assertion failure / double destruction triggered by parser::symbol_type's move constructor

[Wolfgang Thaller]

In bison 3.2.3, given a parser definition that starts like this:

%require "3.0.2"
%defines
%define parser_class_name {RezParser}
%skeleton "lalr1.cc"

%locations;

%define api.token.constructor
%define api.value.type variant
%define parse.assert

%token<int> INTLIT;
//…


the following code crashes:

    std::string filename = "foo";
    yy::location loc(&filename, 0,0);
    auto sym = RezParser::make_INTLIT(42, loc);
    auto sym2 = std::move(sym);


Assertion failed: (yytypeid_), function as, file 
/Users/wolfgang/Projects/Retro68-build/build-host/Rez/RezParser.generated.hh, 
line 370.

The reason is that variant::destroy<int> gets called twice.
When basic_symbol<by_type>::move is used, variant::move is invoked, which 
destroys the source, and by_type::move is invoked, which resets the source’s 
type to empty.
When basic_symbol’s move constructor is used, variant::move is invoked as well, 
but by_type’s constructor is used. Because by_type has no move constructor, it 
never resets the source object’s type to empty, and so the variant will be 
desrtroyed again when basic_symbol’s destructor is invoked.

Suggested fix: by_type needs a move constructor that acts like by_type::move.


I also suggest that the fact that yy:: parser_class_name::symbol_type is no 
longer copy-constructible in bison 3.2 should be listed as a breaking change 
from bison 3.0.



Cheers,

Wolfgang