From 66ffceabc6f413126b75f4efe1205b6aac86eb5e Mon Sep 17 00:00:00 2001
From: Jishnu Bhattacharya <jishnub.github@gmail.com>
Date: Wed, 22 May 2024 15:56:00 +0530
Subject: [PATCH] Reinstate similar for AbstractQ for backward compatibility
 (#52694)

Co-authored-by: Steven G. Johnson <stevenj@alum.mit.edu>
(cherry picked from commit 7ec60b4187321411ee2e6ec23c35f6e5b417fc7b)
---
 stdlib/LinearAlgebra/src/LinearAlgebra.jl |  2 ++
 stdlib/LinearAlgebra/src/abstractq.jl     |  8 ++++++++
 stdlib/LinearAlgebra/test/abstractq.jl    | 24 +++++++++++++++++++++++
 3 files changed, 34 insertions(+)

diff --git a/stdlib/LinearAlgebra/src/LinearAlgebra.jl b/stdlib/LinearAlgebra/src/LinearAlgebra.jl
index 9262c2fddca45..ec0363e1f751c 100644
--- a/stdlib/LinearAlgebra/src/LinearAlgebra.jl
+++ b/stdlib/LinearAlgebra/src/LinearAlgebra.jl
@@ -241,6 +241,8 @@ Note that the [element type](@ref eltype) of the matrix must admit [`norm`](@ref
 """
 struct ColumnNorm <: PivotingStrategy end
 
+using Base: DimOrInd
+
 # Check that stride of matrix/vector is 1
 # Writing like this to avoid splatting penalty when called with multiple arguments,
 # see PR 16416
diff --git a/stdlib/LinearAlgebra/src/abstractq.jl b/stdlib/LinearAlgebra/src/abstractq.jl
index bf4064c907a2d..09beca1dd960d 100644
--- a/stdlib/LinearAlgebra/src/abstractq.jl
+++ b/stdlib/LinearAlgebra/src/abstractq.jl
@@ -78,6 +78,14 @@ axes(Q::AbstractQ, d::Integer) = d in (1, 2) ? axes(Q)[d] : Base.OneTo(1)
 copymutable(Q::AbstractQ{T}) where {T} = lmul!(Q, Matrix{T}(I, size(Q)))
 copy(Q::AbstractQ) = copymutable(Q)
 
+# legacy compatibility
+similar(Q::AbstractQ) = similar(Q, eltype(Q), size(Q))
+similar(Q::AbstractQ, ::Type{T}) where {T} = similar(Q, T, size(Q))
+similar(Q::AbstractQ, size::DimOrInd...) = similar(Q, eltype(Q), size...)
+similar(Q::AbstractQ, ::Type{T}, size::DimOrInd...) where {T} = similar(Q, T, Base.to_shape(size))
+similar(Q::AbstractQ, size::Tuple{Vararg{DimOrInd}}) = similar(Q, eltype(Q), Base.to_shape(size))
+similar(Q::AbstractQ, ::Type{T}, size::NTuple{N,Integer}) where {T,N} = Array{T,N}(undef, size)
+
 # getindex
 @inline function getindex(Q::AbstractQ, inds...)
     @boundscheck Base.checkbounds_indices(Bool, axes(Q), inds) || Base.throw_boundserror(Q, inds)
diff --git a/stdlib/LinearAlgebra/test/abstractq.jl b/stdlib/LinearAlgebra/test/abstractq.jl
index 0eb88324e8c20..e46f4d2600eb2 100644
--- a/stdlib/LinearAlgebra/test/abstractq.jl
+++ b/stdlib/LinearAlgebra/test/abstractq.jl
@@ -102,6 +102,30 @@ n = 5
     @test Q ≈ Prect
     @test Q ≈ Psquare
     @test Q ≈ F.Q*I
+
+    @testset "similar" begin
+        QS = similar(Q)
+        @test QS isa Matrix{eltype(Q)}
+        @test size(QS) == size(Q)
+
+        QS = similar(Q, Int8)
+        @test QS isa Matrix{Int8}
+        @test size(QS) == size(Q)
+
+        QS = similar(Q, 1)
+        @test QS isa Vector{eltype(Q)}
+        @test size(QS) == (1,)
+
+        QS = similar(Q, Int8, 2)
+        @test QS isa Vector{Int8}
+        @test size(QS) == (2,)
+
+        QS = similar(Q, Int8, ())
+        @test QS isa Array{Int8,0}
+
+        QS = similar(Q, ())
+        @test QS isa Array{eltype(Q),0}
+    end
 end
 
 end # module