From 31ed8d293ddb79d34306e0b23e76d8a361c8856f Mon Sep 17 00:00:00 2001
From: Veikka Tuominen <git@vexu.eu>
Date: Thu, 2 Feb 2023 14:39:13 +0200
Subject: [PATCH] Sema: add missing peer type resolution for error unions

Closes #14077
---
 src/Sema.zig           | 19 +++++++++++++++++++
 test/behavior/cast.zig | 12 ++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/src/Sema.zig b/src/Sema.zig
index 2dba678931..ddcdecb6b4 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -29761,6 +29761,25 @@ fn resolvePeerTypes(
                     continue;
                 }
             },
+            .ErrorSet => {
+                chosen = candidate;
+                chosen_i = candidate_i + 1;
+                if (err_set_ty) |chosen_set_ty| {
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(block, chosen_set_ty, chosen_ty, src, src)) {
+                        continue;
+                    }
+                    if (.ok == try sema.coerceInMemoryAllowedErrorSets(block, chosen_ty, chosen_set_ty, src, src)) {
+                        err_set_ty = chosen_ty;
+                        continue;
+                    }
+
+                    err_set_ty = try chosen_set_ty.errorSetMerge(sema.arena, chosen_ty);
+                    continue;
+                } else {
+                    err_set_ty = chosen_ty;
+                    continue;
+                }
+            },
             else => {},
         }
 
diff --git a/test/behavior/cast.zig b/test/behavior/cast.zig
index dbb4c07f64..30889aef04 100644
--- a/test/behavior/cast.zig
+++ b/test/behavior/cast.zig
@@ -1541,3 +1541,15 @@ test "single item pointer to pointer to array to slice" {
     const z1 = @as([]const i32, @as(*[1]i32, &x));
     try expect(z1[0] == 1234);
 }
+
+test "peer type resolution forms error union" {
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+
+    var foo: i32 = 123;
+    const result = if (foo < 0) switch (-foo) {
+        0 => unreachable,
+        42 => error.AccessDenied,
+        else => unreachable,
+    } else @intCast(u32, foo);
+    try expect(try result == 123);
+}