Add rmul!

README.md

@@ -300,6 +300,9 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `Compat.qr` takes `pivot` as a `Val` _instance_ and keyword argument `full` ([#22475], [#24279]).
 
+* `Compat.rmul!` provides a subset of the functionality of `LinearAlgebra.rmul!` for
+  use with Julia 0.6 ([#25701], [#25812]).
+
 ## Renaming
 
 * `Display` is now `AbstractDisplay` ([#24831]).
@@ -607,10 +610,12 @@ includes this fix. Find the minimum version from there.
 [#25647]: https://github.com/JuliaLang/julia/issues/25647
 [#25654]: https://github.com/JuliaLang/julia/issues/25654
 [#25662]: https://github.com/JuliaLang/julia/issues/25662
+[#25701]: https://github.com/JuliaLang/julia/issues/25701
 [#25705]: https://github.com/JuliaLang/julia/issues/25705
 [#25706]: https://github.com/JuliaLang/julia/issues/25706
 [#25738]: https://github.com/JuliaLang/julia/issues/25738
 [#25780]: https://github.com/JuliaLang/julia/issues/25780
+[#25812]: https://github.com/JuliaLang/julia/issues/25812
 [#25819]: https://github.com/JuliaLang/julia/issues/25819
 [#25873]: https://github.com/JuliaLang/julia/issues/25873
 [#25896]: https://github.com/JuliaLang/julia/issues/25896

src/Compat.jl

@@ -1843,6 +1843,31 @@ else
     using LinearAlgebra: qr
 end
 
+# rmul! (NOTE: Purposefully not exported)
+if VERSION < v"0.7.0-DEV.3563" # scale! not deprecated
+    if VERSION >= v"0.7.0-DEV.3449" # LinearAlgebra in the stdlib
+        using LinearAlgebra: UnitUpperTriangular, UnitLowerTriangular, scale!
+    else
+        using Base.LinAlg: UnitUpperTriangular, UnitLowerTriangular, scale!
+    end
+    const Triangle = Union{UpperTriangular, UnitUpperTriangular,
+                           LowerTriangular, UnitLowerTriangular}
+    if VERSION < v"0.7.0-DEV.3204" # A_mul_B! not deprecated
+        rmul!(A::AbstractMatrix, B::Triangle) = A_mul_B!(A, A, B)
+    else
+        rmul!(A::AbstractMatrix, B::Triangle) = mul!(A, A, B)
+    end
+    rmul!(A::AbstractArray, s::Number) = scale!(A, s)
+    rmul!(A::AbstractMatrix, D::Diagonal) = scale!(A, D.diag)
+    rmul!(A::Diagonal, B::Diagonal) = Diagonal(A.diag .*= B.diag)
+    rmul!(A::Triangle, B::Diagonal) = typeof(A)(rmul!(A.data, B))
+elseif v"0.7.0-DEV.3563" <= VERSION < v"0.7.0-DEV.3665" # scale! -> mul1!
+    using LinearAlgebra: mul1!
+    const rmul! = mul1!
+elseif VERSION >= v"0.7.0-DEV.3665" # mul1! -> rmul!
+    using LinearAlgebra: rmul!
+end
+
 # https://github.com/JuliaLang/julia/pull/27077
 @static if VERSION < v"0.7.0-DEV.5087"
     export isletter

test/runtests.jl

@@ -1730,6 +1730,14 @@ let A = [1 2; 1 2; 1 2]
     @test f[1] * [f[2]; [0 0]] ≈ A[:,f[3]]
 end
 
+let A = [1 2; 3 4]
+    @test rmul!(A, 2) == [2 4; 6 8]
+    @test rmul!(A, Diagonal([1, 2])) == [2 8; 6 16]
+    @test rmul!(A, UpperTriangular([2 2; 3 3])) == [4 28; 12 60]
+    @test rmul!(LowerTriangular(A), Diagonal([1, 2])) == LowerTriangular([4 0; 12 120])
+    @test rmul!(Diagonal(A), Diagonal([2, 1])) == Diagonal([8, 120])
+end
+
 # 0.7.0-DEV.5087
 @test isletter('a')
 @test isletter('β')