forbid Order for non-endo group homomorphisms (gap-system#6156)

Author: ThomasBreuer

lib/morpheus.gi

@@ -26,6 +26,9 @@ MORPHEUSELMS := 50000;
 InstallMethod(Order,"for automorphisms",true,[IsGroupHomomorphism],0,
 function(hom)
 local map,phi,o,lo,i,j,start,img,d,nat,ser,jord,first;
+  if not IsEndoGeneralMapping( hom ) then
+    Error( "Source and Range of <hom> must be equal" );
+  fi;
   d:=Source(hom);
   if not (HasIsFinite(d) and IsFinite(d)) then
     TryNextMethod();

tst/testinstall/mapphomo.tst

@@ -71,6 +71,20 @@ fail
 gap> GroupHomomorphismByImages(G, H, [], []);
 fail
 
+# Order for homomorphisms
+gap> G:= SymmetricGroup( 3 );;
+gap> hom:= IdentityMapping( G );;
+gap> Order( hom );
+1
+gap> One( hom ) = hom;
+true
+gap> H:= Group( (1,2) );;
+gap> hom:= RestrictedMapping( hom, H );;
+gap> Order( hom );
+Error, Source and Range of <hom> must be equal
+gap> One( hom );
+fail
+
 # Check that group homomorphisms created by a function can compute preimages.
 gap> for G in [ SymmetricGroup(5), SmallGroup( 24, 12 ), GL(2,3) ] do
 >      hom:= GroupHomomorphismByFunction( G, G, x -> x );