disallow alignment on packed union fields

Author: Rexicon226

lib/std/zig/AstGen.zig

@@ -5386,6 +5386,9 @@ fn unionDeclInner(
             return astgen.failNode(member_node, "union field missing type", .{});
         }
         if (member.ast.align_expr.unwrap()) |align_expr| {
+            if (layout == .@"packed") {
+                return astgen.failNode(align_expr, "unable to override alignment of packed union fields", .{});
+            }
             const align_inst = try expr(&block_scope, &block_scope.base, coerced_align_ri, align_expr);
             wip_members.appendToField(@intFromEnum(align_inst));
             any_aligned_fields = true;

test/behavior/type.zig

@@ -433,8 +433,8 @@ test "Type.Union" {
             .layout = .@"packed",
             .tag_type = null,
             .fields = &.{
-                .{ .name = "signed", .type = i32, .alignment = @alignOf(i32) },
-                .{ .name = "unsigned", .type = u32, .alignment = @alignOf(u32) },
+                .{ .name = "signed", .type = i32, .alignment = 0 },
+                .{ .name = "unsigned", .type = u32, .alignment = 0 },
             },
             .decls = &.{},
         },

test/cases/compile_errors/packed_struct_field_alignment_unavailable_for_reify_type.zig

@@ -0,0 +1,9 @@
+const U = packed union {
+    x: f32,
+    y: u8 align(10),
+    z: u32,
+};
+
+// error
+//
+// :3:17: error: unable to override alignment of packed union fields

test/cases/compile_errors/packed_union_alignment_override.zig

@@ -75,4 +75,5 @@ comptime {
 // :16:5: error: tuple field name '3' does not match field index 0
 // :30:5: error: comptime field without default initialization value
 // :44:5: error: extern struct fields cannot be marked comptime
-// :58:5: error: alignment in a packed struct field must be set to 0
+// :58:5: error: alignment of a packed struct field must be set to 0
+