Ambiguity warnings

[andreasnoack]

I have just merged #6265. It provokes some ambiguity warnings that cannot be printed correctly. Cc: @JeffBezanson.

Warning: New definition 
    convert could not show value of type Tuple at linalg/ldlt.jl:9
is ambiguous with: 
    convert could not show value of type Tuple at linalg/ldlt.jl:8.
To fix, define 
    convert could not show value of type Tuple
before the new definition.
Warning: New definition 
    * could not show value of type Tuple at linalg/sparse.jl:128
is ambiguous with: 
    * could not show value of type Tuple at linalg/tridiag.jl:247.
To fix, define 
    * could not show value of type Tuple

[carlobaldassi]

One of the warnings was spurious (a manifestation of bug #6383), the other was legitimate (it needed to specify what to do with Tridiagonal * Sparse).

About the second one, note that I resolved it in favour of the method defined in tridiagonal.jl, since that keeps the sparseness in the result, but a more efficient method could be written for that particular case.

[carlobaldassi]

Oh and BTW a tip on how to debug these things: look at the lines printed in the warnings, and if the ambiguity is not obvious define a function with the same two signatures at the REPL, which will then print what other signature is needed to disambiguate.

[StefanKarpinski]

That's a good tip.

[andreasnoack]

@carlobaldassi Thank you for fixing this and for the tip.