From 6cbb732b59d1ded92c7272a8b06f645e1c8acfd2 Mon Sep 17 00:00:00 2001
From: LemonBoy <thatlemon@gmail.com>
Date: Thu, 10 Oct 2019 10:47:59 +0200
Subject: [PATCH] Extern unions should not trigger active field check

Fixes #3378
---
 src/ir.cpp                     |  2 +-
 test/stage1/behavior/union.zig | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/ir.cpp b/src/ir.cpp
index 8fecbf7a18..7dd141423c 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -17951,7 +17951,7 @@ static IrInstruction *ir_analyze_container_field_ptr(IrAnalyze *ira, Buf *field_
                     union_val->special = ConstValSpecialStatic;
                     bigint_init_bigint(&union_val->data.x_union.tag, &field->enum_field->value);
                     union_val->data.x_union.payload = payload_val;
-                } else {
+                } else if (bare_type->data.unionation.layout != ContainerLayoutExtern) {
                     TypeUnionField *actual_field = find_union_field_by_tag(bare_type, &union_val->data.x_union.tag);
                     if (actual_field == nullptr)
                         zig_unreachable();
diff --git a/test/stage1/behavior/union.zig b/test/stage1/behavior/union.zig
index d340a52d1e..f5e9ec09a8 100644
--- a/test/stage1/behavior/union.zig
+++ b/test/stage1/behavior/union.zig
@@ -511,3 +511,13 @@ test "union with comptime_int tag" {
     };
     comptime expect(@TagType(@TagType(Union)) == comptime_int);
 }
+
+test "extern union doesn't trigger field check at comptime" {
+    const U = extern union {
+        x: u32,
+        y: u8,
+    };
+
+    const x = U{ .x = 0x55AAAA55 };
+    comptime expect(x.y == 0x55);
+}