Negating SymTridiagonal yields dense matrix

[jagot]

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julia> using LinearAlgebra
julia> -Tridiagonal(1:10, 1:11, 1:10)
11×11 Tridiagonal{Int64,Vector{Int64}}:
 -1  -1   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅    ⋅    ⋅
 -1  -2  -2   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅    ⋅    ⋅
  ⋅  -2  -3  -3   ⋅   ⋅   ⋅   ⋅   ⋅    ⋅    ⋅
  ⋅   ⋅  -3  -4  -4   ⋅   ⋅   ⋅   ⋅    ⋅    ⋅
  ⋅   ⋅   ⋅  -4  -5  -5   ⋅   ⋅   ⋅    ⋅    ⋅
  ⋅   ⋅   ⋅   ⋅  -5  -6  -6   ⋅   ⋅    ⋅    
  ⋅   ⋅   ⋅   ⋅   ⋅  -6  -7  -7   ⋅    ⋅    ⋅
  ⋅   ⋅   ⋅   ⋅   ⋅   ⋅  -7  -8  -8    ⋅    ⋅
  ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅  -8  -9   -9    ⋅
  ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅  -9  -10  -10
  ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅   ⋅  -10  -11
julia> -SymTridiagonal(1:11, 1:10)
11×11 Matrix{Int64}:
 -1  -1   0   0   0   0   0   0   0    0    0
 -1  -2  -2   0   0   0   0   0   0    0    0
  0  -2  -3  -3   0   0   0   0   0    0    0
  0   0  -3  -4  -4   0   0   0   0    0    0
  0   0   0  -4  -5  -5   0   0   0    0    0
  0   0   0   0  -5  -6  -6   0   0    0    0
  0   0   0   0   0  -6  -7  -7   0    0    0
  0   0   0   0   0   0  -7  -8  -8    0    0
  0   0   0   0   0   0   0  -8  -9   -9    0
  0   0   0   0   0   0   0   0  -9  -10  -10
  0   0   0   0   0   0   0   0   0  -10  -11

[StefanKarpinski]

Seems like a simple missing method. Would be a good first issue!