fix: prevent toexpr from rewriting Const values via st.rewrites#870
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AayushSabharwal merged 1 commit intoJuliaSymbolics:masterfrom Feb 28, 2026
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In v4.18.4, the refactored `toexpr(O::BasicSymbolic{T}, st)` function
started checking `st.rewrites` for all BasicSymbolic types including
Const values. The old code had a guard `(issym(O) || iscall(O))` in
`substitute_name` that prevented constants from being rewritten.
This caused a bug where hash-consed constant values (e.g., Const(0))
that appeared both in an expression body AND in function argument
arrays (via DestructuredArgs with create_bindings=false) would be
incorrectly aliased. The constant 0 in the expression would be
replaced with a reference to the argument position (e.g., arg2[12]),
producing wrong results at runtime when that argument had non-zero
values.
This manifested as incorrect jacobian computations in
DifferentiationInterface.jl when using Symbolics.jl's build_function
with cache contexts (where cache variables are erased to zero before
building the function).
Fixes JuliaSymbolics/Symbolics.jl#1811
Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
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AayushSabharwal
approved these changes
Feb 28, 2026
AayushSabharwal
merged commit c370e77
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JuliaSymbolics:master
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Summary
toexpr(O::BasicSymbolic{T}, st)incorrectly rewritesConstvalues throughst.rewrites, causing hash-consed constants to be aliased with function argument references(issym(O) || iscall(O))insubstitute_namethat prevented constants from being rewritten; the v4.18.4 refactor dropped this guardRoot Cause
In v4.18.4, PR #862 refactored
toexprfor performance. The newtoexpr(O::BasicSymbolic{T}, st)checkshaskey(st.rewrites, O)for allBasicSymbolictypes, includingConst. The old code routed throughsubstitute_namewhich had a guard:When
build_functionis called with argument arrays containing constant values (e.g., zeros from erased cache variables in DifferentiationInterface.jl),handle_let_pair!adds these constants tost.rewrites:Due to hash consing, all
Const(0)values are the same object. So whentoexprencounters a literal0in the expression body (e.g., off-diagonal entries of a sparse Jacobian), it finds the rewrite and replaces it withˍ₋arg2[12]. At runtime,arg2contains actual (non-zero) data, producing incorrect results.Impact
This broke DifferentiationInterface.jl's Symbolics backend when computing Jacobians with cache contexts:
Test plan
test/code.jlthat reproduces the exact scenario🤖 Generated with Claude Code