BUGFIX: genmatmul! for empty input matrices (#33874)

* correct genmatmul! for empty input matrices

* tests for getmatmul!

Author: KlausC

stdlib/LinearAlgebra/src/generic.jl

@@ -75,6 +75,9 @@ julia> C
 end
 
 @inline function _rmul_or_fill!(C::AbstractArray, beta::Number)
+    if isempty(C)
+        return C
+    end
     if iszero(beta)
         fill!(C, zero(eltype(C)))
     else

stdlib/LinearAlgebra/src/matmul.jl

@@ -738,10 +738,8 @@ function _generic_matmatmul!(C::AbstractVecOrMat{R}, tA, tB, A::AbstractVecOrMat
     if size(C,1) != mA || size(C,2) != nB
         throw(DimensionMismatch("result C has dimensions $(size(C)), needs ($mA,$nB)"))
     end
-    if isempty(A) || isempty(B)
-        return C
-    end
-    if iszero(_add.alpha)
+
+    if iszero(_add.alpha) || isempty(A) || isempty(B)
         return _rmul_or_fill!(C, _add.beta)
     end
 

stdlib/LinearAlgebra/test/matmul.jl

@@ -618,4 +618,10 @@ end
     @test_throws MethodError zero(eltype(C))
 end
 
+@testset "Issue #33873: genmatmul! with empty operands" begin
+    @test Matrix{Any}(undef, 0, 2) * Matrix{Any}(undef, 2, 3) == Matrix{Any}(undef, 0, 3)
+    @test_throws MethodError Matrix{Any}(undef, 2, 0) * Matrix{Any}(undef, 0, 3)
+    @test Matrix{Int}(undef, 2, 0) * Matrix{Int}(undef, 0, 3) == zeros(Int, 2, 3)
+end
+
 end # module TestMatmul