From a8ec306b493ddac701f279bfe82808525956f3f2 Mon Sep 17 00:00:00 2001
From: Jacob Young <jacobly0@users.noreply.github.com>
Date: Sat, 16 Nov 2024 19:40:55 -0500
Subject: [PATCH] Sema: fix peer resolution alignment between slice and empty
 struct

An empty struct that coerces to an empty array should not force
`align(1)` on the resulting slice type.
---
 src/Sema.zig            | 26 +++++++++++++++-----------
 test/behavior/slice.zig |  8 ++++++++
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/src/Sema.zig b/src/Sema.zig
index b99852fb0a..70212e0a63 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -35010,6 +35010,7 @@ fn resolvePeerTypesInner(
             // if there were no actual slices. Else, we want the slice index to report a conflict.
             var opt_slice_idx: ?usize = null;
 
+            var any_abi_aligned = false;
             var opt_ptr_info: ?InternPool.Key.PtrType = null;
             var first_idx: usize = undefined;
             var other_idx: usize = undefined; // We sometimes need a second peer index to report a generic error
@@ -35054,17 +35055,14 @@ fn resolvePeerTypesInner(
                 } };
 
                 // Note that the align can be always non-zero; Type.ptr will canonicalize it
-                ptr_info.flags.alignment = Alignment.min(
-                    if (ptr_info.flags.alignment != .none)
-                        ptr_info.flags.alignment
-                    else
-                        try Type.fromInterned(ptr_info.child).abiAlignmentSema(pt),
-
-                    if (peer_info.flags.alignment != .none)
-                        peer_info.flags.alignment
-                    else
-                        try Type.fromInterned(peer_info.child).abiAlignmentSema(pt),
-                );
+                if (peer_info.flags.alignment == .none) {
+                    any_abi_aligned = true;
+                } else if (ptr_info.flags.alignment == .none) {
+                    any_abi_aligned = true;
+                    ptr_info.flags.alignment = peer_info.flags.alignment;
+                } else {
+                    ptr_info.flags.alignment = ptr_info.flags.alignment.minStrict(peer_info.flags.alignment);
+                }
 
                 if (ptr_info.flags.address_space != peer_info.flags.address_space) {
                     return generic_err;
@@ -35312,6 +35310,12 @@ fn resolvePeerTypesInner(
                 },
             }
 
+            if (any_abi_aligned and opt_ptr_info.?.flags.alignment != .none) {
+                opt_ptr_info.?.flags.alignment = opt_ptr_info.?.flags.alignment.minStrict(
+                    try Type.fromInterned(pointee).abiAlignmentSema(pt),
+                );
+            }
+
             return .{ .success = try pt.ptrTypeSema(opt_ptr_info.?) };
         },
 
diff --git a/test/behavior/slice.zig b/test/behavior/slice.zig
index 15db19f1e8..4f5367ad85 100644
--- a/test/behavior/slice.zig
+++ b/test/behavior/slice.zig
@@ -995,3 +995,11 @@ test "sentinel-terminated 0-length slices" {
     try expect(comptime_known_array_value[0] == 2);
     try expect(runtime_array_value[0] == 2);
 }
+
+test "peer slices keep abi alignment with empty struct" {
+    var cond: bool = undefined;
+    cond = false;
+    const slice = if (cond) &[1]u32{42} else &.{};
+    comptime assert(@TypeOf(slice) == []const u32);
+    try expect(slice.len == 0);
+}