From 559efec8a20eb773fa5a7d1f9ed76b7c68ea2dc9 Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Thu, 8 Oct 2026 15:44:19 +0200
Subject: [PATCH] kernel: check mutability where a tnum implied it

Two places test for an exact tnum and rely on it implying that the
object is mutable: the interpreter and compiled code assign directly
to a list with tnum T_PLIST, and MigrateObjects in HPC-GAP sorts the
components of a record with tnum T_PREC. Test the mutability as well,
so that this keeps working once mutability is no longer part of the
tnum (see #6025).

Assisted-by: Claude Code (Opus 5.5)
---
 src/compiled.h                |  6 ++++--
 src/hpc/threadapi.c           |  2 +-
 src/vars.c                    |  2 +-
 tst/testinstall/listindex.tst | 13 ++++++++++++-
 4 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/src/compiled.h b/src/compiled.h
index ef5c8cb214..2f157ac56f 100644
--- a/src/compiled.h
+++ b/src/compiled.h
@@ -124,7 +124,8 @@ static inline void CHECK_FUNC(Obj obj)
   else ASSB_LIST(list, p, rhs);
 
 #define C_ASS_LIST_FPL(list,p,rhs) \
- if ( IS_POS_INTOBJ(p) && TNUM_OBJ(list) == T_PLIST ) { \
+ if ( IS_POS_INTOBJ(p) && TNUM_OBJ(list) == T_PLIST && \
+      IS_MUTABLE_OBJ(list) ) { \
   if ( LEN_PLIST(list) < INT_INTOBJ(p) ) { \
    GROW_PLIST( list, (UInt)INT_INTOBJ(p) ); \
    SET_LEN_PLIST( list, INT_INTOBJ(p) ); \
@@ -136,7 +137,8 @@ static inline void CHECK_FUNC(Obj obj)
  }
 
 #define C_ASS_LIST_FPL_INTOBJ(list,p,rhs) \
- if ( IS_POS_INTOBJ(p) && TNUM_OBJ(list) == T_PLIST) { \
+ if ( IS_POS_INTOBJ(p) && TNUM_OBJ(list) == T_PLIST && \
+      IS_MUTABLE_OBJ(list) ) { \
   if ( LEN_PLIST(list) < INT_INTOBJ(p) ) { \
    GROW_PLIST( list, (UInt)INT_INTOBJ(p) ); \
    SET_LEN_PLIST( list, INT_INTOBJ(p) ); \
diff --git a/src/hpc/threadapi.c b/src/hpc/threadapi.c
index 4abb6f6a5e..a5010699eb 100644
--- a/src/hpc/threadapi.c
+++ b/src/hpc/threadapi.c
@@ -2023,7 +2023,7 @@ MigrateObjects(int count, Obj * objects, Region * target, int retype)
     // they need to be sorted, as sorting upon access may prove impossible.
     for (i = 0; i < count; i++) {
         Obj obj = objects[i];
-        if (TNUM_OBJ(obj) == T_PREC) {
+        if (TNUM_OBJ(obj) == T_PREC && IS_MUTABLE_OBJ(obj)) {
             SortPRecRNam(obj);
         }
         SET_REGION(obj, target);
diff --git a/src/vars.c b/src/vars.c
index 335492c62e..7de1530d18 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -494,7 +494,7 @@ static ExecStatus ExecAssList(Expr stat)
         p = INT_INTOBJ(pos);
 
         // special case for plain list
-        if ( TNUM_OBJ(list) == T_PLIST ) {
+        if ( TNUM_OBJ(list) == T_PLIST && IS_MUTABLE_OBJ(list) ) {
             if ( LEN_PLIST(list) < p ) {
                 GROW_PLIST( list, p );
                 SET_LEN_PLIST( list, p );
diff --git a/tst/testinstall/listindex.tst b/tst/testinstall/listindex.tst
index 0c4571b94a..d28ccf057a 100644
--- a/tst/testinstall/listindex.tst
+++ b/tst/testinstall/listindex.tst
@@ -5,7 +5,7 @@
 ##
 ##
 ##
-#@local foo,l,o,r,res,s,t,x
+#@local f,foo,l,o,r,res,s,t,x
 gap> START_TEST("listindex.tst");
 
 # custom list implementation, pretending to have infinite length
@@ -408,5 +408,16 @@ function ( x )
     return 1;
 end
 
+# assignment inside a function to an immutable list that the kernel has not
+# classified beyond "plain list"
+gap> l := [ 1, "a", 3 ];; l[5] := 1;; Unbind(l[5]);; MakeImmutable(l);;
+gap> f := function(l, i) l[i] := 0; end;;
+gap> f(l, 1);
+Error, List Assignment: <list> must be a mutable list
+gap> f(l, 5);
+Error, List Assignment: <list> must be a mutable list
+gap> l;
+[ 1, "a", 3 ]
+
 # that's all, folks
 gap> STOP_TEST("listindex.tst");