From f2d13f6d074139c01184e170fe24127a2d9db944 Mon Sep 17 00:00:00 2001
From: David Rubin <sinon@vortan.dev>
Date: Tue, 9 Jun 2026 18:58:47 -0700
Subject: [PATCH] Sema: preserve union alignment to field on switch

---
 src/Sema.zig             | 15 ++++++---------
 test/behavior/switch.zig | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index d5ca3362eff259d909c3cbd8236923ceab9291c6..b27d452b7dd6b4223c569bc0b22d838e6adeda16 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -12083,15 +12083,12 @@ fn resolveSwitchPayloadCaptureTaggedUnion(
     const field_ty: Type = .fromInterned(union_obj.field_types.get(ip)[field_index]);
     const payload_ref: Air.Inst.Ref = payload_ref: {
         if (capture_by_ref) {
-            const operand_ptr_info = sema.typeOf(loaded_operand).ptrInfo(zcu);
-            const ptr_field_ty = try pt.ptrType(.{
-                .child = field_ty.toIntern(),
-                .flags = .{
-                    .is_const = operand_ptr_info.flags.is_const,
-                    .is_volatile = operand_ptr_info.flags.is_volatile,
-                    .address_space = operand_ptr_info.flags.address_space,
-                },
-            });
+            const ptr_field_ty = try sema.typeOf(loaded_operand).fieldPtrType(field_index, pt);
+            if (try sema.resolveDefinedValue(case_block, operand_src, loaded_operand)) |op_ptr_val| {
+                if (op_ptr_val.isUndef(zcu)) break :payload_ref try pt.undefRef(ptr_field_ty);
+                const field_ptr_val = try op_ptr_val.ptrField(field_index, pt);
+                break :payload_ref .fromValue(try pt.getCoerced(field_ptr_val, ptr_field_ty));
+            }
             break :payload_ref try case_block.addStructFieldPtr(loaded_operand, field_index, ptr_field_ty);
         }
         if (try sema.resolveDefinedValue(case_block, operand_src, loaded_operand)) |union_val| {
diff --git a/test/behavior/switch.zig b/test/behavior/switch.zig
index e9923bac27db1c6060d3485daed18ac20d271f47..e56da92324e578190ea4119e04b5379abf687c2d 100644
--- a/test/behavior/switch.zig
+++ b/test/behavior/switch.zig
@@ -1574,3 +1574,22 @@ test "repeated switch analysis overrides previous analysis results" {
         };
     }
 }
+
+test "union field pointer capture preserves alignment in inline prong" {
+    const U = union(enum) {
+        a: u32,
+        b: u32,
+        fn doTheTest(u: *align(1) const @This()) !void {
+            switch (u.*) {
+                inline .a, .b => |*a_ptr| {
+                    comptime assert(@TypeOf(a_ptr) == *align(1) const u32);
+                    try expect(a_ptr.* == 123);
+                },
+            }
+        }
+    };
+    try U.doTheTest(&.{ .a = 123 });
+    try U.doTheTest(&.{ .b = 123 });
+    try comptime U.doTheTest(&.{ .a = 123 });
+    try comptime U.doTheTest(&.{ .b = 123 });
+}
-- 
2.54.0