gh-159184: Do not intern NaN constants on the free-threaded build (GH-161276)

compare_constants() matches floats by bits, but hash_const() uses the object
hash, which for a NaN depends on its address, so each NaN loaded by
marshal.loads() added an immortal entry to the constants table.  Hashing the
bits would merge NaNs and shrink frozensets, so leave them out of the table.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: cmaloney

Misc/NEWS.d/next/Core_and_Builtins/2026-11-03-13-44-26.gh-issue-159184.X_sOdY.rst

@@ -0,0 +1,2 @@
+Fix a leak of NaN constants in code objects created by :func:`marshal.loads`
+on the free-threaded build.

Objects/codeobject.c

@@ -173,8 +173,16 @@ should_immortalize_constant(PyObject *v)
                 _Py_IsImmortal(slice->stop) &&
                 _Py_IsImmortal(slice->step));
     }
-    return (PyLong_CheckExact(v) || PyFloat_CheckExact(v) ||
-            PyComplex_CheckExact(v) || PyBytes_CheckExact(v));
+    // The hash of a NaN depends on its address, so the table would keep every
+    // NaN object it gets (and merging them would shrink frozensets).
+    if (PyFloat_CheckExact(v)) {
+        return !isnan(PyFloat_AS_DOUBLE(v));
+    }
+    if (PyComplex_CheckExact(v)) {
+        Py_complex c = ((PyComplexObject *)v)->cval;
+        return !isnan(c.real) && !isnan(c.imag);
+    }
+    return PyLong_CheckExact(v) || PyBytes_CheckExact(v);
 }
 #endif