new_with_undefined_constructor_default

The class '{0}' doesn't have an unnamed constructor.

The analyzer produces this diagnostic when an unnamed constructor is invoked on a class that defines named constructors but the class doesn't have an unnamed constructor.

The following code produces this diagnostic because A doesn't define an unnamed constructor:

class A {
  A.a();
}

A f() => A();

If one of the named constructors does what you need, then use it:

class A {
  A.a();
}

A f() => A.a();

If none of the named constructors does what you need, and you're able to add an unnamed constructor, then add the constructor:

class A {
  A();
  A.a();
}

A f() => A();

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