Don't let unknown match arms fall back to !

crates/ra_hir_ty/src/infer/expr.rs

@@ -165,7 +165,11 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             Expr::Match { expr, arms } => {
                 let input_ty = self.infer_expr(*expr, &Expectation::none());
 
-                let mut result_ty = self.table.new_maybe_never_type_var();
+                let mut result_ty = if arms.len() == 0 {
+                    Ty::simple(TypeCtor::Never)
+                } else {
+                    self.table.new_type_var()
+                };
 
                 for arm in arms {
                     let _pat_ty = self.infer_pat(arm.pat, &input_ty, BindingMode::default());

crates/ra_hir_ty/src/tests/never_type.rs

@@ -101,6 +101,7 @@ fn test() {
     );
     assert_eq!(t, "Option<i32>");
 }
+
 #[test]
 fn never_type_can_be_reinferred3() {
     let t = type_at(
@@ -137,6 +138,22 @@ fn test(a: Void) {
     assert_eq!(t, "!");
 }
 
+#[test]
+fn match_unknown_arm() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test(a: Option) {
+    let t = match 0 {
+        _ => unknown,
+    };
+    t<|>;
+}
+"#,
+    );
+    assert_eq!(t, "{unknown}");
+}
+
 #[test]
 fn if_never() {
     let t = type_at(