matmul: don't assume the existence of type-conversions

[timholy]

I've been playing around with number types whose product is a real number but for which I cannot define a conversion to Reals. This fixes a bug in our algorithm for matrix multiplication.

julia> immutable RootInt
           i::Int
       end

julia> import Base: *

julia> (*)(x::RootInt, y::RootInt) = x.i*y.i
* (generic function with 143 methods)

julia> a = [RootInt(3)]
1-element Array{RootInt,1}:
 RootInt(3)

julia> C = [0]
1-element Array{Int64,1}:
 0

julia> A_mul_Bt!(C, a, a)
ERROR: MethodError: `convert` has no method matching convert(::Type{Int64}, ::RootInt)
This may have arisen from a call to the constructor Int64(...),
since type constructors fall back to convert methods.
Closest candidates are:
  call{T}(::Type{T}, ::Any)
  convert(::Type{Int64}, ::Int8)
  convert(::Type{Int64}, ::UInt8)
  ...
 in copy_transpose! at abstractarray.jl:383
 in copy_transpose! at linalg/matmul.jl:355
 in generic_matmatmul! at linalg/matmul.jl:465
 in A_mul_Bt! at linalg/matmul.jl:168
 in eval at ./boot.jl:263

[tkelman]

put these in a let or give them issue-number-specific names

nevermind guess it isn't necessary here, the same file is already using these names above

[tkelman]

I wonder how this passed without defining zero(::Type{RootInt}) ?

oh right this also depends on #13803

[timholy]

If you want to backport this, we could add that method (for use by the test).

[tkelman]

without the promote_op changes I don't think this will work?

[timholy]

#14618