[NFC] Override-check decls by property, not kind

Rather than figuring out which kind of decl we’re using and then testing the traits it has, check each trait once on the decls where the AST can express it.

This change is NFC currently, but will support static subscripts.

Author: beccadax

lib/Sema/TypeCheckDeclOverride.cpp

@@ -234,30 +234,21 @@ static bool areOverrideCompatibleSimple(ValueDecl *decl,
   if (parentDecl->isInvalid())
     return false;
 
-  if (auto func = dyn_cast<FuncDecl>(decl)) {
-    // Specific checking for methods.
-    auto parentFunc = cast<FuncDecl>(parentDecl);
-    if (func->isStatic() != parentFunc->isStatic())
-      return false;
-    if (func->isGeneric() != parentFunc->isGeneric())
-      return false;
-  } else if (auto ctor = dyn_cast<ConstructorDecl>(decl)) {
-    auto parentCtor = cast<ConstructorDecl>(parentDecl);
-    if (ctor->isGeneric() != parentCtor->isGeneric())
+  // If their staticness is different, they aren't compatible.
+  if (decl->isStatic() != parentDecl->isStatic())
+    return false;
+
+  // If their genericity is different, they aren't compatible.
+  if (auto genDecl = decl->getAsGenericContext()) {
+    auto genParentDecl = parentDecl->getAsGenericContext();
+    if (genDecl->isGeneric() != genParentDecl->isGeneric())
       return false;
+  }
 
-    // Factory initializers cannot be overridden.
+  // Factory initializers cannot be overridden.
+  if (auto parentCtor = dyn_cast<ConstructorDecl>(parentDecl))
     if (parentCtor->isFactoryInit())
       return false;
-  } else if (auto var = dyn_cast<VarDecl>(decl)) {
-    auto parentVar = cast<VarDecl>(parentDecl);
-    if (var->isStatic() != parentVar->isStatic())
-      return false;
-  } else if (auto subscript = dyn_cast<SubscriptDecl>(decl)) {
-    auto parentSubscript = cast<SubscriptDecl>(parentDecl);
-    if (subscript->isGeneric() != parentSubscript->isGeneric())
-      return false;
-  }
 
   return true;
 }