Error message does not indicate conflicting type parameters

[tyleringebrand]

Originally found here, the issue is a error message for incorrect code is very misleading. Here is a minimum working example:

struct X{T, A<:AbstractArray{T}}
    t::T
    a::A
end

Vector{X{Int, Vector{Float64}}}(undef, 2)

The issue is that Float64 does not match Int, which causes an error. However, the error message is:

ERROR: LoadError: TypeError: in X, in A, expected A<:(AbstractArray{T, N} where N), got Type{Vector{Float64}}
Stacktrace:
 [1] top-level scope
   @ c:\Users\Tyler\Documents\Julia\Learning\test.jl:6
in expression starting at c:\Users\Tyler\Documents\Julia\Learning\test.jl:6

The reason why this is confusing is because it says the issue is with A being of type AbstractArray{T, N} and that the actual type is Vector{Float64}. Vector is of type AbstractArray, and Float64 is a type. Therefore, I could not figure out why this is wrong because it does seem like the type of A given, Vector{Float64}, is a valid type of A, AbstractArray{T}.
It would be nice if the error message specified that the given Types were wrong because Float64 does not match Int and therefore T is being redefined.

Note in this example it may seem obvious, but this can become quite confusing for a more complicated struct.

If version is relevant:

Julia Version 1.6.2
Commit 1b93d53fc4 (2021-07-14 15:36 UTC)      
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i5-4590 CPU @ 3.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, haswell)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 

[zsunberg]

@tyleringebrand you may want to make the title more informative, e.g. "Error message does not indicate conflicting type parameters" (I would change it myself but I don't have sufficient permission)

[zsunberg]

@tyleringebrand , it would also be nice to test this with version 1.7 to make sure it hasn't been fixed already.

[zsunberg]

To perhaps make it more clear why this is confusing, consider:

struct X{T<:Real, A<:AbstractArray{T}}
    t::T
    a::A
end

Vector{X{Int, Vector{Float64}}}(undef, 2)

which throws

ERROR: TypeError: in X, in A, expected A<:(AbstractArray{T<:Real, N} where N), got Type{Vector{Float64}}

Vector{Float64} is a subtype of AbstractArray{<:Real, N} where N, but the code will not run because T actually has more constraints than just <:Real. It actually has to be Int to run.

[vtjnash]

julia/src/jltypes.c

Lines 1223 to 1226 in 7544b18

	if (!within_typevar(params[i], bounds[2*i], bounds[2*i+1])) {
	// TODO: pass a new version of `tv` containing the instantiated bounds
	jl_type_error_rt(jl_symbol_name(tn->name), jl_symbol_name(tv->name), (jl_value_t*)tv, params[i]);
	}