py: Reraising exception possible only in except block.

py/vm.c

@@ -697,6 +697,9 @@ unwind_return:
                         unum = *ip++;
                         assert(unum <= 1);
                         if (unum == 0) {
+                            if (!currently_in_except_block) {
+                                nlr_jump(mp_obj_new_exception_msg(&mp_type_RuntimeError, "No active exception to reraise"));
+                            }
                             // This assumes that nlr.ret_val holds last raised
                             // exception and is not overwritten since then.
                             obj1 = nlr.ret_val;

tests/basics/try-reraise.py

-# Re-reraising last exception with raise w/o args
+# Reraising last exception with raise w/o args
 
 def f():
     try:
@@ -10,3 +10,10 @@ try:
     f()
 except ValueError as e:
     print(repr(e))
+
+
+# Can reraise only in except block
+try:
+    raise
+except RuntimeError:
+    print("RuntimeError")