Overload UniformScaling to make I(n) create a Diagonal matrix. (#30298)

Fixes #23156

Author: andreasnoack

NEWS.md

@@ -33,6 +33,7 @@ Standard library changes
 #### LinearAlgebra
 
 * Added keyword arguments `rtol`, `atol` to `pinv` and `nullspace` ([#29998]).
+* `UniformScaling` instances are now callable such that e.g. `I(3)` will produce a `Diagonal` matrix ([#30298]).
 
 #### SparseArrays
 

stdlib/LinearAlgebra/src/uniformscaling.jl

@@ -48,6 +48,31 @@ julia> [1 2im 3; 1im 2 3] * I
 """
 const I = UniformScaling(true)
 
+"""
+    (I::UniformScaling)(n::Integer)
+
+Construct a `Diagonal` matrix from a `UniformScaling`.
+
+!!! compat "Julia 1.2"
+     This method is available as of Julia 1.2.
+
+# Examples
+```jldoctest
+julia> I(3)
+3×3 Diagonal{Bool,Array{Bool,1}}:
+ 1  ⋅  ⋅
+ ⋅  1  ⋅
+ ⋅  ⋅  1
+
+julia> (0.7*I)(3)
+3×3 Diagonal{Float64,Array{Float64,1}}:
+ 0.7   ⋅    ⋅
+  ⋅   0.7   ⋅
+  ⋅    ⋅   0.7
+```
+"""
+(I::UniformScaling)(n::Integer) = Diagonal(fill(I.λ, n))
+
 eltype(::Type{UniformScaling{T}}) where {T} = T
 ndims(J::UniformScaling) = 2
 Base.has_offset_axes(::UniformScaling) = false

stdlib/LinearAlgebra/test/uniformscaling.jl

@@ -309,4 +309,10 @@ end
     @test rmul!(copyto!(C, A), J) == target
 end
 
+@testset "Construct Diagonal from UniformScaling" begin
+    @test size(I(3)) === (3,3)
+    @test I(3) isa Diagonal
+    @test I(3) == [1 0 0; 0 1 0; 0 0 1]
+end
+
 end # module TestUniformscaling