Inconsistent behaviour in brace expansion

[andgra2]

Brace expansion seems a bit inconsistent in its behaviour, see below. The documentation is also not so clear about this case.

Reproduction Steps:

1. Run: begin; set -l arr a b; echo x{$arr,c}y; end
2. Run: begin; set -l arr a b; echo x{{a,b},c}y; end

Expected behavior:

I expect 1. to give the same output as 2, ie: xay xby xcy.

That's the same output as echo x{a,b,c}y (ie things (arrays or other brace expressions) inside a brace expression should be unioned togheter when expanding).

This for consistency and simplicity reasons. I expect $arr and {a,b} to work in the same way in cases when they are both expanded.

Observed behavior:

1. Outputs: xay xcy xby xcy
2. Outputs: xay xby xcy

Additional information:

---

Fish version: fish, version 2.2.0-876-g1c6f6df
Operating system: ubuntu 64-bit, compiled fish from github source

Terminal or terminal emulator: The builtin terminal emulator of neovim.

[krader1961]

Those examples are just variations of the problem reported in issue #3002.

[andgra2]

I think this is quite different than #3002.

Here we have only one , in the outermost brace expression, but the problem in #3002 needs two , in order to reproduce.

jakwings added other examples in #3002, but I think those examples are more related to this issue (#3004).

Because of this I think this issue should be reopened. However it doesn't matter so much, I can check if this issue #3004 is solved when #3002 is solved, but I'm not sure that may be the case.

fish seems like doing echo x{$arr,c}y this way:

Step 1: join(x, join&merge([a, b], c), y)
Step 2: join(x, {a, c, b, c}, y)
Step 3: join(x, a, y)  join(x, c, y)  join(x, b, y)  join(x, c, y)
Result: xay xcy xby xcy

As to echo x{{a,b},c}y, I imagine fish is doing this:

Step 1: join(x, merge({a, b}, c), y)
Step 2: join(x, {a, b, c}, y)
Step 3: join(x, a, y)  join(x, b, y)  join(x, c, y)
Result: xay xby xcy