fix ImmutableMatrix for IsMatrixObj (#6186)

According to the documentation, `ImmutableMatrix` does the same as
`ChangedBaseDomain` if the 2nd argument is in `IsMatrixObj`.
This was not the case up to now.

(And we are allowed to call `MakeImmutable` instead of `Immutable`
if the 3rd argument is `true`.)

Author: ThomasBreuer

hpcgap/lib/vecmat.gi

@@ -1584,9 +1584,13 @@ local sf, rep, ind, ind2, row, i,big,l,nr;
   if IsMatrixObj(matrix) then
     # result is a matrix object iff 'matrix' is
     if field=BaseDomain(matrix) then
-      return Immutable(matrix);
+      if change then
+        return MakeImmutable(matrix);
+      else
+        return Immutable(matrix);
+      fi;
     else
-      return ImmutableMatrix(field,Unpack(matrix));
+      return MakeImmutable(ChangedBaseDomain(matrix, field));
     fi;
   elif not (IsPlistRep(matrix) or IsGF2MatrixRep(matrix) or
     Is8BitMatrixRep(matrix)) then

lib/vecmat.gi

@@ -1581,9 +1581,13 @@ local sf, rep, ind, ind2, row, i,big,l,nr;
   if IsMatrixObj(matrix) then
     # result is a matrix object iff 'matrix' is
     if field=BaseDomain(matrix) then
-      return Immutable(matrix);
+      if change then
+        return MakeImmutable(matrix);
+      else
+        return Immutable(matrix);
+      fi;
     else
-      return ImmutableMatrix(field,Unpack(matrix));
+      return MakeImmutable(ChangedBaseDomain(matrix, field));
     fi;
   elif not (IsPlistRep(matrix) or IsGF2MatrixRep(matrix) or
     Is8BitMatrixRep(matrix)) then

tst/testinstall/vecmat.tst

@@ -249,6 +249,22 @@ gap> w := ImmutableMatrix( F, m, true );;
 gap> IsMutable(w);
 false
 
+# matrix object
+gap> m:= Matrix( IsPlistMatrixRep, GF(2), [ [ Z(2) ] ] );
+<1x1-matrix over GF(2)>
+gap> w:= ImmutableMatrix( GF(4), m );
+<immutable 1x1-matrix over GF(2^2)>
+gap> IsMutable( w );
+false
+gap> IsMatrixObj( w );
+true
+gap> w:= ImmutableMatrix( GF(2), m, true );
+<immutable 1x1-matrix over GF(2)>
+gap> IsMutable( w );
+false
+gap> IsIdenticalObj( m, w );
+true
+
 # empty matrix
 gap> m := ImmutableMatrix( Rationals, [] );
 [  ]