Long types printed in MethodError

[timholy]

As reported in https://discourse.julialang.org/t/type-parameter-display-in-1-10-for-methoderror/102516

a = view(reinterpret(reshape, UInt8, PermutedDimsArray(rand(5, 7), (2, 1))), 2:3, 2:4, 1:4); # a mildly-complex type
function foo end

julia> foo(a, a, a, a, a, a, a)
ERROR: MethodError: no method matching foo(::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false}, ::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false}, ::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false}, ::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false}, ::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false}, ::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false}, ::SubArray{UInt8, 3, Base.ReinterpretArray{UInt8, 3, Float64, PermutedDimsArray{Float64, 2, (2, 1), (2, 1), Matrix{Float64}}, true}, Tuple{UnitRange{Int64}, UnitRange{Int64}, UnitRange{Int64}}, false})
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

[gdalle]

Thanks for the issue and the MWE 🚀

[timholy]

One big concern: when printing a stacktrace, we can fall back on

Some type information was truncated. Use `show(err)` to see complete types.

for those cases where full type-information is needed to resolve the error. But when the MethodError happens at toplevel (directly in the REPL call itself), there is no err object to query:

julia> foo(a, a, a, a, a, a, a)
ERROR: MethodError: no method matching foo... # manually truncated
Stacktrace:
 [1] top-level scope
   @ REPL[3]:1

julia> err

julia>

[gdalle]

Newbie reac here: can we put the error in ans in that case?

[timholy]

I think we should be able to put it in err, but I'm not immediately sure why we don't have it. Will do a bit of digging.

[timholy]

The answer seems to be

julia/base/client.jl

Lines 105 to 106 in 117ef2e

	istrivialerror(stack::ExceptionStack) =
	length(stack) == 1 && length(stack[1].backtrace) ≤ 1

If I change that to always return false, then we get a useful err object. @BioTurboNick, can you comment on this check? I'm not sure what the consequences will be if we change it.

[BioTurboNick]

The intent there was just to avoid saving a stack trace that was just the top level, partly to avoid overwriting the variable if you make an error trying to access it. So I don't see any harm in deeming all MethodErrors to be not trivial.