From 0bcf29aff6e4ed536406634f03f7bf61eda1a2b8 Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Wed, 6 May 2026 10:37:54 +0100
Subject: [PATCH] compiler: correct ABI size of comptime-only optional type

Resolves: https://codeberg.org/ziglang/zig/issues/31603
---
 src/Type.zig             | 19 ++++++++++++-------
 test/behavior/struct.zig | 13 +++++++++++++
 2 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/src/Type.zig b/src/Type.zig
index 8e8202c023d9d95832f1285839a4654380a23016..4960f60c22520f196e3b3ad3ef31631e33536ddd 100644
--- a/src/Type.zig
+++ b/src/Type.zig
@@ -1094,13 +1094,18 @@ pub fn abiSize(ty: Type, zcu: *const Zcu) u64 {
         },
         .opt_type => |child_ty_ip| {
             const child_ty: Type = .fromInterned(child_ty_ip);
-            if (child_ty.classify(zcu) == .no_possible_value) return 0;
-            if (ty.optionalReprIsPayload(zcu)) return child_ty.abiSize(zcu);
-            // Optional types are represented as a struct with the child type as the first
-            // field and a boolean as the second. Since the child type's abi alignment is
-            // guaranteed to be >= that of bool's (1 byte) the added size is exactly equal
-            // to the child type's ABI alignment.
-            return child_ty.abiSize(zcu) + child_ty.abiAlignment(zcu).toByteUnits().?;
+            switch (child_ty.classify(zcu)) {
+                .no_possible_value => return 0, // we are OPV
+                .fully_comptime => return 0, // we are also fully_comptime (same justification as error unions, see below)
+                .one_possible_value, .partially_comptime, .runtime => {
+                    if (ty.optionalReprIsPayload(zcu)) return child_ty.abiSize(zcu);
+                    // Optional types are represented as a struct with the child type as the first
+                    // field and a boolean as the second. Since the child type's abi alignment is
+                    // guaranteed to be >= that of bool's (1 byte) the added size is exactly equal
+                    // to the child type's ABI alignment.
+                    return child_ty.abiSize(zcu) + child_ty.abiAlignment(zcu).toByteUnits().?;
+                },
+            }
         },
         .error_set_type, .inferred_error_set_type => errorAbiSize(zcu),
         .error_union_type => |error_union| {
diff --git a/test/behavior/struct.zig b/test/behavior/struct.zig
index 973eab39c8d3912d36774375af1da268b36ee438..831f994ed0a36d1a295c48341c9c2c6bcda6d4ff 100644
--- a/test/behavior/struct.zig
+++ b/test/behavior/struct.zig
@@ -2301,3 +2301,16 @@ test "struct queries typeinfo of struct containing pointer back to first struct"
     };
     _ = @as(static.A, undefined);
 }
+
+test "pointer to runtime field of struct containing struct containing comptime-only optional" {
+    const Foo = struct {
+        padding: struct { a: u8, b: ?comptime_int },
+        number: u8,
+    };
+
+    const foo: Foo = .{ .padding = undefined, .number = 123 };
+
+    var ptr: *const u8 = undefined;
+    ptr = &foo.number;
+    try expect(ptr.* == 123);
+}
-- 
2.54.0