Re: [c++-concepts]: Requires expression

[Gabriel Dos Reis <gdr at axiomatics dot org>]

Andrew Sutton <andrew.n.sutton@gmail.com> writes:

| > | +  // Concept extensions
| > | +  case RID_REQUIRES:
| > | +    return cp_parser_requires_expression (parser);
| > | +
| >
| > I think you meant here "nested requirements", not "extensions" as in
| > "GNU extensions" or "vendor lock-ins".  We should continue with "nested
| > requirements" then.
| 
| This denotes a primary expression, not a nested requirement. I was
| trying to subset the requirement like the "Objective-C++ expressions"
| comment just below it.
| 
| I changed the comment to just "C++ concepts". We may get other primary
| expressions related to concepts, although I don't have any planned.

OK.

| > | +static inline tree
| > | +cp_parser_requires_clause (cp_parser *parser)
| > | +{
| > | +  // Parse the constant expression.
| > | +  tree expr =
| > | +    cp_parser_binary_expression (parser, false, false, PREC_NOT_OPERATOR, NULL);
| > | +  if (!require_potential_rvalue_constant_expression (expr))
| > | +    return error_mark_node;
| > | +  return expr;
| >
| > The grammar says "constant-expression".  You should use
| > cp_parser_constant_expression.
| 
| When we started talking about requirements, we didn't want the full
| breadth of an expression, so I limited it to a logical-or-expression
| that is also a constant expression. Commas, assignments, and
| conditions at the top-level seemed counter-intuitive.

I guess what I was saying earlier was that the comment preceding the
function said "constant-expression", and we want the code to match that.

I am not sure it is less counter-intuitive to allow non-toplevel comma
constant expression  but disallow toplevel ones; they will all end up
being logical literal terms anyway.  We shouldn't worry about
assignments, for we are enforcing the semantics constraints of constant
expressions.   

| Do we want to allow any constant expression, or should we just change
| the docs to logical-or-expression that is also a constant expression?

Yes; and change the comments to say the grammar term is
logical-or-expression,  with additional semantics constraint that they
must be constant expressions.

-- Gaby