wrong method called in @testset

[martinholters]

I think I found a small typo in the test of #21666, but it's curious that it hasn't lead to problems:

julia> using Base.Test

julia> struct F21666{T <: Base.TypeArithmetic}
           x::Float32
       end

julia> @testset "Reduced exactness of cumsum # 21666" begin
           Base.TypeArithmetic(::Type{F21666{T}}) where {T} = T # this should be T() I think
           Base.:+(x::F, y::F) where {F <: F21666} = F(x.x + y.x)
           Base.convert(::Type{Float64}, x::F21666) = Float64(x.x)
           v = zeros(300)
           cumsum(F21666{Base.ArithmeticRounds}.(v))
       end
Test Summary:                       |
Reduced exactness of cumsum # 21666 | No tests
Base.Test.DefaultTestSet("Reduced exactness of cumsum # 21666", Any[], 0, false)

julia> @testset "Reduced exactness of cumsum # 21666" begin # try again
           Base.TypeArithmetic(::Type{F21666{T}}) where {T} = T
           Base.:+(x::F, y::F) where {F <: F21666} = F(x.x + y.x)
           Base.convert(::Type{Float64}, x::F21666) = Float64(x.x)
           v = zeros(300)
           cumsum(F21666{Base.ArithmeticRounds}.(v))
       end
Reduced exactness of cumsum # 21666: Error During Test
  Got an exception of type MethodError outside of a @test
  MethodError: no method matching _cumsum!(::Array{F21666{Base.ArithmeticRounds},1}, ::Array{F21666{Base.ArithmeticRounds},1}, ::Int64, ::Type{Base.ArithmeticRounds})
  Closest candidates are:
    _cumsum!(::Any, ::Any, ::Any, ::Base.ArithmeticRounds) at multidimensional.jl:582
    _cumsum!(::Any, ::Any, ::Any, ::Base.ArithmeticUnknown) at multidimensional.jl:585
    _cumsum!(::Any, ::Any, ::Any, ::Base.TypeArithmetic) at multidimensional.jl:588
  Stacktrace:
   [1] macro expansion at ./REPL[10]:6 [inlined]
   [2] macro expansion at ./test.jl:856 [inlined]
   [3] anonymous at ./<missing>:?
   [4] eval(::Module, ::Any) at ./boot.jl:235
   [5] eval_user_input(::Any, ::Base.REPL.REPLBackend) at ./REPL.jl:66
   [6] macro expansion at ./REPL.jl:97 [inlined]
   [7] (::Base.REPL.##1#2{Base.REPL.REPLBackend})() at ./event.jl:73
Test Summary:                       | Error  Total
Reduced exactness of cumsum # 21666 |     1      1
ERROR: Some tests did not pass: 0 passed, 0 failed, 1 errored, 0 broken.

The first time the @testset runs, the new method for Base.TypeArithmetic is ignored and the Base.ArithmeticUnknown() fallback is used. The second time the @testset runs, the new, faulty method for Base.TypeArithmetic is used leading to an error. It looks like the whole @testset runs in the same world - but that is not generally the case, is it?

BTW, I'd open a PR fixing the test, but I'm not sure whether the method definitions should be moved outside the @testset. They have global effect anyway, so it wouldn't hurt, but OTOH it's nice to have them in there just for grouping's sake.