From fca51c81bc5fd83a3d4a87092696df80790003e9 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Fri, 11 Mar 2022 11:53:01 -0800
Subject: [PATCH] stage2: is_non_error always returns comptime true for empty
 error set

---
 src/Sema.zig            | 9 +++++++++
 test/behavior/error.zig | 6 +++++-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index 00553710df..44b2f33d56 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -18144,6 +18144,15 @@ fn analyzeIsNonErr(
     if (ot != .ErrorSet and ot != .ErrorUnion) return Air.Inst.Ref.bool_true;
     if (ot == .ErrorSet) return Air.Inst.Ref.bool_false;
     assert(ot == .ErrorUnion);
+
+    // exception if the error union error set is known to be empty,
+    // we allow the comparison but always make it comptime known.
+    const set_ty = operand_ty.errorUnionSet();
+    switch (set_ty.tag()) {
+        .anyerror, .error_set_inferred => {},
+        else => if (set_ty.errorSetNames().len == 0) return Air.Inst.Ref.bool_true,
+    }
+
     const result_ty = Type.bool;
     if (try sema.resolveMaybeUndefVal(block, src, operand)) |err_union| {
         if (err_union.isUndef()) {
diff --git a/test/behavior/error.zig b/test/behavior/error.zig
index 73e03b1c3e..0a55b34121 100644
--- a/test/behavior/error.zig
+++ b/test/behavior/error.zig
@@ -215,7 +215,11 @@ fn testExplicitErrorSetCast(set1: Set1) !void {
 }
 
 test "comptime test error for empty error set" {
-    if (builtin.zig_backend != .stage1) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
 
     try testComptimeTestErrorEmptySet(1234);
     comptime try testComptimeTestErrorEmptySet(1234);