Incorrect warning about unreachable code.

[blishko]

Description

For the following example

abstract contract A {
    bool s;

    function f() public view mod {
        assert(s); // holds for C, but fails for B
    }
    modifier mod() virtual;
}

contract B is A {
    modifier mod() virtual override {
        s = false;
        _;
    }
}

contract C is B {
    modifier mod() override {
        s = true;
        _;
    }
}

The compiler reports the following warning:

Warning: Unreachable code.
 --> /Users/mblicha/projects/solidity/build/test.sol:4:34:
  |
4 |     function f() public view mod {
  |                                  ^ (Relevant source part starts here and spans across multiple lines).

This warning is very confusing and most likely also incorrect. I would expect this to be compiled without any warning.

Environment

• Compiler version: 0.7.5, also current develop
• Target EVM version (as per compiler settings): N/A
• Framework/IDE (e.g. Truffle or Remix): N/A
• EVM execution environment / backend / blockchain client: N/A
• Operating system: MacOs 10.13

Steps to Reproduce

Just compile with solc.

[chriseth]

Might be related to / duplicate of #10084

[chriseth]

Reduced the example to

abstract contract A {
    function f() public view mod {
        require(block.timestamp > 10);
    }
    modifier mod() virtual;
}

The assert can be replaced by a require but not by a regular statement.