[cfe-dev] Help with parsing C++ member-declarator

[Jan Korous via cfe-dev]

Hi all,

I would like to ask for help with Parser. My goal is to detect grammatically invalid order of *override* and *noexcept* keywords in parser and suggest a fixit.

C++ grammar of member-declarator allows virt-specifier-seq (e. g. override) only after declarator (which contains exception-specification - e. g. noexcept).

For this code:

struct Base { virtual void foo(); };
struct Derived : Base { virtual void foo() override noexcept; };

we currently display this error:

exp.cpp:2:52: error: expected ';' at end of declaration list

from which the issue is not really obvious.

Current implementation works like this.

The starting point is ParseCXXMemberDeclaratorBeforeInitializer() which calls (in this order):
1. ParseFunctionDeclarator() which parses all of these
    cv-qualifier-seq,
    ref-qualifier,
    exception-specification,
    attribute-specifier-seq,
    exception-specification,
    dynamic-exception-specification,
    noexcept-specification

2. ParseOptionalCXX11VirtSpecifierSeq() which parses
    virt-specifier-seq

The call graph looks like this:

ParseCXXMemberDeclaratorBeforeInitializer()
    ParseDeclarator()
        ParseDeclaratorInternal( &ParseDirectDeclarator ) // pointer to method as an argument
            ParseDirectDeclarator()
                ParseFunctionDeclarator()

ParseCXXMemberDeclaratorBeforeInitializer()
    ParseOptionalCXX11VirtSpecifierSeq()

There's also this related FIXME:

ParseDecl.cpp:6028
      // FIXME: Accept these components in any order, and produce fixits to
      // correct the order if the user gets it wrong. Ideally we should deal
      // with the pure-specifier in the same way.

What do you guys recommend as the best approach here? I would expect some refactoring of relevant methods and then just a small if block detecting the wrong order of keywords, outputting diagnostics and fixit and swapping those tokens. I am still just getting familiar with the codebase and would appreciate some guidance.

Thank you.

Best,

Jan