diff --git a/base/deprecated.jl b/base/deprecated.jl
index 0bac394c53cef8..9883f16c7c083f 100644
--- a/base/deprecated.jl
+++ b/base/deprecated.jl
@@ -778,6 +778,20 @@ end
 @deprecate cholfact!(A::Base.LinAlg.HermOrSym, uplo::Symbol, ::Type{Val{false}}) cholfact!(A, Val{false})
 @deprecate cholfact!(A::Base.LinAlg.HermOrSym, uplo::Symbol = :U) cholfact!(A)
 
+# #18218
+eval(Base.LinAlg, quote
+  function arithtype(T)
+      depwarn("arithtype is deprecated as it is no longer needed in Julia Base. It will not be available in future release.",
+          :arithtype)
+      T
+  end
+  function arithtype(::Type{Bool})
+      depwarn("arithtype is deprecated as it is no longer needed in Julia Base. It will not be available in future release.",
+          :arithtype)
+      Int
+  end
+end
+
 # During the 0.5 development cycle, do not add any deprecations below this line
 # To be deprecated in 0.6
 
diff --git a/base/linalg/matmul.jl b/base/linalg/matmul.jl
index 2f44c5c2c7585f..562ed140a13215 100644
--- a/base/linalg/matmul.jl
+++ b/base/linalg/matmul.jl
@@ -2,9 +2,6 @@
 
 # matmul.jl: Everything to do with dense matrix multiplication
 
-arithtype(T) = T
-arithtype(::Type{Bool}) = Int
-
 matprod(x, y) = x*y + x*y
 
 # multiply by diagonal matrix as vector
@@ -78,11 +75,11 @@ At_mul_B{T<:BlasComplex}(x::StridedVector{T}, y::StridedVector{T}) = [BLAS.dotu(
 
 # Matrix-vector multiplication
 function (*){T<:BlasFloat,S}(A::StridedMatrix{T}, x::StridedVector{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     A_mul_B!(similar(x, TS, size(A,1)), A, convert(AbstractVector{TS}, x))
 end
 function (*){T,S}(A::AbstractMatrix{T}, x::AbstractVector{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     A_mul_B!(similar(x,TS,size(A,1)),A,x)
 end
 (*)(A::AbstractVector, B::AbstractMatrix) = reshape(A,length(A),1)*B
@@ -101,22 +98,22 @@ end
 A_mul_B!(y::AbstractVector, A::AbstractVecOrMat, x::AbstractVector) = generic_matvecmul!(y, 'N', A, x)
 
 function At_mul_B{T<:BlasFloat,S}(A::StridedMatrix{T}, x::StridedVector{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     At_mul_B!(similar(x,TS,size(A,2)), A, convert(AbstractVector{TS}, x))
 end
 function At_mul_B{T,S}(A::AbstractMatrix{T}, x::AbstractVector{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     At_mul_B!(similar(x,TS,size(A,2)), A, x)
 end
 At_mul_B!{T<:BlasFloat}(y::StridedVector{T}, A::StridedVecOrMat{T}, x::StridedVector{T}) = gemv!(y, 'T', A, x)
 At_mul_B!(y::AbstractVector, A::AbstractVecOrMat, x::AbstractVector) = generic_matvecmul!(y, 'T', A, x)
 
 function Ac_mul_B{T<:BlasFloat,S}(A::StridedMatrix{T}, x::StridedVector{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     Ac_mul_B!(similar(x,TS,size(A,2)),A,convert(AbstractVector{TS},x))
 end
 function Ac_mul_B{T,S}(A::AbstractMatrix{T}, x::AbstractVector{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     Ac_mul_B!(similar(x,TS,size(A,2)), A, x)
 end
 
@@ -127,7 +124,7 @@ Ac_mul_B!(y::AbstractVector, A::AbstractVecOrMat, x::AbstractVector) = generic_m
 # Matrix-matrix multiplication
 
 function (*){T,S}(A::AbstractMatrix{T}, B::AbstractMatrix{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     A_mul_B!(similar(B, TS, (size(A,1), size(B,2))), A, B)
 end
 A_mul_B!{T<:BlasFloat}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = gemm_wrapper!(C, 'N', 'N', A, B)
@@ -144,14 +141,14 @@ end
 A_mul_B!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic_matmatmul!(C, 'N', 'N', A, B)
 
