From 0ac2984a154f1a3f47dd054e2522b8aaee9b2cd9 Mon Sep 17 00:00:00 2001
From: Karl Meakin <karlwfmeakin@gmail.com>
Date: Tue, 17 Jan 2023 05:40:33 +0000
Subject: [PATCH] Don't report check reachability for patterns with errors

---
 fathom/src/surface/elaboration/patterns.rs                | 8 ++++++++
 fathom/src/surface/elaboration/patterns/coverage.rs       | 8 +++++++-
 tests/fail/elaboration/boolean-literal/not-supported.snap | 6 ------
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/fathom/src/surface/elaboration/patterns.rs b/fathom/src/surface/elaboration/patterns.rs
index d441cab39..0e0747785 100644
--- a/fathom/src/surface/elaboration/patterns.rs
+++ b/fathom/src/surface/elaboration/patterns.rs
@@ -63,6 +63,10 @@ impl<'arena> CheckedPattern<'arena> {
             CheckedPattern::RecordLit(_, _, _) => false,
         }
     }
+
+    pub fn is_error(&self) -> bool {
+        matches!(self, CheckedPattern::ReportedError(_))
+    }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
@@ -625,6 +629,10 @@ impl<'arena> PatRow<'arena> {
     pub fn append(&mut self, mut other: Self) {
         self.pairs.append(&mut other.pairs);
     }
+
+    pub fn patterns(&self) -> impl ExactSizeIterator<Item = &CheckedPattern<'arena>> {
+        self.pairs.iter().map(|(pattern, _)| pattern)
+    }
 }
 
 impl<'arena> CheckedPattern<'arena> {
diff --git a/fathom/src/surface/elaboration/patterns/coverage.rs b/fathom/src/surface/elaboration/patterns/coverage.rs
index adb50c89d..93fd43fd3 100644
--- a/fathom/src/surface/elaboration/patterns/coverage.rs
+++ b/fathom/src/surface/elaboration/patterns/coverage.rs
@@ -36,13 +36,19 @@ pub fn check_coverage<'arena>(
     let mut rows = Vec::with_capacity(matrix.num_rows());
     for (row, _) in matrix.iter() {
         let matrix = PatMatrix::new(rows.clone());
+        rows.push(row.clone());
+
+        // Don't check reachability for patterns with errors
+        if row.patterns().any(|pattern| pattern.is_error()) {
+            continue;
+        }
+
         if !is_useful(ctx, &matrix, row) {
             let range = row.first().unwrap().0.range();
             ctx.push_message(Message::UnreachablePattern {
                 range: ctx.file_range(range),
             });
         }
-        rows.push(row.clone());
     }
 }
 
diff --git a/tests/fail/elaboration/boolean-literal/not-supported.snap b/tests/fail/elaboration/boolean-literal/not-supported.snap
index 666177fa8..0b362cde7 100644
--- a/tests/fail/elaboration/boolean-literal/not-supported.snap
+++ b/tests/fail/elaboration/boolean-literal/not-supported.snap
@@ -12,10 +12,4 @@ error: boolean literal not supported for expected type
 7 │     true => 1,
   │     ^^^^
 
-warning: unreachable pattern
-  ┌─ tests/fail/elaboration/boolean-literal/not-supported.fathom:7:5
-  │
-7 │     true => 1,
-  │     ^^^^
-
 '''