diff --git a/crates/ra_hir_ty/src/expr.rs b/crates/ra_hir_ty/src/expr.rs
index 19323fda12..1e7395b164 100644
--- a/crates/ra_hir_ty/src/expr.rs
+++ b/crates/ra_hir_ty/src/expr.rs
@@ -88,6 +88,11 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
         let mut seen = Matrix::empty();
         for pat in pats {
             // We skip any patterns whose type we cannot resolve.
+            //
+            // This could lead to false positives in this diagnostic, so
+            // it might be better to skip the entire diagnostic if we either
+            // cannot resolve a match arm or determine that the match arm has
+            // the wrong type.
             if let Some(pat_ty) = infer.type_of_pat.get(pat) {
                 // We only include patterns whose type matches the type
                 // of the match expression. If we had a InvalidMatchArmPattern
@@ -97,6 +102,8 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
                 // When comparing the types, we also have to consider that rustc
                 // will automatically de-reference the match expression type if
                 // necessary.
+                //
+                // FIXME we should use the type checker for this.
                 if pat_ty == match_expr_ty
                     || match_expr_ty
                         .as_reference()