Emit runtime error when tag unions have an error type

Closes #3266

crates/ast/src/solve_type.rs

@@ -1084,7 +1084,8 @@ fn type_to_union_tags<'a>(
 
     let ext = {
         let (it, ext) =
-            roc_types::types::gather_tags_unsorted_iter(subs, UnionTags::default(), temp_ext_var);
+            roc_types::types::gather_tags_unsorted_iter(subs, UnionTags::default(), temp_ext_var)
+                .expect("not a tag union");
 
         tag_vars.extend(it.map(|(n, v)| (n.clone(), v)));
         tag_vars.sort_unstable_by(|(a, _), (b, _)| a.cmp(b));

crates/compiler/mono/src/layout.rs

@@ -3056,7 +3056,9 @@ pub fn ext_var_is_empty_tag_union(subs: &Subs, ext_var: Variable) -> bool {
     // the ext_var is empty
     let mut ext_fields = std::vec::Vec::new();
     match roc_types::pretty_print::chase_ext_tag_union(subs, ext_var, &mut ext_fields) {
-        Ok(()) | Err((_, Content::FlexVar(_) | Content::RigidVar(_))) => ext_fields.is_empty(),
+        Ok(()) | Err((_, Content::FlexVar(_) | Content::RigidVar(_) | Content::Error)) => {
+            ext_fields.is_empty()
+        }
         Err(content) => panic!("invalid content in ext_var: {:?}", content),
     }
 }

crates/compiler/solve/src/solve.rs

@@ -2807,7 +2807,8 @@ fn type_to_variable<'a>(
                     subs,
                     UnionTags::default(),
                     temp_ext_var,
-                );
+                )
+                .expect("extension var could not be seen as a tag union");
 
                 for _ in it {
                     unreachable!("we assert that the ext var is empty; otherwise we'd already know it was a tag union!");
@@ -3351,7 +3352,8 @@ fn type_to_union_tags<'a>(
                 subs,
                 UnionTags::default(),
                 temp_ext_var,
-            );
+            )
+            .expect("extension var could not be seen as tag union");
 
             tag_vars.extend(it.map(|(n, v)| (n.clone(), v)));
 

crates/compiler/test_gen/src/gen_tags.rs

@@ -1861,3 +1861,27 @@ fn issue_3560_newtype_tag_constructor_has_nested_constructor_with_no_payload() {
         RocStr
     )
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+#[should_panic(expected = r#"Roc failed with message: "Erroneous: Expr::Closure""#)]
+fn error_type_in_tag_union_payload() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            f : ([] -> Bool) -> Bool
+            f = \fun ->
+              if True then
+                fun 42
+              else
+                False
+
+            f (\x -> x)
+            "#
+        ),
+        0,
+        u8,
+        |x| x,
+        true // ignore type errors
+    )
+}

crates/compiler/types/src/subs.rs

@@ -2702,7 +2702,8 @@ impl UnionTags {
         subs: &'a Subs,
         ext: Variable,
     ) -> impl Iterator<Item = (&TagName, &[Variable])> + 'a {
-        let (it, _) = crate::types::gather_tags_unsorted_iter(subs, *self, ext);
+        let (it, _) =
+            crate::types::gather_tags_unsorted_iter(subs, *self, ext).expect("not a tag union");
 
         let f = move |(label, slice): (_, SubsSlice<Variable>)| (label, subs.get_subs_slice(slice));
 
@@ -2715,7 +2716,8 @@ impl UnionTags {
         subs: &'a Subs,
         ext: Variable,
     ) -> (UnsortedUnionLabels<'a, TagName>, Variable) {
-        let (it, ext) = crate::types::gather_tags_unsorted_iter(subs, *self, ext);
+        let (it, ext) =
+            crate::types::gather_tags_unsorted_iter(subs, *self, ext).expect("not a tag union");
         let f = move |(label, slice): (_, SubsSlice<Variable>)| (label, subs.get_subs_slice(slice));
         let it = it.map(f);
 
@@ -2741,7 +2743,8 @@ impl UnionTags {
                 ext,
             )
         } else {
-            let union_structure = crate::types::gather_tags(subs, *self, ext);
+            let union_structure =
+                crate::types::gather_tags(subs, *self, ext).expect("not a tag union");
 
