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Loosen recursion var rank restriction in presence of errors

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Ayaz Hafiz 2022-05-10 11:15:21 -04:00
parent 3497237c99
commit 3de35f7aa2
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2 changed files with 70 additions and 5 deletions
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40
compiler/solve/src/solve.rs
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@ -2685,19 +2685,49 @@ fn adjust_rank_content(
}
}
// THEORY: the recursion var has the same rank as the tag union itself
// The recursion var may have a higher rank than the tag union itself, if it is
// erroneous and escapes into a region where it is let-generalized before it is
// constrained back down to the rank it originated from.
//
// For example, see the `recursion_var_specialization_error` reporting test -
// there, we have
//
// Job a : [ Job (List (Job a)) a ]
//
// job : Job Str
//
// when job is
// Job lst _ -> lst == ""
//
// In this case, `lst` is generalized and has a higher rank for the type
// `(List (Job a)) as a` - notice that only the recursion var `a` is active
// here, not the entire recursive tag union. In the body of this branch, `lst`
// becomes a type error, but the nested recursion var `a` is left untouched,
// because it is nested under the of `lst`, not the surface type that becomes
// an error.
//
// Had this not become a type error, `lst` would then be constrained against
// `job`, and its rank would get pulled back down. So, this can only happen in
// the presence of type errors.
//
// In all other cases, the recursion var has the same rank as the tag union itself
// all types it uses are also in the tags already, so it cannot influence the
// rank
if cfg!(debug_assertions) {
// rank.
if cfg!(debug_assertions)
&& !matches!(
subs.get_content_without_compacting(*rec_var),
Content::Error | Content::FlexVar(..)
)
{
let rec_var_rank =
adjust_rank(subs, young_mark, visit_mark, group_rank, *rec_var);
debug_assert!(
rank >= rec_var_rank,
"rank was {:?} but recursion var {:?} has higher rank {:?}",
"rank was {:?} but recursion var <{:?}>{:?} has higher rank {:?}",
rank,
rec_var,
subs.get_content_without_compacting(*rec_var),
rec_var_rank
);
}