Strange error in constructor to Base.Fix2

[jlumpe]

I originally encountered this while playing around with the type system in IJulia. Here is a minimum example in that context:

In[1]:   T = Tuple{A} where A

Out[1]:  Tuple{A} where A

In[2]:   print(T.body)

Out[2]:  Tuple{A}

In[3]:  display(T.body)

TypeError: in new, expected UnionAll, got Type{Tuple{A}}

Stacktrace:
 [1] Base.Fix2(::typeof(isequal), ::Type{Tuple{A} where A}) at ./operators.jl:910
 [2] isequal(::Type{T} where T) at ./operators.jl:925
 [3] undisplay at /Users/student/.julia/packages/IJulia/yLI42/src/display.jl:125 [inlined]
 [4] display(::IJulia.InlineDisplay, ::Type{T} where T) at /Users/student/.julia/packages/IJulia/yLI42/src/inline.jl:94
 [5] display(::Any) at ./multimedia.jl:323
 [6] top-level scope at In[3]:1

I can shorten this to:

In[1]:   T = Tuple{A} where A

Out[1]:  Tuple{A} where A

In[2]:   Base.Fix2(isequal, T)

TypeError: in new, expected UnionAll, got Type{Tuple{A}}

Stacktrace:
 [1] Base.Fix2(::typeof(isequal), ::Type{Tuple{A} where A}) at ./operators.jl:910
 [2] top-level scope at In[2]:1

At first I could not recreate this in the REPL, but after a lot of playing around I figured out it is dependent on display(T) and thus Fix2(isequal, T) being called when showing the result of the first cell.

The following script is a minimal reproducible example in versions 1.2 to 1.4:

T = Tuple{A} where A
@show Base.Fix2(isequal, T)
@show Base.Fix2(isequal, T.body)

Base.Fix2(isequal, T) = Base.Fix2{typeof(isequal),UnionAll}(isequal, Tuple{A} where A)
ERROR: LoadError: TypeError: in new, expected UnionAll, got Type{Tuple{A}}
Stacktrace:
 [1] Base.Fix2(::typeof(isequal), ::Type{Tuple{A} where A}) at ./operators.jl:910
 [2] top-level scope at show.jl:613
 [3] include(::Module, ::String) at ./Base.jl:377
 [4] exec_options(::Base.JLOptions) at ./client.jl:288
 [5] _start() at ./client.jl:484
in expression starting at /Users/student/git/jl-julia/ex-minimal.jl:3

This error occurs only in very specific but reproducible circumstances. It does not occur in the following examples:

Fix2 not called on T first:

T = Tuple{A} where A
@show Base.Fix2(isequal, T.body)

Base.Fix2(isequal, T.body) = Base.Fix2{typeof(isequal),DataType}(isequal, Tuple{A})

Called with different functions (functions other than isequal do cause an error if used for both):

T = Tuple{A} where A
@show Base.Fix2(isequal, T)
@show Base.Fix2(+, T.body)

Base.Fix2(isequal, T) = Base.Fix2{typeof(isequal),UnionAll}(isequal, Tuple{A} where A)
Base.Fix2(+, T.body) = Base.Fix2{typeof(+),DataType}(+, Tuple{A})

A second UnionAll identical to the first:

T = Tuple{A} where A
@show Base.Fix2(isequal, T)
try
	@show Base.Fix2(isequal, T.body)
catch e
	@error e
end

T2 = Tuple{A} where A
@assert T2 === T1
@show Base.Fix2(isequal, T2)
try
	@show Base.Fix2(isequal, T2.body)
catch e
	@error e
end

Base.Fix2(isequal, T) = Base.Fix2{typeof(isequal),UnionAll}(isequal, Tuple{A} where A)
┌ Error: TypeError(:new, "", UnionAll, Tuple{A})
└ @ Main ~/git/jl-julia/ex-t2.jl:6
Base.Fix2(isequal, T2) = Base.Fix2{typeof(isequal),UnionAll}(isequal, Tuple{A} where A)
Base.Fix2(isequal, T2.body) = Base.Fix2{typeof(isequal),DataType}(isequal, Tuple{A})

[JeffBezanson]

Similar to #28983; types with free variables misbehave a bit sometimes. We should try to fix this soon.

[JeffBezanson]

Also related: #35130

[jakobnissen]

Seems to be fixed on master?

[vtjnash]

no, confirmed not fixed