Extern unions should not trigger active field check

Fixes #3378

Author: LemonBoy

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();

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);
+}