Constraining of list patterns

crates/reporting/src/error/type.rs

@@ -1945,8 +1945,8 @@ fn to_pattern_report<'b>(
                     severity: Severity::RuntimeError,
                 }
             }
-            PReason::TagArg { .. } | PReason::PatternGuard => {
-                unreachable!("I didn't think this could trigger. Please tell Folkert about it!")
+            PReason::TagArg { .. } | PReason::PatternGuard | PReason::ListElem => {
+                internal_error!("We didn't think this could trigger. Please tell us about it on Zulip if it does!")
             }
         },
     }

crates/reporting/tests/test_reporting.rs

@@ -11821,4 +11821,30 @@ All branches in an `if` must have the same type!
     I would have to crash if I saw one of those! Add branches for them!
     "###
     );
+
+    test_report!(
+        #[ignore = "must implement exhaustiveness sketching for lists first"]
+        mismatch_within_list_pattern,
+        indoc!(
+            r#"
+            when [] is
+                [A, 1u8] -> ""
+            "#
+        ),
+    @r###"
+    "###
+    );
+
+    test_report!(
+        #[ignore = "must implement exhaustiveness sketching for lists first"]
+        mismatch_list_pattern_vs_condition,
+        indoc!(
+            r#"
+            when [A, B] is
+                ["foo", "bar"] -> ""
+            "#
+        ),
+    @r###"
+    "###
+    );
 }