type assertion in convert

[jishnub]

Since something similar is implemented on Julia nightly now, it seems to be the preferred way forward. After this, the following is inferred:

julia> @inferred (x -> convert(OffsetArray{Float64,2,Matrix{Float64}}, x[1]))(Any[zeros(1,1)])
1×1 OffsetArray(::Matrix{Float64}, 1:1, 1:1) with eltype Float64 with indices 1:1×1:1:
 0.0

This may help with inference downstream. Mildly breaking if T(x) doesn't return an object of type T, but then the conversion fails anyway, so this may be considered a bug. It's better to error if the conversion fails.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.45%. Comparing base (9c3b73f) to head (88073a4).
⚠️ Report is 46 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #312   +/-   ##
=======================================
  Coverage   96.45%   96.45%           
=======================================
  Files           5        5           
  Lines         451      451           
=======================================
  Hits          435      435           
  Misses         16       16           

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