qapi: Move conditional code from QAPISchemaVariants to its subtypes

QAPISchemaVariants.check()'s code is almost entirely conditional on
union vs. alternate type.

Move the conditional code to QAPISchemaBranches.check() and
QAPISchemaAlternatives.check(), where the conditions are always
satisfied.

Attribute QAPISchemaVariants.tag_name is now only used by
QAPISchemaBranches.  Move it there.

Refactor the three types' .__init__() to make them a bit simpler.

Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/schema.py

@@ -719,20 +719,11 @@ def visit(self, visitor: QAPISchemaVisitor) -> None:
 class QAPISchemaVariants:
     def __init__(
         self,
-        tag_name: Optional[str],
         info: QAPISourceInfo,
-        tag_member: Optional[QAPISchemaObjectTypeMember],
         variants: List[QAPISchemaVariant],
     ):
-        # Unions pass tag_name but not tag_member.
-        # Alternates pass tag_member but not tag_name.
-        # After check(), tag_member is always set.
-        assert bool(tag_member) != bool(tag_name)
-        assert (isinstance(tag_name, str) or
-                isinstance(tag_member, QAPISchemaObjectTypeMember))
-        self._tag_name = tag_name
         self.info = info
-        self._tag_member = tag_member
+        self._tag_member: Optional[QAPISchemaObjectTypeMember] = None
         self.variants = variants
 
     @property
@@ -751,7 +742,21 @@ def set_defined_in(self, name: str) -> None:
     def check(
             self, schema: QAPISchema, seen: Dict[str, QAPISchemaMember]
     ) -> None:
-        if self._tag_name:      # union
+        for v in self.variants:
+            v.check(schema)
+
+
+class QAPISchemaBranches(QAPISchemaVariants):
+    def __init__(self,
+                 info: QAPISourceInfo,
+                 variants: List[QAPISchemaVariant],
+                 tag_name: str):
+        super().__init__(info, variants)
+        self._tag_name = tag_name
+
+    def check(
+            self, schema: QAPISchema, seen: Dict[str, QAPISchemaMember]
+    ) -> None:
         # We need to narrow the member type:
         tmp = seen.get(c_name(self._tag_name))
         assert tmp is None or isinstance(tmp, QAPISchemaObjectTypeMember)
@@ -786,12 +791,6 @@ def check(
                 self.info,
                 "discriminator member '%s' of %s must not be conditional"
                 % (self._tag_name, base))
-        else:                   # alternate
-            assert self._tag_member
-            assert isinstance(self.tag_member.type, QAPISchemaEnumType)
-            assert not self.tag_member.optional
-            assert not self.tag_member.ifcond.is_present()
-        if self._tag_name:      # union
         # branches that are not explicitly covered get an empty type
         assert self.tag_member.defined_in
         cases = {v.name for v in self.variants}
@@ -834,20 +833,21 @@ def check_clash(
             v.type.check_clash(info, dict(seen))
 
 
-class QAPISchemaBranches(QAPISchemaVariants):
-    def __init__(self,
-                 info: QAPISourceInfo,
-                 variants: List[QAPISchemaVariant],
-                 tag_name: str):
-        super().__init__(tag_name, info, None, variants)
-
-
 class QAPISchemaAlternatives(QAPISchemaVariants):
     def __init__(self,
                  info: QAPISourceInfo,
                  variants: List[QAPISchemaVariant],
                  tag_member: QAPISchemaObjectTypeMember):
-        super().__init__(None, info, tag_member, variants)
+        super().__init__(info, variants)
+        self._tag_member = tag_member
+
+    def check(
+            self, schema: QAPISchema, seen: Dict[str, QAPISchemaMember]
+    ) -> None:
+        super().check(schema, seen)
+        assert isinstance(self.tag_member.type, QAPISchemaEnumType)
+        assert not self.tag_member.optional
+        assert not self.tag_member.ifcond.is_present()
 
 
 class QAPISchemaMember: