[tombouctou]

Fixes #1381.

Two bugs in matching a pattern against an expression whose head is Flat.

1. A blank before a literal never takes several elements. basic_match_expression pairs the leading pattern elements whose match count is 1 with the expression's elements one by one, as a fast pre-check. With Flat, a blank x_ can take several elements (x_ in f[x_, z] matches f[x, y] in f[x, y, z]), so the pre-check rejected valid matches. It is already skipped for Orderless heads; this skips it for Flat heads too.

In[1]:= SetAttributes[f, Flat]; f[a_, 0] := zero[a]; f[x, y, 0]
master:  f[x, y, 0]
fixed:   zero[f[x, y]]

2. Elements are dropped. After the first pattern element matched, the next one was matched with first still set. In a partial match (fully=False, as ReplaceAll uses for Flat heads) it could then start anywhere, skipping elements, and the skipped elements were dropped from the result. Only the first element may leave elements before it.

In[2]:= f[x, y, z] /. f[a__, z] :> {a}
master:  {x}
fixed:   {x, y}

In[3]:= f[x, y, z] /. f[a___, y, b___] :> {{a}, {b}}
master:  {{}, {z}}
fixed:   {{x}, {z}}

In[4]:= g[l__, SameTest -> s_] := {{l}, s}  (g Flat, as in #1381)
        g[{1, -1}, {2, 3}, SameTest -> t]
master:  {{{1, -1}}, t}
fixed:   {{{1, -1}, {2, 3}}, t}

Together, the inputs from the Scope section of the Flat reference page now work:

f[a, b, c, d, e] /. f[b, c, d] -> x        f[a, x, e]       (master: unchanged)
f[a, b, c, d, e] /. f[b, c, d] -> f[x, y]  f[a, x, y, e]    (master: unchanged)
f[a, b, c, d, e] /. f[b] -> x              f[a, x, c, d, e] (master: unchanged)
f[a, b, c, d, e] /. f[d, b] -> x           f[a, b, c, d, e] (both; order matters)

Tests: test_flat_patterns in test/builtin/test_patterns.py; 11 of its cases fail on master. Full pytest and doctests pass.

Not changed here (separate differences from WMA): f[a, b, c] /. f[x_] -> {x} gives f[{f[a, b]}, c] rather than {f[a, b, c]}, and MatchQ[f[1, 2], f[_]] gives False.

🤖 Generated with Claude Code

[rocky]

Wow - this is complicated.

Not changed here (separate differences from WMA): f[a, b, c] /. f[x_] -> {x} gives f[{f[a, b]}, c] rather than {f[a, b, c]}, and MatchQ[f[1, 2], f[_]] gives False.

@tombouctou would you please open an issue so we have this tracked?

LGTM - thanks for fixing this long-standing bug.

[tombouctou]

Thanks! Opened #2026 for these two (both come from the same thing: with a Flat head, a single blank does not take several elements).