From beb0d58f01f2e94bccc7844130f69359520cfdbc Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Thu, 3 Sep 2020 16:58:04 +0200
Subject: [PATCH] Avoid calling ImmutableVector(0, v)

The code for algebraic extensions passed fam!.rchar to ImmutableVector;
but that value was 0 if the characteristic exceeded 256.
---
 hpcgap/lib/vecmat.gi        | 2 ++
 lib/algfld.gi               | 2 +-
 lib/vecmat.gi               | 2 ++
 tst/teststandard/algext.tst | 9 ++++++++-
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/hpcgap/lib/vecmat.gi b/hpcgap/lib/vecmat.gi
index 645ab8232b..fbd16028c1 100644
--- a/hpcgap/lib/vecmat.gi
+++ b/hpcgap/lib/vecmat.gi
@@ -1705,6 +1705,7 @@ end);
 InstallMethod( ImmutableVector,"general,2",[IsObject,IsRowVector],0,
 function(f,v)
   local v2;
+  Assert(1, IsField(f) or IsPrimePowerInt(f));
   if not IsInt(f) then f := Size(f); fi;
   if f <= 256 then
     v2 := CopyToVectorRep(v,f);
@@ -1716,6 +1717,7 @@ end);
 InstallOtherMethod( ImmutableVector,"general,3",[IsObject,IsRowVector,IsBool],0,
 function(f,v,change)
   local v2;
+  Assert(1, IsField(f) or IsPrimePowerInt(f));
   if not IsInt(f) then f := Size(f); fi;
   if f <= 256 then
     v2 := CopyToVectorRep(v,f);
diff --git a/lib/algfld.gi b/lib/algfld.gi
index 9356eb1553..12b356f533 100644
--- a/lib/algfld.gi
+++ b/lib/algfld.gi
@@ -94,7 +94,7 @@ local fam,i,cof,red,rchar,impattr,deg,tmp;
   else
     rchar:=0;
   fi;
-  fam!.rchar:=rchar;
+  fam!.rchar:=Size(f);
 
   fam!.poly:=p;
   fam!.polCoeffs:=CoefficientsOfUnivariatePolynomial(p);
diff --git a/lib/vecmat.gi b/lib/vecmat.gi
index 5934ba1725..3cdb688aa1 100644
--- a/lib/vecmat.gi
+++ b/lib/vecmat.gi
@@ -1521,12 +1521,14 @@ end);
 ##
 InstallMethod( ImmutableVector,"general,2",[IsObject,IsRowVector],0,
 function(f,v)
+  Assert(1, IsField(f) or IsPrimePowerInt(f));
   ConvertToVectorRepNC(v,f);
   return Immutable(v);
 end);
 
 InstallOtherMethod( ImmutableVector,"general,3",[IsObject,IsRowVector,IsBool],0,
 function(f,v,change)
+  Assert(1, IsField(f) or IsPrimePowerInt(f));
   ConvertToVectorRepNC(v,f);
   if change then
     MakeImmutable(v);
diff --git a/tst/teststandard/algext.tst b/tst/teststandard/algext.tst
index 29160e264f..9b48f78f2c 100644
--- a/tst/teststandard/algext.tst
+++ b/tst/teststandard/algext.tst
@@ -2,7 +2,7 @@
 ##
 ##  Test of algebraic extensions.
 ##
-#@local t, f0, p1, f1, p2, f2, p3, f3, p4, f4, x, l, a, ll, b
+#@local t, f0, p1, f1, p2, f2, p3, f3, p4, f4, x, l, a, ll, b, pol, K, xinv, c
 gap> START_TEST("algext.tst");
 gap> t := Runtime();;
 gap> f0 := GF(3);;
@@ -25,5 +25,12 @@ gap> b := RootOfDefiningPolynomial(ll);;
 gap> (a+b)^5-(a-b)^5;
 20*alpha^2*beta^3+(10*alpha^2-10)*beta+!4
 
+#
+gap> pol := UnivariatePolynomial(GF(293), Z(293)^0 * ConwayPol(293,8));;
+gap> K := AlgebraicExtension(GF(293), pol);;
+gap> xinv := 1/PrimitiveElement(K);
+Z(293)^145*a^7+Z(293)^274*a^3+Z(293)^120*a^2+Z(293)^134*a+Z(293)^179
+gap> c := Random(K);;
+
 #
 gap> STOP_TEST("algext.tst",1);