normal_subgroups for FPGroup broken?

[joschmitt]

Describe the bug

julia> F = free_group(:s, :t);

julia> s, t = F[1], F[2];

julia> G, _ = quo(F, [s^4, t^2, s*t*s*t^-1]);

julia> typeof(G)
FPGroup

julia> normal_subgroups(G)
ERROR: MethodError: Cannot `convert` an object of type SubFPGroup to an object of type FPGroup
The function `convert` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  (::Type{S})(::T) where {S<:Union{FinGenAbGroup, Oscar.GAPGroup}, T<:Oscar.GAPGroup}
   @ Oscar ~/.julia/dev/Oscar/src/Groups/homomorphisms.jl:1004
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:126

Stacktrace:
 [1] setindex!(A::Vector{FPGroup}, x::SubFPGroup, i::Int64)
   @ Base ./array.jl:976
 [2] _as_subgroups
   @ ~/.julia/dev/Oscar/src/Groups/sub.jl:162 [inlined]
 [3] normal_subgroups(G::FPGroup)
   @ Oscar ~/.julia/dev/Oscar/src/Groups/sub.jl:199
 [4] top-level scope
   @ REPL[18]:1

I do not know whether this is supposed to work. If not, the error message should be improved. @ThomasBreuer

System (please complete the following information):

julia> Oscar.versioninfo(full = true)
OSCAR version 1.3.0-DEV - #master, 50d2662afb -- 2024-11-05 18:36:38 +0100
  combining:
    AbstractAlgebra.jl   v0.43.9
    GAP.jl               v0.12.0
    Hecke.jl             v0.34.6
    Nemo.jl              v0.47.3
    Polymake.jl          v0.11.22
    Singular.jl          v0.23.10
  building on:
    FLINT_jll               v300.100.300+0
    GAP_jll                 v400.1300.102+2
    Singular_jll            v404.0.606+0
    libpolymake_julia_jll   v0.13.0+0
    libsingular_julia_jll   v0.46.0+0
    polymake_jll            v400.1300.2+0
See `]st -m` for a full list of dependencies.

Julia Version 1.11.1
Commit 8f5b7ca12ad (2024-10-16 10:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 16 × 12th Gen Intel(R) Core(TM) i5-1250P
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, alderlake)
Threads: 1 default, 0 interactive, 1 GC (on 16 virtual cores)
Environment:
  JULIA_EDITOR = vim
Official https://julialang.org/ release

Additional context
Found by @Santymax98 in #4275.

[ThomasBreuer]

@joschmitt No, this should work.
Thanks for the report.
I will investigate what went wrong.