[devmotion]

Since #328, @grad_from_chainrules k(func, x::TrackedArray) fails with syntax: function argument name not unique: "func", which breaks ImplicitAD and its dependents (#334).

The self argument func is not escaped, so macro hygiene should rename it. It doesn't, because of a quirk in Julia's hygiene (src/macroexpand.scm):

• Escaped argument names that are bare symbols are deliberately left unrenamed (safe-llist-keyword-args: "count escaped argument names as 'keywords' to prevent renaming").
• The name or self position of a definition, e.g. (func::T)(…) or a local function closure(…), is resolved with that same mapping. So an internal name there that coincides with an escaped argument name stays unrenamed as well, and collides.

Minimal example without ReverseDiff:

struct S end
macro a(T, n) quote (func::$(esc(T)))($(esc(n))) = func end end
@macroexpand @a S func  # (func::S)(func) = func

Trying every internal name as an argument name in both macros found one more case: an untyped argument named closure in @grad fails with cannot add method to function argument closure. The existing test missed it because typed arguments are escaped as a whole :: expression, which hygiene does not pin. Contrary to the guess in #334, func works in @grad. All other internal names (args, kwargs, tp, back, output, …) work in both macros.

Hygiene alone cannot fix this: each macro has to put one of its own names in such a position. Dropping the self name and calling $f in the body would let an argument named like the function shadow it again, which #328 avoided on purpose. So this PR uses a gensym for exactly these two names: the self argument in @grad_from_chainrules and the local function in @grad.

Regression tests: an untyped argument func in @grad_from_chainrules (as in ImplicitAD), and untyped closure and func in @grad.

Fixes #334.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.78%. Comparing base (31b0e3d) to head (0dad5d8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #335   +/-   ##
=======================================
  Coverage   94.77%   94.78%           
=======================================
  Files          19       19           
  Lines        1703     1705    +2     
=======================================
+ Hits         1614     1616    +2     
  Misses         89       89           

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[devmotion]

Reported the underlying hygiene bug upstream: JuliaLang/julia#64945.