Fix multiplying Symmetric and Hermitian matrices (#261)

src/dispatch.jl

@@ -474,26 +474,27 @@ Base.:*(A::AbstractArray, α::AbstractMutable) = A .* α
 
 # Needed for Julia v1.0, otherwise, `broadcast(*, α, A)` gives a `Array` and
 # not a `Symmetric`.
+
+_mult_upper(α, A) = parent(α * LinearAlgebra.UpperTriangular(parent(A)))
+_mult_lower(α, A) = parent(α * LinearAlgebra.LowerTriangular(parent(A)))
+
 function Base.:*(α::Number, A::LinearAlgebra.Symmetric{<:AbstractMutable})
-    return LinearAlgebra.Symmetric(
-        α * parent(A),
-        LinearAlgebra.sym_uplo(A.uplo),
-    )
+    c = LinearAlgebra.sym_uplo(A.uplo)
+    B = c == :U ? _mult_upper(α, A) : _mult_lower(α, A)
+    return LinearAlgebra.Symmetric(B, c)
 end
 
 function Base.:*(α::Number, A::LinearAlgebra.Hermitian{<:AbstractMutable})
-    return LinearAlgebra.Hermitian(
-        α * parent(A),
-        LinearAlgebra.sym_uplo(A.uplo),
-    )
+    c = LinearAlgebra.sym_uplo(A.uplo)
+    B = c == :U ? _mult_upper(α, A) : _mult_lower(α, A)
+    return LinearAlgebra.Hermitian(B, c)
 end
 
 # Fix ambiguity identified by Aqua.jl.
 function Base.:*(α::Real, A::LinearAlgebra.Hermitian{<:AbstractMutable})
-    return LinearAlgebra.Hermitian(
-        α * parent(A),
-        LinearAlgebra.sym_uplo(A.uplo),
-    )
+    c = LinearAlgebra.sym_uplo(A.uplo)
+    B = c == :U ? _mult_upper(α, A) : _mult_lower(α, A)
+    return LinearAlgebra.Hermitian(B, c)
 end
 
 # These three have specific methods that just redirect to `Matrix{T}` which

test/dispatch.jl

@@ -43,3 +43,15 @@ end
         @test MA.operate(LinearAlgebra.dot, z, z) == LinearAlgebra.dot(z, z)
     end
 end
+
+@testset "*(::Real, ::Union{Hermitian,Symmetric})" begin
+    A = DummyBigInt[1 2; 2 3]
+    B = DummyBigInt[2 4; 4 6]
+    @test MA.isequal_canonical(2 * A, B)
+    C = LinearAlgebra.Symmetric(B)
+    @test MA.isequal_canonical(2 * LinearAlgebra.Symmetric(A, :U), C)
+    @test MA.isequal_canonical(2 * LinearAlgebra.Symmetric(A, :L), C)
+    D = LinearAlgebra.Hermitian(B)
+    @test all(MA.isequal_canonical.(2 * LinearAlgebra.Hermitian(A, :L), D))
+    @test all(MA.isequal_canonical.(2 * LinearAlgebra.Hermitian(A, :U), D))
+end