Fix finiteness condition in IsomorphismGroups

If one group is finite and the other is infinite, the answer is always `fail`
gap-system · Mar 8, 2019 · 1213dab · 1213dab
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commit 1213dab
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diff --git a/lib/morpheus.gi b/lib/morpheus.gi
@@ -2151,9 +2151,12 @@ local m;
     IsFinite(G);
     IsFinite(H);
   fi;
-  if not IsFinite(G) and IsFinite(H) then
+  if not IsFinite(G) and not IsFinite(H) then
     Error("cannot test isomorphism of infinite groups");
   fi;
+  if IsFinite(G) <> IsFinite(H) then
+    return fail;
+  fi;
 
   #AH: Spezielle Methoden ?
   if Size(G)=1 then

diff --git a/tst/testbugfix/2019-03-08-IsomorphismGroups.tst b/tst/testbugfix/2019-03-08-IsomorphismGroups.tst
@@ -0,0 +1,9 @@
+# see https://github.com/gap-system/gap/pull/3331
+gap> F := FreeGroup( 1 );;
+gap> IsFinite( F );;
+gap> S := SymmetricGroup( 3 );;
+gap> IsFinite( S );;
+gap> IsomorphismGroups( F, S );
+fail
+gap> IsomorphismGroups( S, F );
+fail