             (
                 Box::new(union_structure.fields.into_iter()),
@@ -2767,7 +2770,8 @@ impl UnionTags {
                 ext,
             )
         } else {
-            let (fields, ext) = crate::types::gather_tags_slices(subs, *self, ext);
+            let (fields, ext) =
+                crate::types::gather_tags_slices(subs, *self, ext).expect("not a tag union");
 
             (Box::new(fields.into_iter()), ext)
         }

crates/compiler/types/src/types.rs

@@ -2674,14 +2674,28 @@ pub fn gather_fields(
     })
 }
 
+#[derive(Debug)]
+pub enum GatherTagsError {
+    NotATagUnion(Variable),
+}
+
+/// Gathers tag payloads of a type, assuming it is a tag.
+///
+/// If the given type is unbound or an error, no payloads are returned.
+///
+/// If the given type cannot be seen as a tag, unbound type, or error, this
+/// function returns an error.
 pub fn gather_tags_unsorted_iter(
     subs: &Subs,
     other_fields: UnionTags,
     mut var: Variable,
-) -> (
+) -> Result<
+    (
         impl Iterator<Item = (&TagName, VariableSubsSlice)> + '_,
         Variable,
-) {
+    ),
+    GatherTagsError,
+> {
     use crate::subs::Content::*;
     use crate::subs::FlatType::*;
 
@@ -2711,20 +2725,18 @@ pub fn gather_tags_unsorted_iter(
             }
 
             Alias(_, _, actual_var, _) => {
-                // TODO according to elm/compiler: "TODO may be dropping useful alias info here"
                 var = *actual_var;
             }
 
             Structure(EmptyTagUnion) => break,
             FlexVar(_) => break,
 
-            // TODO investigate this likely can happen when there is a type error
+            // TODO investigate, this likely can happen when there is a type error
             RigidVar(_) => break,
 
-            other => unreachable!(
+            Error => break,
-                "something weird ended up in a tag union type: {:?} at {:?}",
+
-                other, var
+            _ => return Err(GatherTagsError::NotATagUnion(var)),
-            ),
         }
     }
 
@@ -2738,15 +2750,15 @@ pub fn gather_tags_unsorted_iter(
             (tag_name, subs_slice)
         });
 
-    (it, var)
+    Ok((it, var))
 }
 
 pub fn gather_tags_slices(
     subs: &Subs,
     other_fields: UnionTags,
     var: Variable,
-) -> (Vec<(TagName, VariableSubsSlice)>, Variable) {
+) -> Result<(Vec<(TagName, VariableSubsSlice)>, Variable), GatherTagsError> {
-    let (it, ext) = gather_tags_unsorted_iter(subs, other_fields, var);
+    let (it, ext) = gather_tags_unsorted_iter(subs, other_fields, var)?;
 
     let mut result: Vec<_> = it
         .map(|(ref_label, field): (_, VariableSubsSlice)| (ref_label.clone(), field))
@@ -2754,11 +2766,15 @@ pub fn gather_tags_slices(
 
     result.sort_by(|(a, _), (b, _)| a.cmp(b));
 
-    (result, ext)
+    Ok((result, ext))
 }
 
-pub fn gather_tags(subs: &Subs, other_fields: UnionTags, var: Variable) -> TagUnionStructure {
+pub fn gather_tags(
-    let (it, ext) = gather_tags_unsorted_iter(subs, other_fields, var);
+    subs: &Subs,
+    other_fields: UnionTags,
+    var: Variable,
+) -> Result<TagUnionStructure, GatherTagsError> {
+    let (it, ext) = gather_tags_unsorted_iter(subs, other_fields, var)?;
 
     let mut result: Vec<_> = it
         .map(|(ref_label, field): (_, VariableSubsSlice)| {
@@ -2768,10 +2784,10 @@ pub fn gather_tags(subs: &Subs, other_fields: UnionTags, var: Variable) -> TagUn
 
     result.sort_by(|(a, _), (b, _)| a.cmp(b));
 
-    TagUnionStructure {
+    Ok(TagUnionStructure {
         fields: result,
         ext,
-    }
+    })
 }
 
 fn instantiate_lambda_sets_as_unspecialized(