return temporarily _init*-stuff

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -533,6 +533,26 @@ matprod_dest(A::Diagonal, B::Diagonal, TS) = similar(B, TS)
 matprod_dest(A::HermOrSym, B::Diagonal, TS) = similar(A, TS, size(A))
 matprod_dest(A::Diagonal, B::HermOrSym, TS) = similar(B, TS, size(B))
 
+# TODO: remove once not used anymore in SparseArrays.jl
+# some trait like this would be cool
+# onedefined(::Type{T}) where {T} = hasmethod(one, (T,))
+# but we are actually asking for oneunit(T), that is, however, defined for generic T as
+# `T(one(T))`, so the question is equivalent for whether one(T) is defined
+onedefined(::Type) = false
+onedefined(::Type{<:Number}) = true
+
+# initialize return array for op(A, B)
+_init_eltype(::typeof(*), ::Type{TA}, ::Type{TB}) where {TA,TB} =
+ (onedefined(TA) && onedefined(TB)) ?
+ typeof(matprod(oneunit(TA), oneunit(TB))) :
+ promote_op(matprod, TA, TB)
+_init_eltype(op, ::Type{TA}, ::Type{TB}) where {TA,TB} =
+ (onedefined(TA) && onedefined(TB)) ?
+ typeof(op(oneunit(TA), oneunit(TB))) :
+ promote_op(op, TA, TB)
+_initarray(op, ::Type{TA}, ::Type{TB}, C) where {TA,TB} =
+ similar(C, _init_eltype(op, TA, TB), size(C))
+
 # General fallback definition for handling under- and overdetermined system as well as square problems
 # While this definition is pretty general, it does e.g. promote to common element type of lhs and rhs
 # which is required by LAPACK but not SuiteSparse which allows real-complex solves in some cases. Hence,