Replace assert by proper error message

Author: aschwaighofer

include/swift/AST/DiagnosticsSema.def

@@ -3918,6 +3918,8 @@ NOTE(dynamic_replacement_found_function_of_type, none,
       "found function %0 of type %1", (DeclName, Type))
 ERROR(dynamic_replacement_not_in_extension, none,
       "dynamicReplacement(for:) of %0 is not defined in an extension or at the file level", (DeclName))
+ERROR(dynamic_replacement_must_not_be_dynamic, none,
+      "dynamicReplacement(for:) of %0 must not be dynamic itself", (DeclName))
 
 //------------------------------------------------------------------------------
 // MARK: @available

lib/Sema/TypeCheckAttr.cpp

@@ -2080,13 +2080,14 @@ void TypeChecker::checkDynamicReplacementAttribute(ValueDecl *D) {
   if (!isa<ExtensionDecl>(D->getDeclContext()) &&
       !D->getDeclContext()->isModuleScopeContext()) {
     diagnose(attr->getLocation(), diag::dynamic_replacement_not_in_extension,
-                D->getBaseName());
+             D->getBaseName());
     attr->setInvalid();
     return;
   }
 
   if (D->isDynamic() && !D->isObjC()) {
-    assert(false && "dynamic replacement must not be dynamic itself");
+    diagnose(attr->getLocation(), diag::dynamic_replacement_must_not_be_dynamic,
+             D->getBaseName());
     attr->setInvalid();
     return;
   }