Avoid copy in getindex(::AbstractQ, ...) (#44729)

Author: dlfivefifty

stdlib/LinearAlgebra/src/qr.jl

@@ -582,8 +582,9 @@ size(F::Union{QR,QRCompactWY,QRPivoted}) = size(getfield(F, :factors))
 size(Q::AbstractQ, dim::Integer) = size(getfield(Q, :factors), dim == 2 ? 1 : dim)
 size(Q::AbstractQ) = size(Q, 1), size(Q, 2)
 
-copy(Q::AbstractQ{T}) where {T} = lmul!(Q, Matrix{T}(I, size(Q)))
-getindex(Q::AbstractQ, inds...) = copy(Q)[inds...]
+copymutable(Q::AbstractQ{T}) where {T} = lmul!(Q, Matrix{T}(I, size(Q)))
+copy(Q::AbstractQ) = copymutable(Q)
+getindex(Q::AbstractQ, inds...) = copymutable(Q)[inds...]
 getindex(Q::AbstractQ, ::Colon, ::Colon) = copy(Q)
 
 function getindex(Q::AbstractQ, ::Colon, j::Int)

stdlib/LinearAlgebra/test/qr.jl

@@ -449,6 +449,12 @@ end
         @test Q2[:, :] ≈ M[:, :]
         @test Q2[:, :, :] ≈ M[:, :, :]
     end
+    # Check that getindex works if copy returns itself (#44729)
+    struct MyIdentity{T} <: LinearAlgebra.AbstractQ{T} end
+    Base.size(::MyIdentity, dim::Integer) = dim in (1,2) ? 2 : 1
+    Base.copy(J::MyIdentity) = J
+    LinearAlgebra.lmul!(::MyIdentity{T}, M::Array{T}) where {T} = M
+    @test MyIdentity{Float64}()[1,:] == [1.0, 0.0]
 end
 
 end # module TestQR