Don't widen the primal when differentiating x^y with a real base - #824
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devmotion merged 2 commits intoJul 27, 2026
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`2^x` is `Float32` for a `Float32` `x`, but `2^Dual{T,Float32,N}` came back
`Dual{T,Float64,N}`: `expv = x^v` honours Base's promotion, while `log(x)` was
evaluated from `x` alone, so an integer base gave a Float64 partial and the
`Dual` constructor unified value and partials upward. Differentiating should
not change the primal's type.
Take the log in `expv`'s type instead, which is the promotion `^` already chose
for this base/exponent pair. Float64 results are bit-identical; only bases
narrower than the exponent's own promotion (e.g. `Integer`) change, and only in
type. `NaNMath.pow` shares the definition and is fixed too.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017an8Gie5b1ULeSjhep3Sjb
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #824 +/- ##
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Coverage 90.74% 90.74%
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Files 11 11
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2^xisFloat32for aFloat32x, but2^Dual{T,Float32,N}came backDual{T,Float64,N}. Came up on JuliaMolSim/DftFunctionals.jl#33