subtype_of_disallowed_type

'{0}' can't be used as a superclass constraint.

Classes and mixins can't implement '{0}'.

Classes can't extend '{0}'.

Classes can't mixin '{0}'.

The analyzer produces this diagnostic when one of the restricted classes is used in either an extends, implements, with, or on clause. The classes bool, double, FutureOr, int, Null, num, and String are all restricted in this way, to allow for more efficient implementations.

The following code produces this diagnostic because String is used in an extends clause:

class A extends String {}

The following code produces this diagnostic because String is used in an implements clause:

class B implements String {}

The following code produces this diagnostic because String is used in a with clause:

class C with String {}

The following code produces this diagnostic because String is used in an on clause:

mixin M on String {}

If a different type should be specified, then replace the type:

class A extends Object {}

If there isn't a different type that would be appropriate, then remove the type, and possibly the whole clause:

class B {}