Fix linear indexing for ReshapedArray if the parent has offset axes (#41232)

This PR fixes
```julia
julia> r = reshape(Base.IdentityUnitRange(3:4), 2, 1)
2×1 reshape(::Base.IdentityUnitRange{UnitRange{Int64}}, 2, 1) with eltype Int64:
 3
 4

julia> collect(r) == r
false

julia> collect(r)
2×1 Matrix{Int64}:
 3258125826116431922
 3688512103538242609
```

After this PR,
```julia
julia> collect(r)
2×1 Matrix{Int64}:
 3
 4
```

Author: jishnub

base/reshapedarray.jl

@@ -238,7 +238,8 @@ offset_if_vec(i::Integer, axs::Tuple) = i
 
 @inline function isassigned(A::ReshapedArrayLF, index::Int)
     @boundscheck checkbounds(Bool, A, index) || return false
-    @inbounds ret = isassigned(parent(A), index)
+    indexparent = index - firstindex(A) + firstindex(parent(A))
+    @inbounds ret = isassigned(parent(A), indexparent)
     ret
 end
 @inline function isassigned(A::ReshapedArray{T,N}, indices::Vararg{Int, N}) where {T,N}
@@ -251,7 +252,8 @@ end
 
 @inline function getindex(A::ReshapedArrayLF, index::Int)
     @boundscheck checkbounds(A, index)
-    @inbounds ret = parent(A)[index]
+    indexparent = index - firstindex(A) + firstindex(parent(A))
+    @inbounds ret = parent(A)[indexparent]
     ret
 end
 @inline function getindex(A::ReshapedArray{T,N}, indices::Vararg{Int,N}) where {T,N}
@@ -275,7 +277,8 @@ end
 
 @inline function setindex!(A::ReshapedArrayLF, val, index::Int)
     @boundscheck checkbounds(A, index)
-    @inbounds parent(A)[index] = val
+    indexparent = index - firstindex(A) + firstindex(parent(A))
+    @inbounds parent(A)[indexparent] = val
     val
 end
 @inline function setindex!(A::ReshapedArray{T,N}, val, indices::Vararg{Int,N}) where {T,N}

test/abstractarray.jl

@@ -2060,3 +2060,33 @@ end
     test_unsetindex(MyMatrixUnsetIndexCartInds)
     test_unsetindex(MyMatrixUnsetIndexLinInds)
 end
+
+@testset "reshape for offset arrays" begin
+    p = Base.IdentityUnitRange(3:4)
+    r = reshape(p, :, 1)
+    @test r[eachindex(r)] == UnitRange(p)
+    @test collect(r) == r
+
+    struct ZeroBasedArray{T,N,A<:AbstractArray{T,N}} <: AbstractArray{T,N}
+        a :: A
+        function ZeroBasedArray(a::AbstractArray)
+            Base.require_one_based_indexing(a)
+            new{eltype(a), ndims(a), typeof(a)}(a)
+        end
+    end
+    Base.parent(z::ZeroBasedArray) = z.a
+    Base.size(z::ZeroBasedArray) = size(parent(z))
+    Base.axes(z::ZeroBasedArray) = map(x -> Base.IdentityUnitRange(0:x - 1), size(parent(z)))
+    Base.getindex(z::ZeroBasedArray{<:Any, N}, i::Vararg{Int,N}) where {N} = parent(z)[map(x -> x + 1, i)...]
+    Base.setindex!(z::ZeroBasedArray{<:Any, N}, val, i::Vararg{Int,N}) where {N} = parent(z)[map(x -> x + 1, i)...] = val
+
+    z = ZeroBasedArray(collect(1:4))
+    r2 = reshape(z, :, 1)
+    @test r2[CartesianIndices(r2)] == r2[LinearIndices(r2)]
+    r2[firstindex(r2)] = 34
+    @test z[0] == 34
+    r2[eachindex(r2)] = r2 .* 2
+    for (i, j) in zip(eachindex(r2), eachindex(z))
+        @test r2[i] == z[j]
+    end
+end