"Doug Gregor" <[EMAIL PROTECTED]> writes:
| On 10 Nov 2006 19:15:45 +0100, Gabriel Dos Reis
| <[EMAIL PROTECTED]> wrote:
| > "Doug Gregor" <[EMAIL PROTECTED]> writes:
| > | With concepts, there are cases where we end up creating two different
| > | type nodes that we later find out should have been the same type node.
| > | Here's an example:
| > |
| > | template<typename T, typename U>
| > | where LessThanComparable<T> && SameType<T, U>
| > | const T& weird_min(const T& t, const U& u) {
| > | return t < u? t : u;
| > | }
| > |
| > | When we parse the template header, we create two different type nodes
| > | for T and U. Then we start parsing the where clause, and create a type
| > | node for LessThanComparable<T>. Now we hit the SameType requirement,
| > | which says that T and U are the same type. It's a little late to make
| > | T and U actually have the same type node (unless we want to re-parse
| > | the template or re-write the AST).
| >
| > I don't think that implies rewriting the AST or reparsing. The
| > same-type constraints reads to me that "it is assume T and U have the
| > same canonical type", e.g. the predicate SameType<T, U> translates to
| > the constraints
| >
| > TYPE_CANONICAL(T) == TYPE_CANONICAL(U)
| >
| > this equation can be added the constraint set without reparsing (it is
| > a semantic constraint).
|
| Yes, but there are types built from 'T' and 'U' that also need to be
| "merged" in this way.
I don't see why you need to merge the types, as opposed to setting
their canonical types.
| For instance, say we have built the types T* and
| U* before seeing that same-type constraint. Now, we also need
| TYPE_CANONICAL (T*) == TYPE_CANONICAL (U*).
| And TYPE_CANONICAL(LessThanComparable<T>) ==
| TYPE_CANONICAL(LessThanComparable<U>).
| If you know about all of these other types that have been built from T
| and U, you can use Nelson and Oppen's algorithm to update the
| TYPE_CANONICAL information relatively quickly. If you don't have that
| information...
In a template definition, one has that information.
-- Gaby
