address routerAddress = 0x7a250d5630b4cf539739df2c5dacb4c659f2488d;
address _WETH = address(0);

function setWETHOnChain(address routerAddress) external {
        IUniswapV2Router02 router = IUniswapV2Router02(routerAddress);
        _WETH = router.WETH();
    }

This line of code no matter what will revert upon execution, to my surprise. Remix debugger said _WETH = router.WETH(); is the last executed statement before revert.

I also have problems with @uniswap/v2-periphery/contracts/libraries/UniswapV2Library.sol, specifically getReserve().

function getReserves(address factory, address tokenA, address tokenB) internal view returns (uint reserveA, uint reserveB) {
    (address token0,) = sortTokens(tokenA, tokenB);
    (uint reserve0, uint reserve1,) = IUniswapV2Pair(pairFor(factory, tokenA, tokenB)).getReserves();
    (reserveA, reserveB) = tokenA == token0 ? (reserve0, reserve1) : (reserve1, reserve0);
}

I would always revert on this line. (uint reserve0, uint reserve1,) = IUniswapV2Pair(pairFor(factory, tokenA, tokenB)).getReserves();, directly after pairFor() has returned successfully.

I tried the first code on Solidity 0.6.12, 0.7.6, and 0.8.7 without any success.

address routerAddress = 0x7a250d5630b4cf539739df2c5dacb4c659f2488d;
address _WETH = address(0);

function setWETHOnChain(address routerAddress) external {
        IUniswapV2Router02 router = IUniswapV2Router02(routerAddress);
        _WETH = router.WETH();
    }

This line of code no matter what will revert upon execution, to my surprise. Remix debugger said _WETH = router.WETH(); is the last executed statement before revert.

I also have problems with @uniswap/v2-periphery/contracts/libraries/UniswapV2Library.sol, specifically getReserve().

function getReserves(address factory, address tokenA, address tokenB) internal view returns (uint reserveA, uint reserveB) {
    (address token0,) = sortTokens(tokenA, tokenB);
    (uint reserve0, uint reserve1,) = IUniswapV2Pair(pairFor(factory, tokenA, tokenB)).getReserves();
    (reserveA, reserveB) = tokenA == token0 ? (reserve0, reserve1) : (reserve1, reserve0);
}

I would always revert on this line. (uint reserve0, uint reserve1,) = IUniswapV2Pair(pairFor(factory, tokenA, tokenB)).getReserves();, directly after pairFor() has returned successfully.

I tried the first code on Solidity 0.6.12, 0.7.6, and 0.8.7 without any success.

The weirdest thing is, querying such values using a block explorer or an EOA using Ethers.js works just fine.