fill[stored]! for Symmetric and Hermitian matrices (#25152)

base/linalg/symmetric.jl

@@ -201,6 +201,21 @@ function copyto!(dest::Hermitian, src::Hermitian)
     return dest
 end
 
+# fill[stored]!
+fill!(A::HermOrSym, x) = fillstored!(A, x)
+function fillstored!(A::HermOrSym{T}, x) where T
+    xT = convert(T, x)
+    if isa(A, Hermitian)
+        isreal(xT) || throw(ArgumentError("cannot fill Hermitian matrix with a nonreal value"))
+    end
+    if A.uplo == 'U'
+        fillband!(A.data, xT, 0, size(A,2)-1)
+    else # A.uplo == 'L'
+        fillband!(A.data, xT, 1-size(A,1), 0)
+    end
+    return A
+end
+
 function Base.isreal(A::HermOrSym)
     n = size(A, 1)
     @inbounds if A.uplo == 'U'

test/linalg/symmetric.jl

@@ -469,3 +469,18 @@ end
     @test_throws TypeError Symmetric{Float64,Matrix{Float32}}(A, 'U')
     @test_throws TypeError Hermitian{Float64,Matrix{Float32}}(A, 'U')
 end
+
+@testset "fill[stored]!" begin
+    for uplo in (:U, :L)
+        # Hermitian
+        A = Hermitian(fill(1.0+0im, 2, 2), uplo)
+        @test fill!(A, 2) == fill(2, 2, 2)
+        @test A.data == (uplo == :U ? [2 2; 1.0+0im 2] : [2 1.0+0im; 2 2])
+        @test_throws ArgumentError fill!(A, 2+im)
+
+        # Symmetric
+        A = Symmetric(fill(1.0+im, 2, 2), uplo)
+        @test fill!(A, 2) == fill(2, 2, 2)
+        @test A.data == (uplo == :U ? [2 2; 1.0+im 2] : [2 1.0+im; 2 2])
+    end
+end