[devmotion]

Fixes #327.

deepcopy calls deepcopy_internal when it recurses, so the existing Base.deepcopy(::TrackedReal) method only applied at the top level. Tracked numbers inside a Vector, Tuple or NamedTuple, and every TrackedArray, were copied together with their InstructionTape. Later operations recorded onto that orphan tape, and the gradients silently came back as zero.

Following the deepcopy docstring, this PR specialises deepcopy_internal instead of deepcopy:

• While its tape is recording, a tracked value is returned unchanged. It acts as a number on the tape, matching copy, ForwardDiff (whose Duals are isbits) and Zygote (where the derivative of deepcopy is the identity).
• Once the tape has finished, it is copied together with its tape, so deepcopy(tape) stays independent. Returning the value unchanged in all cases would break this, as noted in the issue.
• Copies keep NULL_TAPE itself, since ReverseDiff recognises it by identity.
• Copies of a CompiledTape are recompiled. Copied FunctionWrappers still point to the original tape's instructions, so before this PR deepcopy(compile(tape)) returned zero gradients as well. This affects e.g. AbstractMCMC's MCMCThreads, which deepcopies models, together with LogDensityProblemsAD's compiled ReverseDiff tapes.

deepcopy_internal has no docstring and is not public, but the deepcopy docstring names it as the method to specialise, so it is added to the ExplicitImports ignore list.

The new tests cover the cases from the issue with gradient and with replays of plain and compiled tapes, plus independence of copied tapes. Without the fix, 17 of the 24 new tests fail.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.32%. Comparing base (f6c9ddd) to head (93c9995).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   94.29%   94.32%   +0.03%     
==========================================
  Files          19       19              
  Lines        1699     1710      +11     
==========================================
+ Hits         1602     1613      +11     
  Misses         97       97              

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