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Allow uninhabited type extension to happen on either unification side

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Ayaz Hafiz 2022-09-20 14:20:47 -05:00
parent 49ba8b92b4
commit f96c825aa4
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2 changed files with 22 additions and 23 deletions
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30
crates/compiler/unify/src/unify.rs
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@ -2096,10 +2096,6 @@ enum Rec {
///
/// NB: the tag union extension to grow should be `ext1`, and the candidate
/// NB: uninhabited type should be `candidate_type`.
/// NB:
/// NB: as such, it is generally expected that
/// NB: - the BRANCH TYPE is passed on the LEFT
/// NB: - the CONDITION TYPE is passed on the RIGHT
/// of the variables under unification
fn should_extend_ext_with_uninhabited_type(
subs: &Subs,
@ -2123,7 +2119,7 @@ fn unify_tag_unions<M: MetaCollector>(
initial_ext2: Variable,
recursion_var: Rec,
) -> Outcome<M> {
let (separate, mut ext1, ext2) =
let (separate, mut ext1, mut ext2) =
separate_union_tags(env.subs, tags1, initial_ext1, tags2, initial_ext2);
let shared_tags = separate.in_both;
@ -2222,15 +2218,29 @@ fn unify_tag_unions<M: MetaCollector>(
shared_tags_outcome
}
} else if separate.only_in_2.is_empty() {
let unique_tags1 = UnionTags::insert_slices_into_subs(env.subs, separate.only_in_1);
let flat_type = FlatType::TagUnion(unique_tags1, ext1);
let sub_record = fresh(env, pool, ctx, Structure(flat_type));
let extra_tags_in_1 = {
let unique_tags1 = UnionTags::insert_slices_into_subs(env.subs, separate.only_in_1);
let flat_type = FlatType::TagUnion(unique_tags1, ext1);
fresh(env, pool, ctx, Structure(flat_type))
};
let mut total_outcome = Outcome::default();
// In a presence context, we don't care about ext2 being equal to tags1
if ctx.mode.is_eq() {
let ext_outcome = unify_pool(env, pool, sub_record, ext2, ctx.mode);
// SPECIAL-CASE: if we can grow empty extensions with uninhabited types,
// patch `ext2` to grow accordingly.
if should_extend_ext_with_uninhabited_type(env.subs, ext2, extra_tags_in_1) {
let new_ext = fresh(env, pool, ctx, Content::FlexVar(None));
let new_union = Structure(FlatType::TagUnion(tags2, new_ext));
let mut new_desc = ctx.second_desc;
new_desc.content = new_union;
env.subs.set(ctx.second, new_desc);
ext2 = new_ext;
}
let ext_outcome = unify_pool(env, pool, extra_tags_in_1, ext2, ctx.mode);
if !ext_outcome.mismatches.is_empty() {
return ext_outcome;
@ -2244,7 +2254,7 @@ fn unify_tag_unions<M: MetaCollector>(
ctx,
shared_tags,
OtherTags2::Empty,
sub_record,
extra_tags_in_1,
recursion_var,
);
total_outcome.union(shared_tags_outcome);