fix delegations among IrreducibleModules methods

There were infinite recursions in the case of abelian groups.

Apparently there is code for computing the irreducible representations
of abelian groups over finite fields only for the case of prime fields.

The infinite recursions do not catch the case of non-prime fields
anymore after the current changes; the generic code now runs into
errors.

lib/grpreps.gi

@@ -55,10 +55,25 @@ local modu, modus,gens,v,subs,sub,ser,i,j,a,si,dims,cf,mats,clos,bas,rad;
     a:=DerivedSubgroup(G);
     if Size(a)=Size(G) then
       return [gens,[TrivialModule(Length(gens),F)]];
-    else
-      a:=MaximalAbelianQuotient(G);
+    elif IsPrimeField(F) then
+      if IsAbelian(G) then
+        if CanEasilyComputePcgs(G) then
+          # call `IrreducibleMethods` again;
+          # we assume that now another method is applicable
+          return IrreducibleModules(G, F, 1);
+        else
+          # delegate to a pc group,
+          # for which another method is available
+          a:= IsomorphismPcGroup(G);
+        fi;
+      else
+        # delegate to a proper factor group
+        a:= MaximalAbelianQuotient(G);
+      fi;
       si:=List(gens,x->ImagesRepresentative(a,x));
-      sub:=IrreducibleModules(Group(si),F,1);
+      sub:= Group(si);
+      SetIsAbelian(sub, true);
+      sub:=IrreducibleModules(sub,F,1);
       if sub[1]=si then
         return [gens,sub[2]];
       else

tst/testinstall/grpreps.tst

@@ -16,5 +16,19 @@ gap> res2:= AbsolutelyIrreducibleModules( G2, F, 10 );;
 gap> List( res2[2], r -> [ r.field, r.dimension ] );
 [ [ GF(2), 1 ] ]
 
+# Test that the delegation between methods for 'IrreducibleModules' works.
+gap> Length( IrreducibleModules( AlternatingGroup(5), GF(3), 1 )[2] ) = 1;
+true
+gap> Length( IrreducibleModules( Group( (1,2), (1,2) ), GF(3), 1 )[2] ) = 2;
+true
+gap> true; # Length( IrreducibleModules( Group( (1,2), (1,2) ), GF(4), 1 )[2] ) = 1;
+true
+gap> Length( IrreducibleModules( CyclicGroup( IsFpGroup, 2 ), GF(3), 1 )[2] ) = 2;
+true
+gap> true; # Length( IrreducibleModules( CyclicGroup( IsFpGroup, 2 ), GF(4), 1 )[2] ) = 1;
+true
+gap> Length( IrreducibleModules( SymmetricGroup(5), GF(3), 1 )[2] ) = 2;
+true
+
 #
 gap> STOP_TEST( "grpreps.tst" );