From c58b304170bb25c8db3e085e32ec06ff5bb2cbb0 Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Thu, 8 Oct 2026 14:49:28 +0200
Subject: [PATCH] tst: test that kernel functions keep mutability

Cover places where the mutability of an object travels with its tnum
or its type: converting compressed vectors and matrices back to plain
lists, CLONE_OBJ, turning a plain list into a positional object,
setting the IsMutable filter again after MakeImmutable, and the HPC-GAP
serializer. These are the places that need care when mutability moves
out of the tnum (see #6025).

Assisted-by: Claude Code (Opus 5.5)
---
 tst/testinstall/hpc/serialize.tst  | 13 +++++++++++++
 tst/testinstall/kernel/objects.tst | 24 +++++++++++++++++++++++-
 tst/testinstall/kernel/vec8bit.tst | 11 +++++++++++
 tst/testinstall/kernel/vecgf2.tst  | 11 +++++++++++
 4 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/tst/testinstall/hpc/serialize.tst b/tst/testinstall/hpc/serialize.tst
index decc1745b1..38cc0055d1 100644
--- a/tst/testinstall/hpc/serialize.tst
+++ b/tst/testinstall/hpc/serialize.tst
@@ -137,6 +137,19 @@ gap> CheckSerialization(["abc", MakeImmutable("abc")]);
 gap> CheckSerialization([1,2,,3,,,4,"x","y"]);
 gap> CheckSerializationGeneric([~], x->IsIdenticalObj(x, x[1]));
 
+#
+# mutability
+#
+gap> Mutability := x -> [IsMutable(x), List(x, IsMutable), IsMutable(x[2].a),
+>                        IsIdenticalObj(x[5], x[6])];;
+gap> x := MakeImmutable([1]);;
+gap> x := [[2], rec(a := [3]), "abc", [1..5], x, x, [true,false]];;
+gap> CheckSerializationBy(x, Mutability);
+gap> CheckSerializationBy(MakeImmutable(x), Mutability);
+gap> CheckSerializationBy(MakeImmutable(rec(a := 1)), IsMutable);
+gap> CheckSerializationBy(MakeImmutable([1..5]), IsMutable);
+gap> CheckSerializationBy(MakeImmutable([true,false]), IsMutable);
+
 #
 # object set
 #
diff --git a/tst/testinstall/kernel/objects.tst b/tst/testinstall/kernel/objects.tst
index 88dc4df441..198c0ac1ee 100644
--- a/tst/testinstall/kernel/objects.tst
+++ b/tst/testinstall/kernel/objects.tst
@@ -1,7 +1,7 @@
 #
 # Tests for functions defined in src/objects.c
 #
-#@local x, t1c, t2c, t1p, t2p
+#@local x, y, t1c, t2c, t1p, t2p, tm
 gap> START_TEST("kernel/objects.tst");
 
 #
@@ -69,6 +69,21 @@ Error, PosObj Element: <PosObj>![2] must have an assigned value
 gap> x![4];
 Error, PosObj Element: <PosObj>![4] must have an assigned value
 
+# the type decides whether the resulting object is mutable
+gap> tm := NewType(NewFamily("MockFamily"), IsPositionalObjectRep and IsMutable);;
+gap> x := MakeImmutable([ 1 ]);;
+gap> SET_TYPE_POSOBJ(x, tm);;
+gap> IsMutable(x);
+true
+gap> x := MakeImmutable([ 1 ]);;
+gap> SET_TYPE_POSOBJ(x, t2p);;
+gap> IsMutable(x);
+false
+gap> x := Objectify(tm, [ 1 ]);; MakeImmutable(x);; IsMutable(x);
+false
+gap> SetFilterObj(x, IsMutable);; IsMutable(x);
+true
+
 #
 # CLONE_OBJ
 #
@@ -92,5 +107,12 @@ gap> # TODO: overwriting an immutable object via CLONE_OBJ should probably
 gap> # not be allowed, but InstallValue relies on it...
 #@fi
 
+# the clone has the mutability of the source
+gap> x := [ 1 ];; y := MakeImmutable([ 2 ]);;
+gap> CLONE_OBJ(x, y);
+gap> x; IsMutable(x);
+[ 2 ]
+false
+
 #
 gap> STOP_TEST("kernel/objects.tst");
diff --git a/tst/testinstall/kernel/vec8bit.tst b/tst/testinstall/kernel/vec8bit.tst
index b9fe3df7e4..2e2e7a2d50 100644
--- a/tst/testinstall/kernel/vec8bit.tst
+++ b/tst/testinstall/kernel/vec8bit.tst
@@ -99,6 +99,11 @@ Error, COPY_VEC8BIT: <q> must be a positive small integer (not the value 'fail\
 gap> v := V8([]);; PLAIN_VEC8BIT(v);; IsPlistRep(v); Length(v);
 true
 0
+gap> v := MakeImmutable(V8([o, z]));; v := v + v;; IsMutable(v);
+false
+gap> PLAIN_VEC8BIT(v);; IsPlistRep(v); IsMutable(v);
+true
+false
 
 # bad arguments
 gap> PLAIN_VEC8BIT(fail);
@@ -595,6 +600,12 @@ Error, CONV_MAT8BIT: <list> must be a plain list (not a positional object)
 #
 gap> m := Mx08(2);; PLAIN_MAT8BIT(m);; IsPlistRep(m) and m = [[], []];
 true
+gap> m := MakeImmutable(M8([[o, z], [z, o]]));; m := m * m;; IsMutable(m);
+false
+gap> PLAIN_MAT8BIT(m);; IsPlistRep(m); IsMutable(m); IsMutable(m[1]);
+true
+false
+false
 
 # bad arguments
 gap> PLAIN_MAT8BIT(fail);
diff --git a/tst/testinstall/kernel/vecgf2.tst b/tst/testinstall/kernel/vecgf2.tst
index c3021d52cb..e03466ab07 100644
--- a/tst/testinstall/kernel/vecgf2.tst
+++ b/tst/testinstall/kernel/vecgf2.tst
@@ -74,6 +74,11 @@ Error, COPY_GF2VEC: argument must be a list of GF2 elements
 #
 gap> v := V2([]);; PLAIN_GF2VEC(v);; IsPlistRep(v) and Length(v) = 0;
 true
+gap> v := MakeImmutable(V2([o, z]));; v := v + v;; IsMutable(v);
+false
+gap> PLAIN_GF2VEC(v);; IsPlistRep(v); IsMutable(v);
+true
+false
 
 # bad arguments
 gap> PLAIN_GF2VEC(fail);
@@ -84,6 +89,12 @@ Error, PLAIN_GF2VEC: <list> must be a GF2 vector (not the value 'fail')
 #
 gap> m := Mx02(2);; PLAIN_GF2MAT(m);; IsPlistRep(m) and m = [[], []];
 true
+gap> m := MakeImmutable(M2([[o, z], [z, o]]));; m := m * m;; IsMutable(m);
+false
+gap> PLAIN_GF2MAT(m);; IsPlistRep(m); IsMutable(m); IsMutable(m[1]);
+true
+false
+false
 
 # bad arguments
 gap> PLAIN_GF2MAT(fail);