[tombouctou]

A format rule that wraps the expression it formats in Infix, Prefix or Postfix made printing hit $RecursionLimit:

In[1]:= Format[p[t__]] := Infix[p[t], "~"]; p[a, b]
master:  $RecursionLimit::reclim: Recursion depth of 200 exceeded.
branch:  a ~ b

Before boxing, do_format_element formats every subexpression of the result of a format rule. It therefore formatted p[a, b] inside the Infix again, which gave the same Infix, and so on.

Infix, Prefix and Postfix display only the elements of their first argument, never the argument itself. Now only those elements are formatted. Defining a format this way is the usual way to give an expression an operator form, and code written for WMA relies on it: the Toda-bracket package we are running on Mathics3 (see #2001) has Format[t_o] := Infix[t, "∘"] /; Length[t] > 1.

Format[pLeft[t__]] := Infix[pLeft[t], "~", 300, Left]; pLeft[pLeft[a, b], c]   a ~ b ~ c
Format[pPre[x_]] := Prefix[pPre[x], "!"]; pPre[a]                             !a
Format[pPost[x_]] := Postfix[pPost[x], "!!"]; pPost[a]                        a!!

Tests: five new cases in test_layout in test/format/test_format.py. All five fail on master.

🤖 Generated with Claude Code

[mmatera]

Makes sense. LGTM