Fix 3-arg dot for empty arrays

Author: dkarrasch

src/generic.jl

@@ -1034,6 +1034,8 @@ dot(x, A, y) = dot(x, A*y) # generic fallback for cases that are not covered by
 
 function dot(x::AbstractVector, A::AbstractMatrix, y::AbstractVector)
     (axes(x)..., axes(y)...) == axes(A) || throw(DimensionMismatch())
+    # outermost zero call to avoid spurious sign ambiguity (like 0.0 - 0.0im)
+    any(isempty, (x, y)) && return zero(dot(zero(eltype(x)), zero(eltype(A)), zero(eltype(y))))
     T = typeof(dot(first(x), first(A), first(y)))
     s = zero(T)
     i₁ = first(eachindex(x))

test/generic.jl

@@ -784,6 +784,10 @@ end
         @test dot(x, B', y) ≈ dot(B*x, y)
         elty <: Real && @test dot(x, transpose(B), y) ≈ dot(x, transpose(B)*y)
     end
+    for (m, n) in ((0, 0), (1, 0), (0, 1))
+        v = zeros(ComplexF64, m); a = zeros(ComplexF64, m, n); w = zeros(Float64, n)
+        @test dot(v, a, w) === zero(ComplexF64)
+    end
 end
 
 @testset "condskeel #34512" begin