[BUG] mx.expm1 is wrong for complex inputs (discards imaginary part)

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
cpp
Domain
backend

Research direction

Start with the CPU backend implementation of mx.expm1 and compare its handling with the special cases already used by mx.log1p and mx.log2. Confirm that complex inputs preserve their imaginary part and that mx.expm1(z) matches mx.exp(z) - 1 for the reported example.

Written by the indexing model from the issue text.

Description

bug
Describe the bug

mx.expm1 returns wrong results for complex inputs: it operates on the real part and discards the imaginary part. The CPU backend uses std::expm1, which has no complex overload.

import mlx.core as mx
z = mx.array([0.5 + 0.7j])
print(mx.expm1(z))   # [0.6487+0j]  == expm1(0.5), wrong
print(mx.exp(z) - 1) # [0.261+1.062j]  correct

mx.log1p and mx.log2 already special-case complex; expm1 was missed.

Expected behavior

mx.expm1(z) == mx.exp(z) - 1 for complex z.

Desktop
  • OS: macOS
  • Version: main (0.32.0.dev)
Dominant language
C++
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