abstract class A { abstract foo(): void; }
class B extends A { } // error, does not implement foo
var C = class extends A { }; // no error reported!
(new C).foo(); // fails at runtimeIf we look at the code in checker.ts that creates the relevant error:
// It is an error to inherit an abstract member without implementing it or being declared abstract.
// If there is no declaration for the derived class (as in the case of class expressions),
// then the class cannot be declared abstract.
if ( baseDeclarationFlags & NodeFlags.Abstract && (!derivedClassDecl || !(derivedClassDecl.flags & NodeFlags.Abstract))) {
error(derivedClassDecl, Diagnostics.Non_abstract_class_0_does_not_implement_inherited_abstract_member_1_from_class_2,
typeToString(type), symbolToString(baseProperty), typeToString(baseType));
}
In a class expression, derivedClassDecl is undefined - so the diagnostic is created, but not associated with a source location, so it doesn't get reported.
If we look at the code in checker.ts that creates the relevant error:
In a class expression,
derivedClassDeclis undefined - so the diagnostic is created, but not associated with a source location, so it doesn't get reported.