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### 3052. `visit` is underconstrained

Section: 22.6.7 [variant.visit] Status: Resolved Submitter: Casey Carter Opened: 2018-01-23 Last modified: 2021-05-18

Priority: 2

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Discussion:

`std::visit` accepts a parameter pack of forwarding references named `vars` whose types are the parameter pack `Variants`. Despite that:

1. the names of both packs are variants of "variant",
2. para 1 passes the types in `Variants` (modified) to `variant_size_v`,
3. para 3 uses the expression `varsi.index()`,
4. para 4 says "...if any `variant` in `vars` is `valueless_by_exception`, and
5. para 5 mentions "..the number of alternative types of `Variants0`"

the Requires element imposes no explicit requirements on the types in `Variants`. Notably, the `Variants` are not required to be `variant`s. This lack of constraints appears to be simply an oversight.

[2018-01-24, Daniel comments]

This issue should be reviewed in common with LWG 2970.

[2018-06-18 after reflector discussion]

Priority set to 2; status to LEWG

[2020-11-18; this will be resolved by P2162.]

[2021-04-19 P2162R2 was adopted at February 2021 plenary. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

This wording is relative to N4727.

1. Modify 22.6.7 [variant.visit] as indicated:

   >     template<class Visitor, class... Variants>
   >       constexpr see below visit(Visitor&& vis, Variants&&... vars);

   > > […]

   > > -4- Throws: `bad_variant_access` if any `variant` in `vars` is `valueless_by_exception()``(vars.valueless_by_exception() || ...)` is `true`.

   > > -5- Complexity: […]

   > > -?- Remarks: This function shall not participate in overload resolution unless `remove_cvref_t<Variantsi>` is a specialization of `variant` for all `0 <= i < n`.