[clang] [clang] Remove `__is_layout_compatible` from revertible type traits list (PR #100572)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll created https://github.com/llvm/llvm-project/pull/100572

`__is_layout_compatible` was added in Clang 19 (#81506), and at that time it wasn't entirely clear whether it should be a revertible type trait or not. We decided to follow the example of other type traits. Since then #95969 happened, and now we know that we don't want new revertible type traits.

This patch removes `__is_layout_compatible` from revertible type traits list, and leaves a comment what revertible type traits are, and that new type traits should not be added there.

The intention is to also cherry-pick this to 19 branch.

>From 21e50b62f14461dcb6c19593b3d21c3e97209b58 Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Thu, 25 Jul 2024 16:32:46 +0300
Subject: [PATCH] [clang] Remove `__is_layout_compatible` from revertible type
 traits list

---
 clang/lib/Parse/ParseExpr.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index 0a017ae79de75..e82b565272831 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -763,6 +763,9 @@ class CastExpressionIdValidator final : public CorrectionCandidateCallback {
 bool Parser::isRevertibleTypeTrait(const IdentifierInfo *II,
                                    tok::TokenKind *Kind) {
   if (RevertibleTypeTraits.empty()) {
+// Revertible type trait is a feature for backwards compatibility with older
+// standard libraries that declare their own structs with the same name as
+// the builtins listed below. New builtins should NOT be added to this list.
 #define RTT_JOIN(X, Y) X##Y
 #define REVERTIBLE_TYPE_TRAIT(Name)                                            \
   RevertibleTypeTraits[PP.getIdentifierInfo(#Name)] = RTT_JOIN(tok::kw_, Name)
@@ -790,7 +793,6 @@ bool Parser::isRevertibleTypeTrait(const IdentifierInfo *II,
     REVERTIBLE_TYPE_TRAIT(__is_fundamental);
     REVERTIBLE_TYPE_TRAIT(__is_integral);
     REVERTIBLE_TYPE_TRAIT(__is_interface_class);
-    REVERTIBLE_TYPE_TRAIT(__is_layout_compatible);
     REVERTIBLE_TYPE_TRAIT(__is_literal);
     REVERTIBLE_TYPE_TRAIT(__is_lvalue_expr);
     REVERTIBLE_TYPE_TRAIT(__is_lvalue_reference);