[libc++] Fix triviality of std::pair for trivially copyable types without an assignment operator (#95444)

Since 83ead2b, std::pair would not be trivially copyable when it holds a
trivially copyable type without an assignment operator. That is because
pair gained an elligible copy-assignment-operator (the const version) in
83ead2b in C++ >= 23.

This means that the trivially copyable property of std::pair for such
types would be inconsistent between C++11/14/17/20 (trivially copyable)
and C++23/26 (not trivially copyable). This patch makes std::pair's
behavior consistent in all Standard modes EXCEPT C++03, which is a
pre-existing condition and we have no way of changing (also, it
shouldn't matter because the std::is_trivially_copyable trait was
introduced in C++11).

While this is not technically an ABI break, in practice we do know that
folks sometimes use a different representation based on whether a type
is trivially copyable. So we're treating 83ead2b as an ABI break and
this patch is fixing said breakage.

This patch also adds tests stolen from #89652 that pin down the ABI of
std::pair with respect to being trivially copyable.

Fixes #95428

2 files changed, 5 insertions, 4 deletions

diff --git a/libcxx/utils/libcxx/test/dsl.py b/libcxx/utils/libcxx/test/dsl.py
index 7ac66d4..624a00d 100644
--- a/libcxx/utils/libcxx/test/dsl.py
+++ b/libcxx/utils/libcxx/test/dsl.py
@@ -308,8 +308,8 @@ def compilerMacros(config, flags=""):
         with open(test.getSourcePath(), "w") as sourceFile:
             sourceFile.write(
                 """
-      #if __has_include(<__config_site>)
-      #  include <__config_site>
+      #if __has_include(<__config>)
+      #  include <__config>
       #endif
       """
             )
diff --git a/libcxx/utils/libcxx/test/features.py b/libcxx/utils/libcxx/test/features.py
index 7a9631a..28c9c35 100644
--- a/libcxx/utils/libcxx/test/features.py
+++ b/libcxx/utils/libcxx/test/features.py
@@ -335,10 +335,10 @@ DEFAULT_FEATURES = [
 ]
 
 # Deduce and add the test features that that are implied by the #defines in
-# the <__config_site> header.
+# the <__config> header.
 #
 # For each macro of the form `_LIBCPP_XXX_YYY_ZZZ` defined below that
-# is defined after including <__config_site>, add a Lit feature called
+# is defined after including <__config>, add a Lit feature called
 # `libcpp-xxx-yyy-zzz`. When a macro is defined to a specific value
 # (e.g. `_LIBCPP_ABI_VERSION=2`), the feature is `libcpp-xxx-yyy-zzz=<value>`.
 #
@@ -352,6 +352,7 @@ macros = {
     "_LIBCPP_NO_VCRUNTIME": "libcpp-no-vcruntime",
     "_LIBCPP_ABI_VERSION": "libcpp-abi-version",
     "_LIBCPP_ABI_BOUNDED_ITERATORS": "libcpp-has-abi-bounded-iterators",
+    "_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR": "libcpp-deprecated-abi-disable-pair-trivial-copy-ctor",
     "_LIBCPP_HAS_NO_FILESYSTEM": "no-filesystem",
     "_LIBCPP_HAS_NO_RANDOM_DEVICE": "no-random-device",
     "_LIBCPP_HAS_NO_LOCALIZATION": "no-localization",