[mycpp] Fix check for missing default block

It only happens in str_switch().

In contrast, tagswitch() are often exhaustive.

mycpp/cppgen_pass.py

@@ -1974,8 +1974,9 @@ def _write_cases(self, switch_expr, cases, default_block):
             # an error occurred
             return
         if default_block is False:
-            self.report_error(switch_expr,
-                              'switch got no else: for default block')
+            # This is too restrictive
+            #self.report_error(switch_expr,
+            #                  'switch got no else: for default block')
             return
 
         self.def_write_ind('default: ')
@@ -2056,6 +2057,8 @@ def _write_str_switch(self, expr, o):
         """Write a switch statement over strings."""
         assert len(expr.args) == 1, expr.args
 
+        switch_expr = expr  # for later error
+
         switch_var = expr.args[0]
         if not isinstance(switch_var, NameExpr):
             self.report_error(
@@ -2104,6 +2107,14 @@ def _write_str_switch(self, expr, o):
             self.def_write_ind('}\n')
             self.def_write_ind('  break;\n')
 
+        if default_block is None:
+            # an error occurred
+            return
+        if default_block is False:
+            self.report_error(switch_expr,
+                              'str_switch got no else: for default block')
+            return
+
         self.def_write('\n')
         self.def_write_ind('str_switch_default:\n')
         self.def_write_ind('default: ')

mycpp/examples/invalid_switch.py

@@ -10,9 +10,10 @@
 def NoDefault():
     # type: () -> None
 
-    with switch(42) as case:
-        if case(42):
-            print('42')
+    s = "foo"
+    with str_switch(s) as case:
+        if case("bar"):
+            print('bar')
 
 
 def TagSwitch():