Force specialization on func::F

[Moelf]

by default Julia does not specialize on this, this can be performance-critical in some cases.

[codecov]

Codecov Report

Merging #15 (4be7c4a) into main (40ad3aa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #15   +/-   ##
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  Coverage   91.48%   91.48%           
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  Files           1        1           
  Lines          94       94           
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  Hits           86       86           
  Misses          8        8           

Impacted Files	Coverage Δ
src/MultiThreadedCaches.jl	91.48% <100.00%> (ø)

[NHDaly]

LGTM too, but my understanding is that julia should be specializing on F already, because we call func() inside the body.

My understanding was that the heuristic is only that julia skips specializing on it only of the function isn't called:

Julia will always specialize when the argument is used within the method, but not if the argument is just passed through to another function.

https://docs.julialang.org/en/v1/manual/performance-tips/#Be-aware-of-when-Julia-avoids-specializing

Do you have a specific example where you were seeing issues, and where this helps? It would be great to understand that, and also to add that as a benchmark or as a test.