Add more test cases

crates/compiler/types/src/subs.rs

@@ -5444,12 +5444,16 @@ fn is_inhabited(subs: &Subs, var: Variable) -> bool {
                     }
                 }
                 FlatType::TagUnion(tags, ext) | FlatType::RecursiveTagUnion(_, tags, ext) => {
-                    let mut has_no_tags = true;
+                    let mut is_uninhabited = true;
+                    // If any tag is inhabited, the union is inhabited!
                     for (_tag, vars) in tags.unsorted_iterator(subs, *ext) {
-                        has_no_tags = false;
+                        // Sadly we must recurse here...
-                        stack.extend(vars);
+                        let this_tag_is_inhabited = vars.iter().all(|v| is_inhabited(subs, *v));
+                        if this_tag_is_inhabited {
+                            is_uninhabited = false;
+                        }
                     }
-                    if has_no_tags {
+                    if is_uninhabited {
                         return false;
                     }
                 }

crates/reporting/tests/test_reporting.rs

@@ -10545,10 +10545,60 @@ All branches in an `if` must have the same type!
                 Ok {} -> ""
             "#
         ),
+    // no problem!
     @r###"
     "###
     );
 
+    test_report!(
+        uninhabited_type_is_trivially_exhaustive_nested,
+        indoc!(
+            r#"
+            x : Result (Result [A, B] []) []
+
+            when x is
+                Ok (Ok A) -> ""
+                Ok (Ok B) -> ""
+            "#
+        ),
+    // no problem!
+    @r###"
+    "###
+    );
+
+    test_report!(
+        #[ignore = "TODO https://github.com/roc-lang/roc/issues/4068"]
+        branch_patterns_missing_nested_case,
+        indoc!(
+            r#"
+            x : Result (Result [A, B] {}) {}
+
+            when x is
+                Ok (Ok A) -> ""
+                Err _ -> ""
+            "#
+        ),
+    @r###"
+    TODO
+    "###
+    );
+
+    test_report!(
+        #[ignore = "TODO https://github.com/roc-lang/roc/issues/4068"]
+        branch_patterns_missing_nested_case_with_trivially_exhausted_variant,
+        indoc!(
+            r#"
+            x : Result (Result [A, B] []) []
+
+            when x is
+                Ok (Ok A) -> ""
+            "#
+        ),
+    @r###"
+    TODO
+    "###
+    );
+
     test_report!(
         uninhabited_err_branch_is_redundant_when_err_is_matched,
         indoc!(
@@ -10574,4 +10624,44 @@ All branches in an `if` must have the same type!
     one should be removed.
     "###
     );
+
+    test_report!(
+        uninhabited_err_branch_is_redundant_when_err_is_matched_nested,
+        indoc!(
+            r#"
+            x : Result (Result {} []) []
+
+            when x is
+                Ok (Ok {}) -> ""
+                Ok (Err _) -> ""
+                Err _ -> ""
+            "#
+        ),
+    @r###"
+    ── REDUNDANT PATTERN ───────────────────────────────────── /code/proj/Main.roc ─
+
+    The 2nd pattern is redundant:
+
+    6│       when x is
+    7│           Ok (Ok {}) -> ""
+    8│>          Ok (Err _) -> ""
+    9│           Err _ -> ""
+
+    Any value of this shape will be handled by a previous pattern, so this
+    one should be removed.
+
+    ── REDUNDANT PATTERN ───────────────────────────────────── /code/proj/Main.roc ─
+
+    The 2nd pattern is redundant:
+
+    6│      when x is
+    7│          Ok (Ok {}) -> ""
+    8│          Ok (Err _) -> ""
+    9│          Err _ -> ""
+                ^^^^^
+
+    Any value of this shape will be handled by a previous pattern, so this
+    one should be removed.
+    "###
+    );
 }