extend to symtridiagonal case

Author: dkarrasch

src/tridiag.jl

@@ -107,13 +107,13 @@ julia> SymTridiagonal(B)
  [1 2; 3 4]  [1 2; 2 3]
 ```
 """
-function SymTridiagonal(A::AbstractMatrix)
+function (::Type{SymTri})(A::AbstractMatrix) where {SymTri <: SymTridiagonal}
     checksquare(A)
     du = diag(A, 1)
     d  = diag(A)
     dl = diag(A, -1)
     if _checksymmetric(d, du, dl)
-        SymTridiagonal(d, du)
+        SymTri(d, du)
     else
         throw(ArgumentError("matrix is not symmetric; cannot convert to SymTridiagonal"))
     end

test/tridiag.jl

@@ -1121,6 +1121,12 @@ end
             diagm(-1 => [1,2], 1=>[1,2])]
             B = convert(SymTridiagonal, M)
             @test B == SymTridiagonal(M)
+            B = convert(SymTridiagonal{Int8}, M)
+            @test B == M
+            @test B isa SymTridiagonal{Int8}
+            B = convert(SymTridiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}, M)
+            @test B == M
+            @test B isa SymTridiagonal{Int8, OffsetVector{Int8, Vector{Int8}}}
         end
         @test_throws InexactError convert(SymTridiagonal, fill(5, 4, 4))
         @test_throws InexactError convert(SymTridiagonal, diagm(0=>fill(NaN,4)))