Fix weird return value from index_constants!

[MilesCranmer]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 4927766763

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 88.451%

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Totals
Change from base Build 4916483594:	0.01%
Covered Lines:	965
Relevant Lines:	1091

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💛 - Coveralls

[github-actions]

Benchmark Results

	master	dd81086...	t[master]/t[dd81086...]
eval/ComplexF32/evaluation	11.3 ± 0.81 ms	11.9 ± 0.84 ms	0.947
eval/ComplexF64/evaluation	14.1 ± 1.1 ms	14.7 ± 1 ms	0.958
eval/Float32/derivative	19.4 ± 1.2 ms	19.8 ± 1.1 ms	0.98
eval/Float32/derivative_turbo	20.4 ± 1.2 ms	21.6 ± 1.1 ms	0.945
eval/Float32/evaluation	3.97 ± 0.32 ms	3.85 ± 0.31 ms	1.03
eval/Float32/evaluation_turbo	0.91 ± 0.093 ms	0.937 ± 0.093 ms	0.972
eval/Float64/derivative	27 ± 4.7 ms	27.7 ± 4.7 ms	0.977
eval/Float64/derivative_turbo	29.8 ± 4.4 ms	30.5 ± 4.1 ms	0.975
eval/Float64/evaluation	4.75 ± 0.4 ms	4.83 ± 0.39 ms	0.983
eval/Float64/evaluation_turbo	1.4 ± 0.14 ms	1.46 ± 0.13 ms	0.963
time_to_load	15.4 ± 0.27 s	15.4 ± 0.065 s	1
utils/combine_operators/break_sharing	0.0688 ± 0.0057 ms	0.0735 ± 0.0055 ms	0.936
utils/convert/break_sharing	0.0565 ± 0.0071 ms	0.0572 ± 0.0073 ms	0.988
utils/convert/preserve_sharing	0.279 ± 0.013 ms	0.28 ± 0.025 ms	1
utils/copy/break_sharing	0.0551 ± 0.0067 ms	0.0562 ± 0.0056 ms	0.98
utils/copy/preserve_sharing	0.277 ± 0.023 ms	0.279 ± 0.01 ms	0.993
utils/count_constants/break_sharing	0.0327 ± 0.0023 ms	0.0323 ± 0.0014 ms	1.01
utils/count_depth/break_sharing	0.0323 ± 0.0029 ms	0.0331 ± 0.0011 ms	0.976
utils/count_nodes/break_sharing	30.8 ± 1.4 μs	30.4 ± 2.4 μs	1.01
utils/get_set_constants!/break_sharing	0.125 ± 0.0045 ms	0.125 ± 0.0029 ms	1
utils/has_constants/break_sharing	11.3 ± 1.2 μs	11 ± 0.82 μs	1.03
utils/has_operators/break_sharing	3.31 ± 0.12 μs	2.79 ± 0.09 μs	1.19
utils/index_constants/break_sharing	0.1 ± 0.0054 ms	0.0967 ± 0.0046 ms	1.04
utils/is_constant/break_sharing	9.75 ± 0.87 μs	10.7 ± 0.8 μs	0.911
utils/simplify_tree/break_sharing	0.196 ± 0.024 ms	0.196 ± 0.024 ms	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).