Implement inheritance

[LFsWang]

Add support to inheritance

Parser

• New keywords

• abstract
• is
• override
• virtual
• interface

• structure

• identifier-path
• inheritance-specifier
• override-specifier

• Add new ability on AST for inheritance

Analysis

• Function Overriding

• Base functions can be overridden by inheriting contracts to change their behavior if they are marked as virtual
• The overriding function must then use the override keyword in the function header
• The overriding function may only change the visibility of the overridden function from external to public
• nonpayable can be overridden by view and pure
• view can be overridden by pure
• payable is an exception and cannot be changed to any other mutability.
• For multiple inheritance, the most derived base contracts that define the same function must be specified explicitly after the override
• Functions with the private visibility cannot be virtual
• Public state variables can override external functions if the parameter and return types of the function matches the getter function of the variable:

• Modifier Overriding (The details is as same as Function Overriding)

• modifier support.

• Constructors

• Multiple Inheritance and Linearization

• C3 Linearization (Build inheritance graph)
• Inheriting Different Kinds of Members of the Same Name

• Abstract Contracts

• Interfaces

• They cannot inherit from other contracts, but they can inherit from other interfaces.

• All declared functions must be external

• They cannot declare a constructor.

• They cannot declare state variables.

• All functions declared in interfaces are implicitly virtual

• contract injector