Check in some more work

crates/compiler/can/src/exhaustive.rs

@@ -3,7 +3,8 @@ use crate::pattern::DestructType;
 use roc_collections::all::HumanIndex;
 use roc_error_macros::internal_error;
 use roc_exhaustive::{
-    is_useful, Ctor, CtorName, Error, Guard, Literal, Pattern, RenderAs, TagId, Union,
+    is_useful, Ctor, CtorName, Error, Guard, Literal, Pattern, RedundantReason, RenderAs, TagId,
+    Union,
 };
 use roc_module::ident::{TagIdIntType, TagName};
 use roc_module::symbol::ModuleId;
@@ -338,27 +339,31 @@ fn to_nonredundant_rows(subs: &Subs, rows: SketchedRows) -> NonRedundantSummary
             .map(|pattern| pattern.reify(subs))
             .collect();
 
-        let is_useful = {
+        let is_redundant = if !is_inhabited_row(&next_row) {
-            is_inhabited_row(&next_row)
+            Some(RedundantReason::Uninhabited)
-                && (
+        } else if !(matches!(guard, Guard::HasGuard)
-                    // Anything inhabited with a guard is necessary
+            || is_useful(checked_rows.clone(), next_row.clone()))
-                    matches!(guard, Guard::HasGuard)
+        {
-                    // Make sure that the row doesn't match something we already covered
+            Some(RedundantReason::PreviouslyCovered)
-                        || is_useful(checked_rows.clone(), next_row.clone())
+        } else {
-                )
+            None
         };
 
-        if is_useful {
+        match is_redundant {
+            None => {
                 checked_rows.push(next_row);
-        } else {
+            }
+            Some(reason) => {
                 redundancies.push(redundant_mark);
                 errors.push(Error::Redundant {
                     overall_region,
                     branch_region: region,
                     index: HumanIndex::zero_based(checked_rows.len()),
+                    reason,
                 });
             }
         }
+    }
 
     NonRedundantSummary {
         non_redundant_rows: checked_rows,

crates/compiler/exhaustive/src/lib.rs

@@ -92,9 +92,16 @@ pub enum Error {
         overall_region: Region,
         branch_region: Region,
         index: HumanIndex,
+        reason: RedundantReason,
     },
 }
 
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum RedundantReason {
+    PreviouslyCovered,
+    Uninhabited,
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Context {
     BadArg,

crates/compiler/solve/tests/solve_expr.rs

@@ -7775,4 +7775,20 @@ mod solve_expr {
             "{}",
         );
     }
+
+    #[test]
+    fn match_on_result_with_uninhabited_error_branch() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                x : Result Str []
+                x = Ok "abc"
+
+                when x is
+                    Ok s -> s
+                "#
+            ),
+            "",
+        );
+    }
 }

crates/compiler/test_gen/src/gen_tags.rs

@@ -1064,12 +1064,8 @@ fn result_never() {
                 res : Result I64 []
                 res = Ok 4
 
-                # we should provide this in the stdlib
-                never : [] -> a
-
                 when res is
                     Ok v -> v
-                    Err empty -> never empty
                 #"
         ),
         4,
@@ -1990,3 +1986,21 @@ fn fit_recursive_union_in_struct_into_recursive_pointer() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn match_on_result_with_uninhabited_error_branch() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            x : Result Str []
+            x = Ok "abc"
+
+            when x is
+                Ok s -> s
+            "#
+        ),
+        RocStr::from("abc"),
+        RocStr
+    );
+}

crates/compiler/test_mono/src/tests.rs

@@ -1936,3 +1936,16 @@ fn num_width_gt_u8_layout_as_float() {
         "#
     )
 }
+
+#[mono_test]
+fn match_on_result_with_uninhabited_error_branch() {
+    indoc!(
+        r#"
+        x : Result Str []
+        x = Ok "abc"
+
+        when x is
+            Ok s -> s
+        "#
+    )
+}

crates/reporting/src/error/type.rs

@@ -3889,6 +3889,7 @@ fn exhaustive_problem<'a>(
             overall_region,
             branch_region,
             index,
+            reason: _,
         } => {
             let doc = alloc.stack([
                 alloc.concat([