 function At_mul_B{T,S}(A::AbstractMatrix{T}, B::AbstractMatrix{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     At_mul_B!(similar(B, TS, (size(A,2), size(B,2))), A, B)
 end
 At_mul_B!{T<:BlasFloat}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = is(A,B) ? syrk_wrapper!(C, 'T', A) : gemm_wrapper!(C, 'T', 'N', A, B)
 At_mul_B!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic_matmatmul!(C, 'T', 'N', A, B)
 
 function A_mul_Bt{T,S}(A::AbstractMatrix{T}, B::AbstractMatrix{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     A_mul_Bt!(similar(B, TS, (size(A,1), size(B,1))), A, B)
 end
 A_mul_Bt!{T<:BlasFloat}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = is(A,B) ? syrk_wrapper!(C, 'N', A) : gemm_wrapper!(C, 'N', 'T', A, B)
@@ -168,7 +165,7 @@ end
 A_mul_Bt!(C::AbstractVecOrMat, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic_matmatmul!(C, 'N', 'T', A, B)
 
 function At_mul_Bt{T,S}(A::AbstractMatrix{T}, B::AbstractVecOrMat{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     At_mul_Bt!(similar(B, TS, (size(A,2), size(B,1))), A, B)
 end
 At_mul_Bt!{T<:BlasFloat}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = gemm_wrapper!(C, 'T', 'T', A, B)
@@ -177,7 +174,7 @@ At_mul_Bt!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generi
 Ac_mul_B{T<:BlasReal}(A::StridedMatrix{T}, B::StridedMatrix{T}) = At_mul_B(A, B)
 Ac_mul_B!{T<:BlasReal}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = At_mul_B!(C, A, B)
 function Ac_mul_B{T,S}(A::AbstractMatrix{T}, B::AbstractMatrix{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     Ac_mul_B!(similar(B, TS, (size(A,2), size(B,2))), A, B)
 end
 Ac_mul_B!{T<:BlasComplex}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = is(A,B) ? herk_wrapper!(C,'C',A) : gemm_wrapper!(C,'C', 'N', A, B)
@@ -186,14 +183,14 @@ Ac_mul_B!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic
 A_mul_Bc{T<:BlasFloat,S<:BlasReal}(A::StridedMatrix{T}, B::StridedMatrix{S}) = A_mul_Bt(A, B)
 A_mul_Bc!{T<:BlasFloat,S<:BlasReal}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{S}) = A_mul_Bt!(C, A, B)
 function A_mul_Bc{T,S}(A::AbstractMatrix{T}, B::AbstractMatrix{S})
-    TS = promote_op(matprod, arithtype(T), arithtype(S))
+    TS = promote_op(matprod, T, S)
     A_mul_Bc!(similar(B,TS,(size(A,1),size(B,1))),A,B)
 end
 A_mul_Bc!{T<:BlasComplex}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = is(A,B) ? herk_wrapper!(C, 'N', A) : gemm_wrapper!(C, 'N', 'C', A, B)
 A_mul_Bc!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic_matmatmul!(C, 'N', 'C', A, B)
 
 Ac_mul_Bc{T,S}(A::AbstractMatrix{T}, B::AbstractMatrix{S}) =
-    Ac_mul_Bc!(similar(B, promote_op(matprod, arithtype(T), arithtype(S)), (size(A,2), size(B,1))), A, B)
+    Ac_mul_Bc!(similar(B, promote_op(matprod, T, S), (size(A,2), size(B,1))), A, B)
 Ac_mul_Bc!{T<:BlasFloat}(C::StridedMatrix{T}, A::StridedVecOrMat{T}, B::StridedVecOrMat{T}) = gemm_wrapper!(C, 'C', 'C', A, B)
 Ac_mul_Bc!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic_matmatmul!(C, 'C', 'C', A, B)
 Ac_mul_Bt!(C::AbstractMatrix, A::AbstractVecOrMat, B::AbstractVecOrMat) = generic_matmatmul!(C, 'C', 'T', A, B)
@@ -426,7 +423,7 @@ end
 function generic_matmatmul{T,S}(tA, tB, A::AbstractVecOrMat{T}, B::AbstractMatrix{S})
     mA, nA = lapack_size(tA, A)
     mB, nB = lapack_size(tB, B)
-    C = similar(B, promote_op(matprod, arithtype(T), arithtype(S)), mA, nB)
+    C = similar(B, promote_op(matprod, T, S), mA, nB)
     generic_matmatmul!(C, tA, tB, A, B)
 end