add isassigned methods for reinterpretarray (#53663)

Fixes #52925
Refs #51760

(cherry picked from commit 60d4b7b)

Author: vtjnash

base/reinterpretarray.jl

@@ -386,6 +386,10 @@ check_ptr_indexable(a::AbstractArray, sz) = false
 
 @propagate_inbounds getindex(a::ReinterpretArray) = a[firstindex(a)]
 
+@propagate_inbounds isassigned(a::ReinterpretArray, inds::Integer...) = checkbounds(Bool, a, inds...) && (check_ptr_indexable(a) || _isassigned_ra(a, inds...))
+@propagate_inbounds isassigned(a::ReinterpretArray, inds::SCartesianIndex2) = isassigned(a.parent, inds.j)
+@propagate_inbounds _isassigned_ra(a::ReinterpretArray, inds...) = true # that is not entirely true, but computing exactly which indexes will be accessed in the parent requires a lot of duplication from the _getindex_ra code
+
 @propagate_inbounds function getindex(a::ReinterpretArray{T,N,S}, inds::Vararg{Int, N}) where {T,N,S}
     check_readable(a)
     check_ptr_indexable(a) && return _getindex_ptr(a, inds...)

test/reinterpretarray.jl

@@ -588,3 +588,23 @@ end
 
     @test_throws ArgumentError reinterpret(Tuple{Int32, Int64}, (Int16(1), Int64(4)))
 end
+
+let R = reinterpret(Float32, ComplexF32[1.0f0+2.0f0*im, 4.0f0+3.0f0*im])
+    @test !isassigned(R, 0)
+    @test isassigned(R, 1)
+    @test isassigned(R, 4)
+    @test isassigned(R, Int8(2), Int16(1), Int32(1), Int64(1))
+    @test !isassigned(R, 1, 2)
+    @test !isassigned(R, 5)
+    @test Array(R)::Vector{Float32} == [1.0f0, 2.0f0, 4.0f0, 3.0f0]
+end
+
+let R = reinterpret(reshape, Float32, ComplexF32[1.0f0+2.0f0*im, 4.0f0+3.0f0*im])
+    @test !isassigned(R, 0)
+    @test isassigned(R, 1)
+    @test isassigned(R, 4)
+    @test isassigned(R, Int8(2), Int16(2), Int32(1), Int64(1))
+    @test !isassigned(R, 1, 1, 2)
+    @test !isassigned(R, 5)
+    @test Array(R)::Matrix{Float32} == [1.0f0 4.0f0; 2.0f0 3.0f0]
+end