[translation] Rewrite code that returns variants conditionally

Document the gotcha in mycpp/README.md

mycpp/README.md

@@ -108,9 +108,9 @@ More work in this pass:
 
 Note: I really wish we were not using visitors, but that's inherited from MyPy.
 
-## Translation Notes
+## WARNING: Assumptions Not Checked
 
-### WARNING: Assumptions Not Checked
+### Global Constants Can't Be Mutated
 
 We translate top level constants to statically initialized C data structures
 (zero startup cost):
@@ -122,6 +122,36 @@ We translate top level constants to statically initialized C data structures
 Even though `List` and `Dict` are mutable in general, you should **NOT** mutate
 these global instances!  The C++ code will break at runtime.
 
+### Gotcha about Returning Variants (Subclasses) of a type
+
+MyPy will accept this code:
+
+```
+if cond:
+  sig = proc_sig.Open  # type: proc_sig_t
+                       # bad because mycpp HOISTS this
+else:
+  sig = proc_sig.Closed.CreateNull()
+  sig.words = words    # assignment fails
+return sig
+```
+
+It will translate to C++, but fail to compile.  Instead, rewrite it like this:
+
+```
+sig = None  # type: proc_sig_t
+if cond:
+  sig = proc_sig.Open  # type: proc_sig_t
+                       # bad because mycpp HOISTS this
+else:
+  closed = proc_sig.Closed.CreateNull()
+  closed.words = words    # assignment fails
+  sig = closed
+return sig
+```
+
+## More Translation Notes
+
 ### "Creative Hacks"
 
 - `with tagswitch(d) as case` → `switch / case`

ysh/expr_to_ast.py

@@ -1002,20 +1002,24 @@ def Proc(self, pnode):
 
         #self.p_printer.Print(pnode)
 
+        sig = None  # type: proc_sig_t
+
         n = pnode.NumChildren()
         if n == 1:  # proc f {
-            sig = proc_sig.Open  # type: proc_sig_t
+            sig = proc_sig.Open
         elif n == 3:  # proc f () {
             sig = proc_sig.Closed.CreateNull(alloc_lists=True)  # no params
         else:
-            sig = proc_sig.Closed.CreateNull(alloc_lists=True)  # no params
+            closed = proc_sig.Closed.CreateNull(alloc_lists=True)  # no params
 
-            sig.words, sig.rest_words = self._FuncParams(pnode.GetChild(1))
+            closed.words, closed.rest_words = self._FuncParams(pnode.GetChild(1))
             #sig.typed, sig.rest_typed = self._FuncParams(pnode.GetChild(2))
             #sig.named, sig.rest_named = self._FuncParams(pnode.GetChild(3))
 
             # TODO:
-            sig.block_param = None
+            closed.block_param = None
+
+            sig = closed
 
         return sig