Fix _automorphisms for degree 1 number field

[StevellM]

Before

julia> Qt, t = QQ["t"]
(Univariate polynomial ring in t over QQ, t)

julia> K, a = number_field(t+1)
(Number field of degree 1 over QQ, -1)

julia> automorphism_list(K)
ERROR: Data does not define a morphism
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] #map_data#323
   @ ~/.julia/packages/Hecke/CoyAD/src/Map/NumField.jl:281 [inlined]
 [3] map_data
   @ ~/.julia/packages/Hecke/CoyAD/src/Map/NumField.jl:272 [inlined]
 [4] #hom#322
   @ ~/.julia/packages/Hecke/CoyAD/src/Map/NumField.jl:150 [inlined]
 [5] hom
   @ ~/.julia/packages/Hecke/CoyAD/src/Map/NumField.jl:137 [inlined]
 [6] _automorphisms(K::AbsSimpleNumField; is_abelian::Bool)
   @ Hecke ~/.julia/packages/Hecke/CoyAD/src/Map/automorphisms.jl:29
 [7] automorphism_list(K::AbsSimpleNumField; copy::Bool, is_abelian::Bool)
   @ Hecke ~/.julia/packages/Hecke/CoyAD/src/Map/automorphisms.jl:141
 [8] automorphism_list(K::AbsSimpleNumField)
   @ Hecke ~/.julia/packages/Hecke/CoyAD/src/Map/automorphisms.jl:127
 [9] top-level scope
   @ REPL[3]:1

caused by the code of _automorphisms to create the identity morphism for degree 1 number fields by calling hom(K, K, one(K)). But in the case gen(K) != one(K) this does not work (I guess the code was initially intended to work for QQ defined as a number field by the polynomial t-1). Now with the change, even "exotic" degree 1 polynomials work

julia> Qt, t = QQ["t"]
(Univariate polynomial ring in t over QQ, t)

julia> K, a = number_field(t-984762)
(Number field of degree 1 over QQ, 984762)

julia> automorphism_list(K)
1-element Vector{NumFieldHom{AbsSimpleNumField, AbsSimpleNumField, Hecke.MapDataFromAnticNumberField{AbsSimpleNumFieldElem}, Hecke.MapDataFromAnticNumberField{AbsSimpleNumFieldElem}, AbsSimpleNumFieldElem}}:
 Map: K -> K

julia> s = ans[1]
Map
  from number field of degree 1 over QQ
  to number field of degree 1 over QQ

julia> s(one(K))
1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.75%. Comparing base (f88e78f) to head (94a8d86).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1585      +/-   ##
==========================================
+ Coverage   75.73%   75.75%   +0.02%     
==========================================
  Files         358      358              
  Lines      113694   113693       -1     
==========================================
+ Hits        86105    86127      +22     
+ Misses      27589    27566      -23     

Files	Coverage Δ
src/Map/automorphisms.jl	86.40% <100.00%> (ø)

... and 26 files with indirect coverage changes

[thofma]

Merci. Do you want a quick release? If so, just bump the version number in the Project.toml and I'll take care of the rest.

[StevellM]

No, no need for a quick release. It was just something I remarked while doing some computations; I have already bypassed the problem :-)