Slava Pestov dca00debec RequirementMachine: Same-type requirements imply same-shape requirements ...

We want `T.A == U.B` to imply `shape(T) == shape(U)` if T (and thus U)
is a parameter pack.

To do this, we introduce some new rewrite rules:

1) For each associated type symbol `[P:A]`, a rule `([P:A].[shape] => [P:A])`.
2) For each non-pack generic parameter `τ_d_i`, a rule `τ_d_i.[shape] => [shape]`.

Now consider a rewrite rule `(τ_d_i.[P:A] => τ_D_I.[Q:B])`. The left-hand
side overlaps with the rule `([P:A].[shape] => [shape])` on the term
`τ_d_i.[P:A].[shape]`. Resolving the overlap gives us a new rule

    t_d_i.[shape] => T_D_I.[shape]

If T is a term corresponding to some type parameter, we say that `T.[shape]` is
a shape term. If `T'.[shape]` is a reduced term, we say that T' is the reduced
shape of T.

Recall that shape requirements are represented as rules of the form:

    τ_d_i.[shape] => τ_D_I.[shape]

Now, the rules of the first kind reduce our shape term `T.[shape]` to
`τ_d_i.[shape]`, where `τ_d_i` is the root generic parameter of T.

If `τ_d_i` is not a pack, a rule of the second kind reduces it to `[shape]`,
so the reduced shape of a non-pack parameter T is the empty term.

Otherwise, if `τ_d_i` is a pack, `τ_d_i.[shape]` might reduce to `τ_D_I.[shape]`
via a shape requirement. In this case, `τ_D_I` is the reduced shape of T.

Fixes rdar://problem/101813873.

2023-07-03 15:41:09 -04:00

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