fix deepcopy for non-trivial circular references (#56990)

Resolves #56775 . Credit for this fix rests entirely with bbrehm .

---------

Co-authored-by: Lilith Orion Hafner <[email protected]>
Co-authored-by: Neven Sajko <[email protected]>

Author: 3 people

Diff for: base/deepcopy.jl

@@ -120,11 +120,14 @@ function _deepcopy_memory_t(@nospecialize(x::Memory), T, stackdict::IdDict)
     end
     return dest
 end
-@eval function deepcopy_internal(x::Array{T, N}, stackdict::IdDict) where {T, N}
+function deepcopy_internal(x::Array{T, N}, stackdict::IdDict) where {T, N}
     if haskey(stackdict, x)
         return stackdict[x]::typeof(x)
     end
-    stackdict[x] = $(Expr(:new, :(Array{T, N}), :(deepcopy_internal(x.ref, stackdict)), :(x.size)))
+    y = stackdict[x] = Array{T, N}(undef, ntuple(Returns(0), Val{N}()))
+    setfield!(y, :ref, deepcopy_internal(x.ref, stackdict))
+    setfield!(y, :size, x.size)
+    y
 end
 function deepcopy_internal(x::GenericMemoryRef, stackdict::IdDict)
     if haskey(stackdict, x)

Diff for: test/copy.jl

@@ -253,6 +253,15 @@ end
     @test copyto!(s, String[]) == [1, 2] # No error
 end
 
+@testset "circular reference arrays" begin
+    # issue 56775
+    p = Any[nothing]
+    p[1] = p
+    p2 = deepcopy(p)
+    @test p2 === p2[1]
+    @test p2 !== p
+end
+
 @testset "deepcopy_internal arrays" begin
     @test (@inferred Base.deepcopy_internal(zeros(), IdDict())) == zeros()
 end