RequirementMachine: Same-type requirements imply same-shape requirements #67064
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slavapestov
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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.
… since ASTPrinter already prints 'each'
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We want
T.A == U.Bto implyshape(T) == shape(U)if T (and thus U) is a parameter pack.To do this, we introduce some new rewrite rules:
[P:A], a rule([P:A].[shape] => [P:A]).τ_d_i, a ruleτ_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 termτ_d_i.[P:A].[shape]. Resolving the overlap gives us a new ruleIf T is a term corresponding to some type parameter, we say that
T.[shape]is a shape term. IfT'.[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:
Now, the rules of the first kind reduce our shape term
T.[shape]toτ_d_i.[shape], whereτ_d_iis the root generic parameter of T.If
τ_d_iis 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_iis a pack,τ_d_i.[shape]might reduce toτ_D_I.[shape]via a shape requirement. In this case,τ_D_Iis the reduced shape of T.Fixes rdar://problem/101813873.