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Basic support for match_first declarations. (#2523)

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Add basic support for `match_first` declarations to explorer, as described in https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/generics/details.md#prioritization-rule. The concrete syntax here is not yet approved, so `match_first` is renamed to `__match_first` for now, but the semantics are necessary to implement other approved features in explorer so we're intentionally getting a little ahead of the design here.
This commit is contained in:
Richard Smith
2023-01-17 13:08:38 -08:00
committed by GitHub
parent 08a289753c
commit 74aae0911f
16 changed files with 261 additions and 4 deletions
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explorer/interpreter/impl_scope.cpp
+20
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@@ -215,6 +215,7 @@ static auto CombineResults(Nonnull<const InterfaceType*> iface_type,
if (!a) {
return b;
}
// If either of them was a symbolic result, then they'll end up being
// equivalent. In that case, pick `a`.
const auto* impl_a = dyn_cast<ImplWitness>(*a);
@@ -225,13 +226,32 @@ static auto CombineResults(Nonnull<const InterfaceType*> iface_type,
if (!impl_a) {
return b;
}
// If they refer to the same `impl` declaration, it doesn't matter which one
// we pick, so we pick `a`.
// TODO: Compare the identities of the `impl`s, not the declarations.
if (&impl_a->declaration() == &impl_b->declaration()) {
return a;
}
// TODO: Order the `impl`s based on type structure.
// If the declarations appear in the same `match_first` block, whichever
// appears first wins.
// TODO: Once we support an impl being declared more than once, we will need
// to check this more carefully.
if (impl_a->declaration().match_first() &&
impl_a->declaration().match_first() ==
impl_b->declaration().match_first()) {
for (auto* impl : (*impl_a->declaration().match_first())->impls()) {
if (impl == &impl_a->declaration()) {
return a;
}
if (impl == &impl_b->declaration()) {
return b;
}
}
}
return ProgramError(source_loc)
<< "ambiguous implementations of " << *iface_type << " for " << *type;
}