I have the following code snippet for checking a transfer was successful. This function is taken from an already live project, though that project is using pragam 4.2.0. I am using pragma 6.12.0, so I am assuming that it is some kind of versioning issue with Yul but I am struggling to find the correct information.

The code:

    function checkSuccess(
        returns (bool)
        uint256 returnValue = 0;

        assembly {
            // check number of bytes returned from last function call
            switch returndatasize

            // no bytes returned: assume success
            case 0x0 {
                returnValue := 1

            // 32 bytes returned: check if non-zero
            case 0x20 {
                // copy 32 bytes into scratch space
                returndatacopy(0x0, 0x0, 0x20)

                // load those bytes into returnValue
                returnValue := mload(0x0)

            // not sure what was returned: don't mark as success
            default { }

        return returnValue != 0;


I get the following error when compiling:

ParserError: Expected '(' but got reserved keyword 'case'
            case 0x0 {

Anyone with more expertise with inline assembly who could shed some light on this is a +1 from me!

Thanks, Jim

1 Answer 1


You need to add () to the opcode returndatasize. For 0.6.12, check the EVM Dialect to see inline assembly instruction syntax.

Change this line:

switch returndatasize


switch returndatasize()

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