ERROR: TypeError: in <:, expected Type, got a value of type TypeVar

$TYPEDSIGNATURES causes error when calling Base.return_type when typesig is UnionAll with generic types.

I think the easy hacky solution to this is to wrap the following in a try except.

https://github.com/JuliaDocs/DocStringExtensions.jl/blob/339cd6ddd9549d52503cf02782733a0cecd481da/src/utilities.jl#L308-L311

A better solution is to pass the right typesig to Base.return_types. The issue here is that local typesig = doc.data[:typesig] doesn't exactly return what we want. And I think we are currently missing some corner case. Specifically, generic UnionAll types are converted to Tuples without the TypeVar var. This causes some issues.

I'm not able to reproduce an example in isolation, since I believe it is tied to return_types, which depends on the code path. But the function signature looks something like this.

"""
$TYPEDSIGNATURES
"""
function f(x::ParametricType{M}) where M <: Union{A, B}
...
return result
end

calls Base.return_types throws an exception in the core compiler.

help?> Module.f
ERROR: TypeError: in <:, expected Type, got a value of type TypeVar
Stacktrace:
 [1] ⊑(::Any, ::Any) at ./compiler/typelattice.jl:177
 [2] tmerge(::Any, ::Any) at ./compiler/typelimits.jl:286
 [3] getfield_tfunc(::Any, ::Any) at ./compiler/tfuncs.jl:827
 [4] builtin_tfunction(::Any, ::Array{Any,1}, ::Union{Nothing, Core.Compiler.InferenceState}, ::Core.Compiler.Params) at ./compiler/tfuncs.jl:1428
 [5] builtin_tfunction at ./compiler/tfuncs.jl:1342 [inlined]
 [6] abstract_call_known(::Any, ::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:720
 [7] abstract_call(::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:926
 [8] abstract_call(::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:911
 [9] abstract_eval(::Any, ::Array{Any,1}, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1005
 [10] typeinf_local(::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1270
 [11] typeinf_nocycle(::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1326
 [12] typeinf(::Core.Compiler.InferenceState) at ./compiler/typeinfer.jl:12
 [13] typeinf_edge(::Method, ::Any, ::Core.SimpleVector, ::Core.Compiler.InferenceState) at ./compiler/typeinfer.jl:484
 [14] abstract_call_method(::Method, ::Any, ::Core.SimpleVector, ::Bool, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:419
 [15] abstract_call_gf_by_type(::Any, ::Array{Any,1}, ::Any, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:111
 [16] abstract_call_known(::Any, ::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:904
 [17] abstract_call(::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:926
 [18] abstract_call(::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:911
 [19] abstract_eval(::Any, ::Array{Any,1}, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1005
 [20] typeinf_local(::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1270
 [21] typeinf_nocycle(::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1326
 [22] typeinf(::Core.Compiler.InferenceState) at ./compiler/typeinfer.jl:12
 [23] typeinf_edge(::Method, ::Any, ::Core.SimpleVector, ::Core.Compiler.InferenceState) at ./compiler/typeinfer.jl:484
 [24] abstract_call_method(::Method, ::Any, ::Core.SimpleVector, ::Bool, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:419
 [25] abstract_call_gf_by_type(::Any, ::Array{Any,1}, ::Any, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:111
 [26] abstract_call_known(::Any, ::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:904
 [27] abstract_call(::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState, ::Int64) at ./compiler/abstractinterpretation.jl:926
 [28] abstract_call(::Array{Any,1}, ::Array{Any,1}, ::Array{Any,1}, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:911
 [29] abstract_eval(::Any, ::Array{Any,1}, ::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1005
 [30] typeinf_local(::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1270
 [31] typeinf_nocycle(::Core.Compiler.InferenceState) at ./compiler/abstractinterpretation.jl:1326
 [32] typeinf(::Core.Compiler.InferenceState) at ./compiler/typeinfer.jl:12
 [33] typeinf at ./compiler/typeinfer.jl:8 [inlined]
 [34] typeinf_type(::Method, ::Any, ::Core.SimpleVector, ::Core.Compiler.Params) at ./compiler/typeinfer.jl:592
 [35] return_types(::Any, ::Any) at ./reflection.jl:1133
 [36] printmethod(::Base.GenericIOBuffer{Array{UInt8,1}}, ::Base.Docs.Binding, ::Function, ::Method, ::Type{T} where T) at /Users/USER/.julia/packages/DocStringExtensions/c3W3t/src/utilities.jl:308
 [37] format(::DocStringExtensions.TypedMethodSignatures, ::Base.GenericIOBuffer{Array{UInt8,1}}, ::Base.Docs.DocStr) at /Users/USER/.julia/packages/DocStringExtensions/c3W3t/src/abbreviations.jl:391
 [38] formatdoc(::Base.GenericIOBuffer{Array{UInt8,1}}, ::Base.Docs.DocStr, ::DocStringExtensions.TypedMethodSignatures) at /Users/USER/.julia/packages/DocStringExtensions/c3W3t/src/abbreviations.jl:25
 [39] format(::DocStringExtensions.Template{:before}, ::Base.GenericIOBuffer{Array{UInt8,1}}, ::Base.Docs.DocStr) at /Users/USER/.julia/packages/DocStringExtensions/c3W3t/src/abbreviations.jl:639
 [40] formatdoc(::Base.GenericIOBuffer{Array{UInt8,1}}, ::Base.Docs.DocStr, ::DocStringExtensions.Template{:before}) at /Users/USER/.julia/packages/DocStringExtensions/c3W3t/src/abbreviations.jl:25
 [41] formatdoc(::Base.Docs.DocStr) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/REPL/src/docview.jl:83
 [42] parsedoc(::Base.Docs.DocStr) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/REPL/src/docview.jl:91
 [43] iterate at ./generator.jl:47 [inlined]
 [44] _collect(::Array{Base.Docs.DocStr,1}, ::Base.Generator{Array{Base.Docs.DocStr,1},typeof(Base.Docs.parsedoc)}, ::Base.EltypeUnknown, ::Base.HasShape{1}) at ./array.jl:699
 [45] collect_similar at ./array.jl:628 [inlined]
 [46] map at ./abstractarray.jl:2162 [inlined]
 [47] doc(::Base.Docs.Binding, ::Type{T} where T) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/REPL/src/docview.jl:185
 [48] doc(::Base.Docs.Binding) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/REPL/src/docview.jl:153
 [49] top-level scope at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.5/REPL/src/docview.jl:438

I'll take a closer look at this when I get the chance.

Related: https://github.com/JuliaDocs/DocStringExtensions.jl/pull/88

Start in src/utilities.jl around lines 308-311, where $TYPEDSIGNATURES calls Base.return_types, and review the related pull request 88. Investigate the reported generic UnionAll signature involving ParametricType and TypeVar; done means documenting or testing a change that prevents the TypeError without breaking typed signatures.