Sema: Allow parametrized protocol types in protocol inheritance clauses

And ban them in other inheritance clauses.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -2773,6 +2773,8 @@ ERROR(inheritance_from_non_protocol_or_class,none,
   "inheritance from non-protocol, non-class type %0", (Type))
 ERROR(inheritance_from_non_protocol,none,
   "inheritance from non-protocol type %0", (Type))
+ERROR(inheritance_from_parametrized_protocol,none,
+  "cannot inherit from parametrized protocol type %0", (Type))
 ERROR(superclass_not_first,none,
   "superclass %0 must appear first in the inheritance clause", (Type))
 ERROR(superclass_not_open,none,

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -249,6 +249,14 @@ static void checkInheritanceClause(
       inheritedTy = layout.explicitSuperclass;
     }
 
+    if (inheritedTy->is<ParametrizedProtocolType>()) {
+      if (!isa<ProtocolDecl>(decl)) {
+        decl->diagnose(diag::inheritance_from_parametrized_protocol,
+                       inheritedTy);
+      }
+      continue;
+    }
+
     // If this is an enum inheritance clause, check for a raw type.
     if (isa<EnumDecl>(decl)) {
       // Check if we already had a raw type.