  at -e​:1
  ------> say ⏏4..5
"4"..5
$ perl6 -e 'say |4 R.. 5'
Potential difficulties​:
  To apply a Slip flattener to a range, parenthesize the whole range.
  (Or parenthesize the whole endpoint expression, if you meant that.)
  at -e​:1
  ------> say ⏏|4 R.. 5
5..1
$ perl6 -e 'say 4 R.. 5'
Potential difficulties​:
  To stringify a range, parenthesize the whole range.
  (Or parenthesize the whole endpoint expression, if you meant that.)
  at -e​:1
  ------> say ⏏4 R.. 5
5..4

(don't know why that last one "works", but still worth warning.)

It also handles all the ^..^ variants.

The RT System itself - Status changed from 'new' to 'open'

From @skids

On Fri, 06 Oct 2017 19​:32​:17 -0700, bri@​abrij.org wrote​:

On Fri, 15 Jan 2016 10​:24​:16 -0800, cpan@​zoffix.com wrote​:

The slip (|) before a range has higher precedence and it interprets
the starting point of the range as a single-item list. This leads to
two types of bugs, and both situations should likely be error
messages​:

1) Infinite loop when range contains letters, as the range starts
from
1 and tries to reach the second letter by increasing a number​:

<ZoffixW> m​: .say for |"g".."z";
<camelia> rakudo-moar 3259ba​:
OUTPUT«(timeout)1␤2␤3␤4␤5␤6␤7␤8␤9␤10␤11␤12␤13␤14␤15␤16␤17␤18␤19␤20␤21␤22␤23␤24␤25␤26␤27␤28␤29␤30␤31␤32␤33␤34␤35␤36␤37␤38␤39␤40␤41␤42␤43␤44␤45␤46␤47␤48␤49␤50␤51␤5…»

2) Incorrect range is produced when numbers are used for the range.
It'll always start at 1 and proceed until the end number​:
<ZoffixW> m​: .say for |10..20
<camelia> rakudo-moar 3259ba​:
OUTPUT«1␤2␤3␤4␤5␤6␤7␤8␤9␤10␤11␤12␤13␤14␤15␤16␤17␤18␤19␤20␤»
<ZoffixW> m​: .say for |10..5
<camelia> rakudo-moar 3259ba​: OUTPUT«1␤2␤3␤4␤5␤»

Relevant IRC conversation​: http://irclog.perlgeek.de/perl6/2016-01-
15#i_11894289

I submitted​:

rakudo/rakudo#1182

...which warns on these​:

$ perl6 -e 'say |4..5'
Potential difficulties​:
To apply a Slip flattener to a range, parenthesize the whole
range.
(Or parenthesize the whole endpoint expression, if you meant
that.)
at -e​:1
------> say ⏏|4..5
1..5
$ perl6 -e 'say 4..5'
Potential difficulties​:
To stringify a range, parenthesize the whole range.
(Or parenthesize the whole endpoint expression, if you meant
that.)
at -e​:1
------> say ⏏4..5
"4"..5
$ perl6 -e 'say |4 R.. 5'
Potential difficulties​:
To apply a Slip flattener to a range, parenthesize the whole
range.
(Or parenthesize the whole endpoint expression, if you meant
that.)
at -e​:1
------> say ⏏|4 R.. 5
5..1
$ perl6 -e 'say 4 R.. 5'
Potential difficulties​:
To stringify a range, parenthesize the whole range.
(Or parenthesize the whole endpoint expression, if you meant
that.)
at -e​:1
------> say ⏏4 R.. 5
5..4

(don't know why that last one "works", but still worth warning.)

It also handles all the ^..^ variants.

OK, PR merged in rakudo cb9d2e040

Tests in roast 6e9bab5de

These are just worries, not errors, but good enough?

@zoffixznet - Status changed from 'open' to 'resolved'