Finalizers must be callable

Non-function __gc metamethods are not ignored; if present, the
metamethod will be called even if it is not a function.

lgc.c

@@ -832,7 +832,7 @@ static void GCTM (lua_State *L) {
   lua_assert(!g->gcemergency);
   setgcovalue(L, &v, udata2finalize(g));
   tm = luaT_gettmbyobj(L, &v, TM_GC);
-  if (ttisfunction(tm)) {  /* is the finalizer a function? */
+  if (!notm(tm)) {  /* is there a finalizer? */
     int status;
     lu_byte oldah = L->allowhook;
     int running  = g->gcrunning;

manual/manual.of

@@ -689,10 +689,8 @@ After the collection,
 Lua goes through that list.
 For each object in the list,
 it checks the object's @idx{__gc} metamethod:
-If it is a function,
+If it is present,
-Lua calls it with the object as its single argument;
+Lua calls it with the object as its single argument.
-if the metamethod is not a function,
-Lua simply ignores it.
 
 At the end of each garbage-collection cycle,
 the finalizers for objects are called in
@@ -726,6 +724,9 @@ these marks have no effect.
 
 Finalizers cannot yield.
 
+Any error while running a finalizer generates a warning;
+it is not propagated.
+
 }
 
 @sect3{weak-table| @title{Weak Tables}
@@ -7279,6 +7280,11 @@ that is,
 with maximum alignment of 1 (no alignment)
 and native endianness.
 
+Native endianness assumes that the whole system is
+either big or little endian.
+The packing functions will not emulate correctly the behavior
+of mixed-endian formats.
+
 Alignment works as follows:
 For each option,
 the format gets extra padding until the data starts
@@ -7288,6 +7294,7 @@ this minimum must be a power of 2.
 Options @St{c} and @St{z} are not aligned;
 option @St{s} follows the alignment of its starting integer.
 
+
 All padding is filled with zeros by @Lid{string.pack}
 (and ignored by @Lid{string.unpack}).
 
@@ -8720,6 +8727,14 @@ name is visible, even if this other label is declared in an enclosing
 block.
 }
 
+@item{
+When finalizing an object,
+Lua does not ignore @idx{__gc} metamethods that are not functions.
+Any value will be called, if present.
+(Non-callable values will generate a warning,
+like any other error when calling a finalizer.)
+}
+
 }
 
 }