fix type promotion

Author: dkarrasch

src/matmul.jl

@@ -1067,7 +1067,10 @@ function __generic_matvecmul!(::typeof(identity), C::AbstractVector, A::Abstract
             if length(B) == 0
                 C[i] = zero(eltype(C))
             else
-                @stable_muladdmul _modify!(MulAddMul(alpha, beta), A[i]*B[1], C, i)
+                AiB1 = A[i]*B[1]
+                # the sum is necessary for potential type promotion
+                AiB1C = AiB1 + zero(AiB1)
+                @stable_muladdmul _modify!(MulAddMul(alpha, beta), AiB1C, C, i)
             end
         end
         if !iszero(alpha)