// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

/* Graph of inheritance
    A
   / \\
  B   C
 / \\ /
F  D,E

*/

contract A {
    function foo() public pure virtual returns (string memory) {
        return "A";
    }
}

// Contracts inherit other contracts by using the keyword 'is'.
contract B is A {
    // Override A.foo()
    function foo() public pure virtual override returns (string memory) {
        return "B";
    }
}

contract C is A {
    // Override A.foo()
    function foo() public pure virtual override returns (string memory) {
        return "C";
    }
}

// Contracts can inherit from multiple parent contracts.
// When a function is called that is defined multiple times in
// different contracts, parent contracts are searched from
// right to left, and in a depth-first manner.

contract D is B, C {
    // D.foo() returns "C"
    // since C is the right most parent contract with function foo()
    function foo() public pure override(B, C) returns (string memory) {
        return super.foo();
    }
}

contract E is C, B {
    // E.foo() returns "B"
    // since B is the right most parent contract with function foo()
    function foo() public pure override(C, B) returns (string memory) {
        return super.foo();
    }
}

// Inheritance must be ordered from "most base-like" to "most derived".
// Swapping the order of A and B will throw a compilation error.
contract F is A, B {
    function foo() public pure override(A, B) returns (string memory) {
        return super.foo();
    }
}

Summary of Questions and Answers [Which I had manipulated the contract and below are the Asked Questions :

Question 1: Why does contract F return "B" regardless of override(A, B) or override(B, A)?

Prompt (First Prompt): You provided a Solidity contract with the inheritance graph:

    A
   / \\\\
  B   C
 / \\\\ /
F  D,E

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

contract A {
    function foo() public pure virtual returns (string memory) {
        return "A";
    }
}

contract B is A {
    function foo() public pure virtual override returns (string memory) {
        return "B";
    }
}

contract C is A {
    function foo() public pure virtual override returns (string memory) {
        return "C";
    }
}

contract D is B, C {
    function foo() public pure override(B, C) returns (string memory) {
        return super.foo();
    }
}

contract E is C, B {
    function foo() public pure override(C, B) returns (string memory) {
        return super.foo();
    }
}

contract F is A, B {
    function foo() public pure override(A, B) returns (string memory) {
        return super.foo();
    }
}

You asked why F.foo() returns "B" whether the override is override(A, B) or override(B, A).

Answer:

• Explanation:
  • The override(A, B) or override(B, A) specifier only indicates which parent contracts’ foo() functions are being overridden. It does not affect the resolution of super.foo().
  • super.foo() resolves to the foo() function of the first parent in the C3 linearization that defines foo().
  • For contract F is A, B:
    • Linearization: [F, B, A].
      • Start with F.
      • Include B (second parent, inherits from A).
      • Include A (first parent, but also a base of B).
    • When super.foo() is called, Solidity searches [F, B, A]:
      • F calls super.foo(), so skip F.
      • B defines foo() (returns "B").
      • A defines foo() (returns "A"), but B comes first.
    • Thus, super.foo() resolves to B.foo(), returning "B".
  • Changing override(A, B) to override(B, A) doesn’t change the linearization ([F, B, A]), so the output remains "B".
• Why No Error:
  • F is A, B is valid because A is the base contract, and B inherits from A, satisfying the "most base-like to most derived" rule.
• Code (relevant part):

contract F is A, B {
    function foo() public pure override(A, B) returns (string memory) {
        return super.foo(); // Returns "B"
    }
}

• Linearization:
  • [F, B, A] → super.foo() → B.foo() → "B".

Question 2: Why does contract F is B, A cause a compilation error?

Prompt: You modified contract F to:

contract F is B, A {
    function foo() public pure override(A, B) returns (string memory) {
        return super.foo();
    }
}

And asked why this causes a compilation error.