type: handle the 0-length array case in abiSizeAdvanced

This fixes a panic in `unionAbiSize` when a 0-length array of a union is used as a struct field.

Because `resolveTypeLayout` does not resolve the `elem_ty` if `arrayLenIncludingSentinel` returns
0 for the array, the child union type is not guaranteed to have a resolved layout at this point.

Fixed this case by just returning 0 here.

1 files changed, 18 insertions, 0 deletions

diff --git a/test/behavior/struct.zig b/test/behavior/struct.zig
index 393f23ec6e..f6fb45f46b 100644
--- a/test/behavior/struct.zig
+++ b/test/behavior/struct.zig
@@ -995,6 +995,24 @@ test "struct with union field" {
     try expect(True.kind.Bool);
 }
 
+test "struct with 0-length union array field" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+
+    const U = union {
+        a: u32,
+        b: u64,
+    };
+
+    const S = struct {
+        zero_length: [0]U,
+    };
+
+    var s: S = undefined;
+    try expectEqual(@as(usize, 0), s.zero_length.len);
+}
+
 test "type coercion of anon struct literal to struct" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO