py/emitnative: Initialise locals as Python object type for native code.

In @micropython.native code the types of variables and expressions are
always Python objects, so they can be initialised as such.  This prevents
problems with compiling optimised code like while-loops where a local may
be referenced before it is assigned to.

Signed-off-by: Damien George <damien@micropython.org>

py/emitnative.c

@@ -401,7 +401,7 @@ STATIC void emit_native_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scop
 
     // local variables begin unbound, and have unknown type
     for (mp_uint_t i = num_args; i < emit->local_vtype_alloc; i++) {
-        emit->local_vtype[i] = VTYPE_UNBOUND;
+        emit->local_vtype[i] = emit->do_viper_types ? VTYPE_UNBOUND : VTYPE_PYOBJ;
     }
 
     // values on stack begin unbound

tests/basics/assign_expr.py

@@ -12,6 +12,17 @@ x = 1
 print(x, x := 5, x)
 print(x)
 
+# Test "while" with assignment expression as conditional, assigning to a new local.
+# The while conditional is compiled after the while body, so this tests how the
+# compiler handles the case of an unbound local being compiled before it is assigned.
+def f():
+    l = [0, 1]
+    while local := len(l):
+        print(local, l.pop())
+
+
+f()
+
 
 def foo():
     print("any", any((hit := i) % 5 == 3 and (hit % 2) == 0 for i in range(10)))

tests/basics/assign_expr.py.exp

@@ -5,6 +5,8 @@ True
 5
 1 5 5
 5
+2 1
+1 0
 any True
 8
 123