type_annotation_deferred_class

The deferred type '{0}' can't be used in a declaration, cast, or type test.

The analyzer produces this diagnostic when the type annotation is in a variable declaration, or the type used in a cast (as) or type test (is) is a type declared in a library that is imported using a deferred import. These types are required to be available at compile time, but aren't.

For more information, check out Lazily loading a library.

The following code produces this diagnostic because the type of the parameter f is imported from a deferred library:

import 'dart:io' deferred as io;

void f(io.File f) {}

If you need to reference the imported type, then remove the deferred keyword:

import 'dart:io' as io;

void f(io.File f) {}

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