1.7-beta2: type instability for matrix multiplication

[j-fu]

Hi, I encountered a type instability in matrix multiplication

julia> A=Rational[1 1 1; 2 2 2; 3 3 3]
3×3 Matrix{Rational}:
 1//1  1//1  1//1
 2//1  2//1  2//1
 3//1  3//1  3//1

On 1.7-beta2:

julia> A*A
3×3 Matrix{Any}:
  6//1   6//1   6//1
 12//1  12//1  12//1
 18//1  18//1  18//1

On 1.6.1:

julia> A*A
3×3 Matrix{Rational}:
  6//1   6//1   6//1
 12//1  12//1  12//1
 18//1  18//1  18//1

[simeonschaub]

Note that in general, return type calculations with non-concrete types are always very fragile, because intermediary calls often have quite a few matching methods, so those calls will infer to Any, which in many cases means the overall result will also infer to Any. The reason this used to work is probably more by accident due to how +(::Rational, Rational) was implemented. It is easy enough to fix in this case though, so I will make a PR.

[j-fu]

Ah ok, so to be on the safe side one should start with a concrete type in the first place.

[simeonschaub]

Yeah, that's always better when you have the choice.