Undeclared yylex compiler error for C++ scanner with builtin main

[zmajeed]

This minimal flex file with C++ and main options causes a compiler error

/* cxxmain.l */
/*
a minimal C++ flex file that gives an undeclared yylex compiler error
run with: flex cxxmain.l && g++ lex.yy.cc
error is:
lex.yy.cc: In function 'int main()':
lex.yy.cc:1572:2: error: 'yylex' was not declared in this scope
*/

%option c++
%option main

%%

$ flex cxxmain.l && g++ lex.yy.cc
lex.yy.cc: In function 'int main()':
lex.yy.cc:1572:2: error: 'yylex' was not declared in this scope

flex and gcc versions

$ flex --version
flex 2.6.4

$ g++ --version
g++ (GCC) 8.2.0

The issue is simple. The generated main in lex.yy.cc calls the free function yylex instead of the yyFlexLexer::yylex method.

// lex.yy.cc

int main ()
{

	yylex();

	return 0;
}

The reason is flex.skl does not handle the case for a C++ builtin main

%if-not-reentrant
	yylex();
%endif

Here's a proposed fix. I'll submit a PR if it's acceptable.

%if-not-reentrant
%if-c-only
	yylex();
%endif
%endif

%if-c++-only
  yyFlexLexer l;
  l.yylex();
%endif