c++: wrong error with MVP and pushdecl [PR64679]

This patch fixes the second half of 64679.  Here we issue a wrong
"redefinition of 'int x'" for the following:

  struct Bar {
    Bar(int, int, int);
  };

  int x = 1;
  Bar bar(int(x), int(x), int{x}); // #1

cp_parser_parameter_declaration_list does pushdecl every time it sees
a named parameter, so the second "int(x)" causes the error.  That's
premature, since this turns out to be a constructor call after the
third argument!

If the first parameter is parenthesized, we can't push until we've
established we're looking at a function declaration.  Therefore this
could be fixed by some kind of lookahead.  I thought about introducing a
lightweight variant of cp_parser_parameter_declaration_list that would
not have any side effects and would return as soon as it figures out
whether it's looking at a declaration or expression.  Since that would
require fairly nontrivial changes, I wanted something simpler.

Something like delaying the pushdecl until we've reached the ')'
following the parameter-declaration-clause.  But we must push the
parameters before processing a default argument, as in:

  Bar bar(int(a), int(b), int c = sizeof(a));  // valid

Moreover, this code should still be accepted

  Bar f(int(i), decltype(i) j = 42);

so this patch stashes parameters into a vector when parsing tentatively
only when pushdecl-ing a parameter would result in a clash and an error
about redefinition/redeclaration.  The stashed parameters are pushed at
the end of a parameter-declaration-clause if it's followed by a ')', so
that we still diagnose redefining a parameter.

	PR c++/64679

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_parameter_declaration_clause): Maintain
	a vector of parameters that haven't been pushed yet.  Push them at the
	end of a valid parameter-declaration-clause.
	(cp_parser_parameter_declaration_list): Take a new auto_vec parameter.
	Do not pushdecl while parsing tentatively when pushdecl-ing a parameter
	would result in a hard error.
	(cp_parser_cache_defarg): Adjust the call to
	cp_parser_parameter_declaration_list.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/ambig11.C: New test.
	* g++.dg/parse/ambig12.C: New test.
	* g++.dg/parse/ambig13.C: New test.
	* g++.dg/parse/ambig14.C: New test.

0 files changed, 0 insertions, 0 deletions