The behavior of type parameter constraint inheritance is not documented #53380
Description
If you have an abstract type with a constrained type parameter, subtypes whose parameters inherit from the type's parameters do not inherit the constraints.
abstract type AbstractFoo{T<:Real} end struct Foo{T} <: AbstractFoo{T} endjulia> supertype(Foo) AbstractFoo{T} where T julia> supertype(Foo{<:Real}} AbstractFoo julia> Foo <: AbstractFoo # This one is very much unexpected! false julia> Foo{<:Real} <: AbstractFoo trueThis is true even though you can't construct a type which violates the constraints on its parent's parameters:
julia> Foo{String} ERROR: TypeError: in AbstractFoo, in T, expected T<:Real, got Type{String} Stacktrace: [1] top-level scope @ REPL[5]:1This is a subtle point that is not included in the documentation on abstract parametric types, but has caused me some headaches when some subtype relations broke unexpectedly.
The above examples were tested on Julia 1.10 (version info below) running on Arch Linux in WSL2, installed with the AUR provided julia-bin package that provides the official release:
julia> versioninfo() Julia Version 1.10.0 Commit 3120989f39b (2023-12-25 18:01 UTC) Build Info: Official https://julialang.org/ release Platform Info: OS: Linux (x86_64-linux-gnu) CPU: 20 × 13th Gen Intel(R) Core(TM) i5-13600K WORD_SIZE: 64 LIBM: libopenlibm LLVM: libLLVM-15.0.7 (ORCJIT, goldmont) Threads: 1 on 20 virtual cores