Fix negation of SymTridiagonal (fixes #36680) (#36681)

Author: jagot

stdlib/LinearAlgebra/src/tridiag.jl

@@ -204,6 +204,7 @@ end
 
 +(A::SymTridiagonal, B::SymTridiagonal) = SymTridiagonal(A.dv+B.dv, A.ev+B.ev)
 -(A::SymTridiagonal, B::SymTridiagonal) = SymTridiagonal(A.dv-B.dv, A.ev-B.ev)
+-(A::SymTridiagonal) = SymTridiagonal(-A.dv, -A.ev)
 *(A::SymTridiagonal, B::Number) = SymTridiagonal(A.dv*B, A.ev*B)
 *(B::Number, A::SymTridiagonal) = A*B
 /(A::SymTridiagonal, B::Number) = SymTridiagonal(A.dv/B, A.ev/B)

stdlib/LinearAlgebra/test/tridiag.jl

@@ -294,6 +294,11 @@ end
                 @test_throws DimensionMismatch LinearAlgebra.mul!(Cnm,B,Cnn)
             end
         end
+        @testset "Negation" begin
+            mA = -A
+            @test mA isa mat_type
+            @test -mA == A
+        end
         if mat_type == SymTridiagonal
             @testset "Tridiagonal/SymTridiagonal mixing ops" begin
                 B = convert(Tridiagonal{elty}, A)