fix isprimitive bug

[cscherrer]

This PR makes some small changes to isprimitive to work around some errors I was seeing on my local machine. Then again, by my understanding the previous code shouldn't work at all. But it clearly passes the existing tests, so at the very least my understanding is incomplete.

The code in the repo contains the call Core.Compiler.return_type(rrule, (YotaRuleConfig, Ts...,)). This passes a Tuple as the second argument to return_type. I've never seen it work this way - it usually requires a Type{Tuple}. For example, compare

julia> Core.Compiler.return_type(+, (Int, Int))
ERROR: MethodError: no method matching return_type(::typeof(+), ::Tuple{DataType, DataType})
Closest candidates are:
  return_type(::Any, ::DataType) at ~/julia/julia-1.8.0-beta1/share/julia/base/compiler/typeinfer.jl:989
  return_type(::Any, ::DataType, ::UInt64) at ~/julia/julia-1.8.0-beta1/share/julia/base/compiler/typeinfer.jl:995
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1

julia> Core.Compiler.return_type(+, Tuple{Int, Int})
Int64

The repo code also includes the line

Ts = [a isa DataType ? Type{a} : typeof(a) for a in (f, args...)]

This misses the UnionAll case. For example

julia> [a isa DataType ? Type{a} : typeof(a) for a in (Array{Float64},)]
1-element Vector{DataType}:
 UnionAll

I assume it's preferred here to return Type{<:Array{Float64}}. So this PR works instead in terms of a small helper function instance_type, defined as

@inline instance_type(f::F) where {F} = F
@inline instance_type(T::UnionAll) = Type{<:T}
@inline instance_type(T::DataType) = Type{T}

[codecov-commenter]

Codecov Report

Merging #108 (78db740) into main (1d73ef5) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   85.35%   85.47%   +0.12%     
==========================================
  Files           9        9              
  Lines         471      475       +4     
==========================================
+ Hits          402      406       +4     
  Misses         69       69              

Impacted Files	Coverage Δ
src/cr_api.jl	96.34% <100.00%> (+0.18%)	⬆️

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[dfdx]

That's interesting. What version of Julia do you use? For me both versions work on Julia 1.7.2:

julia> Core.Compiler.return_type(+, (Int, Int))
Int64

julia> Core.Compiler.return_type(+, Tuple{Int, Int})
Int64

[cscherrer]

What version of Julia do you use? For me both versions work on Julia 1.7.2

Oh that's interesting, I didn't realize this had changed. I was on Julia 1.8.0-beta1

[dfdx]

Makes sense. So we need it anyways.
Thanks for the PR and thorough explanation of the problem!