fix 5-arg mul! for vectors of vectors (#47665)

Co-authored-by: N5N3 <2642243996@qq.com>
(cherry picked from commit 902e8a7)

stdlib/LinearAlgebra/src/matmul.jl

@@ -807,7 +807,7 @@ function generic_matvecmul!(C::AbstractVector{R}, tA, A::AbstractVecOrMat, B::Ab
         end
         for k = 1:mB
             aoffs = (k-1)*Astride
-            b = _add(B[k], false)
+            b = _add(B[k])
             for i = 1:mA
                 C[i] += A[aoffs + i] * b
             end

stdlib/LinearAlgebra/test/matmul.jl

@@ -156,6 +156,58 @@ end
     end
 end
 
+@testset "generic_matvecmul for vectors of vectors" begin
+    @testset "matrix of scalars" begin
+        u = [[1, 2], [3, 4]]
+        A = [1 2; 3 4]
+        v = [[0, 0], [0, 0]]
+        Au = [[7, 10], [15, 22]]
+        @test A * u == Au
+        mul!(v, A, u)
+        @test v == Au
+        mul!(v, A, u, 2, -1)
+        @test v == Au
+    end
+
+    @testset "matrix of matrices" begin
+        u = [[1, 2], [3, 4]]
+        A = Matrix{Matrix{Int}}(undef, 2, 2)
+        A[1, 1] = [1 2; 3 4]
+        A[1, 2] = [5 6; 7 8]
+        A[2, 1] = [9 10; 11 12]
+        A[2, 2] = [13 14; 15 16]
+        v = [[0, 0], [0, 0]]
+        Au = [[44, 64], [124, 144]]
+        @test A * u == Au
+        mul!(v, A, u)
+        @test v == Au
+        mul!(v, A, u, 2, -1)
+        @test v == Au
+    end
+end
+
+@testset "generic_matmatmul for matrices of vectors" begin
+    B = Matrix{Vector{Int}}(undef, 2, 2)
+    B[1, 1] = [1, 2]
+    B[2, 1] = [3, 4]
+    B[1, 2] = [5, 6]
+    B[2, 2] = [7, 8]
+    A = [1 2; 3 4]
+    C = Matrix{Vector{Int}}(undef, 2, 2)
+    AB = Matrix{Vector{Int}}(undef, 2, 2)
+    AB[1, 1] = [7, 10]
+    AB[2, 1] = [15, 22]
+    AB[1, 2] = [19, 22]
+    AB[2, 2] = [43, 50]
+    @test A * B == AB
+    mul!(C, A, B)
+    @test C == AB
+    mul!(C, A, B, 2, -1)
+    @test C == AB
+    LinearAlgebra._generic_matmatmul!(C, 'N', 'N', A, B, LinearAlgebra.MulAddMul(2, -1))
+    @test C == AB
+end
+
 @testset "fallbacks & such for BlasFloats" begin
     AA = rand(Float64, 6, 6)
     BB = rand(Float64, 6, 6)