un-revert "Simplify some views of Adjoint matrices" (#1122)

This copies JuliaLang/julia#43719 to the new
LinearAlgebra.jl repository (partly to see if this all works!) The
original PR was JuliaLang/julia#39467

Author: mcabbott

src/adjtrans.jl

@@ -382,6 +382,20 @@ parent(A::AdjOrTrans) = A.parent
 vec(v::TransposeAbsVec{<:Number}) = parent(v)
 vec(v::AdjointAbsVec{<:Real}) = parent(v)
 
+Base.reshape(v::TransposeAbsVec{<:Number}, ::Val{1}) = parent(v)
+Base.reshape(v::AdjointAbsVec{<:Real}, ::Val{1}) = parent(v)
+
+ # these make eachrow(A') produce simpler views
+@inline Base.unsafe_view(A::Transpose{<:Number, <:AbstractMatrix}, i::Integer, j::AbstractArray) =
+    Base.unsafe_view(parent(A), j, i)
+@inline Base.unsafe_view(A::Transpose{<:Number, <:AbstractMatrix}, i::AbstractArray, j::Integer) =
+    Base.unsafe_view(parent(A), j, i)
+
+@inline Base.unsafe_view(A::Adjoint{<:Real, <:AbstractMatrix}, i::Integer, j::AbstractArray) =
+    Base.unsafe_view(parent(A), j, i)
+@inline Base.unsafe_view(A::Adjoint{<:Real, <:AbstractMatrix}, i::AbstractArray, j::Integer) =
+    Base.unsafe_view(parent(A), j, i)
+
 ### concatenation
 # preserve Adjoint/Transpose wrapper around vectors
 # to retain the associated semantics post-concatenation

test/adjtrans.jl

@@ -308,6 +308,35 @@ end
     @test vec(adjoint(mvec))[1] == adjoint(mvec[1])
 end
 
+@testset "Adjoint and Transpose view methods" begin
+    intvec, intmat = [1, 2], [1 2 3; 4 5 6]
+    # overload of reshape(v, Val(1)) simplifies views of row vectors:
+    @test view(adjoint(intvec), 1:2) isa SubArray{Int, 1, Vector{Int}}
+    @test view(transpose(intvec), 1:2) isa SubArray{Int, 1, Vector{Int}}
+    cvec = [1, 2im, 3, 4im]
+    @test view(transpose(cvec), 2:3) === view(cvec, 2:3)
+    @test view(adjoint(cvec), 2:3) == conj(view(cvec, 2:3))
+
+    # vector slices of transposed matrices are simplified:
+    @test view(adjoint(intmat), 1, :) isa SubArray{Int, 1, Matrix{Int}}
+    @test view(transpose(intmat), 1, :) isa SubArray{Int, 1, Matrix{Int}}
+    @test view(adjoint(intmat), 1, :) == permutedims(intmat)[1, :]
+    @test view(transpose(intmat), 1:1, :) == permutedims(intmat)[1:1, :] # not simplified
+    @test view(adjoint(intmat), :, 2) isa SubArray{Int, 1, Matrix{Int}}
+    @test view(transpose(intmat), :, 2) isa SubArray{Int, 1, Matrix{Int}}
+    @test view(adjoint(intmat), :, 2) == permutedims(intmat)[:, 2]
+    @test view(transpose(intmat), :, 2:2) == permutedims(intmat)[:, 2:2] # not simplified
+    cmat = [1 2im 3; 4im 5 6im]
+    @test view(transpose(cmat), 1, :) isa SubArray{Complex{Int}, 1, Matrix{Complex{Int}}}
+    @test view(transpose(cmat), :, 2) == cmat[2, :]
+    @test view(adjoint(cmat), :, 2) == conj(cmat[2, :]) # not simplified
+
+    # bounds checks happen before this
+    @test_throws BoundsError view(adjoint(intvec), 0:3)
+    @test_throws BoundsError view(transpose(cvec), 0:3)
+    @test_throws BoundsError view(adjoint(intmat), :, 3)
+end
+
 @testset "horizontal concatenation of Adjoint/Transpose-wrapped vectors and Numbers" begin
     # horizontal concatenation of Adjoint/Transpose-wrapped vectors and Numbers
     # should preserve the Adjoint/Transpose-wrapper to preserve semantics downstream