Reinstate similar for AbstractQ for backward compatibility (#52694)

Author: jishnub

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -18,7 +18,7 @@ import Base: USE_BLAS64, abs, acos, acosh, acot, acoth, acsc, acsch, adjoint, as
     vec, view, zero
 using Base: IndexLinear, promote_eltype, promote_op, promote_typeof, print_matrix,
     @propagate_inbounds, reduce, typed_hvcat, typed_vcat, require_one_based_indexing,
-    splat
+    splat, DimOrInd
 using Base.Broadcast: Broadcasted, broadcasted
 using Base.PermutedDimsArrays: CommutativeOps
 using OpenBLAS_jll

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)

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