Improve error message for trailing commas in function signatures

[federicobond]

Current message:

test.sol:4:31: Error: Expected type name
  function foo(uint a, uint b,) {}
                              ^

Improved message:

test.sol:4:31: Error: Unexpected trailing comma in function arguments
  function foo(uint a, uint b,) {}
                             ^

Also applies to return types:

test.sol:4:45: Error: Expected type name
  function foo(uint a, uint b) returns(uint,) {}
                                            ^

[axic]

Should be simple enough to change the parser (check if the next token is rparen).

[federicobond]

Yes, wrote it here to remember it, plus, it's a nice one for beginners to tackle.

[axic]

In libsolidity/parser/Parser.cpp change parseParameterList. This will "fix" it for:

• function arguments (in function definitions and function types)
• return parameters (in function definitions and function types)
• event arguments
• modifier arguments

Special care must be taken (i.e. enough tests must be added) to ensure it doesn't break anything else.

[JohnAllen]

Is this issue a) changing the parser, b) improving an error message or c) both?

[federicobond]

Only improving the error message.