[libc++] Bump OS version for macOS backdeployment CI jobs

.github/workflows/libcxx-build-and-test.yaml

@@ -197,10 +197,20 @@ jobs:
           os: macos-15
         - config: apple-configuration
           os: macos-15
+        # TODO: These jobs are intended to test back-deployment (building against ToT libc++ but running against an
+        #       older system-provided libc++.dylib). Doing this properly would require building the test suite on a
+        #       recent macOS using a recent Clang (hence recent Xcode), and then running the actual test suite on an
+        #       older mac. We could do that by e.g. sharing artifacts between the two jobs.
+        #
+        #       However, our Lit configuration currently doesn't provide a good way to do that in a batch, so our only
+        #       alternative is to actually build on the same host that we're going to run on. Sadly, that doesn't work
+        #       since older macOSes don't support newer Xcodes. For now, we run the "backdeployment" jobs on recent
+        #       macOS versions as a way to avoid rotting that configuration, but it doesn't provide a lot of additional
+        #       coverage.
         - config: apple-system
-          os: macos-13
+          os: macos-15
         - config: apple-system-hardened
-          os: macos-13
+          os: macos-15
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2

libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp

@@ -8,11 +8,6 @@
 
 // <string>
 
-// This test demonstrates the smaller allocation sizes when the alignment
-// requirements of std::string are dropped from 16 to 8.
-//
-// XFAIL: using-built-library-before-llvm-19
-
 #include <algorithm>
 #include <cassert>
 #include <cstddef>

libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/setbuf.pass.cpp

@@ -10,8 +10,9 @@
 
 // basic_streambuf<charT, traits>* setbuf(char_type* s, streamsize n) override;
 
-// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
-// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+// This test requires the fix to https://github.com/llvm/llvm-project/issues/60509 in the dylib,
+// which landed in 5afb937d8a30445642ccaf33866ee4cdd0713222.
+// XFAIL: using-built-library-before-llvm-19
 
 #include <fstream>
 #include <cstddef>

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_double.pass.cpp

@@ -6,6 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed behavior of
+// FP parsing. This requires 3e15c97fa3812993bdc319827a5c6d867b765ae8 in the dylib.
+// XFAIL: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_float.pass.cpp

@@ -6,6 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed behavior of
+// FP parsing. This requires 3e15c97fa3812993bdc319827a5c6d867b765ae8 in the dylib.
+// XFAIL: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>

libcxx/test/std/localization/locale.categories/category.numeric/locale.num.get/facet.num.get.members/get_long_double.pass.cpp

@@ -6,6 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// The fix for LWG2381 (https://github.com/llvm/llvm-project/pull/77948) changed behavior of
+// FP parsing. This requires 3e15c97fa3812993bdc319827a5c6d867b765ae8 in the dylib.
+// XFAIL: using-built-library-before-llvm-19
+
 // <locale>
 
 // class num_get<charT, InputIterator>

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -10,12 +10,6 @@
 
 // XFAIL: FROZEN-CXX03-HEADERS-FIXME
 
-// After changing the alignment of the allocated pointer from 16 to 8, the exception
-// thrown is no longer `bad_alloc` but instead length_error on systems using new
-// headers but a dylib that doesn't contain 04ce0ba.
-//
-// XFAIL: using-built-library-before-llvm-19
-
 // <string>
 
 // size_type max_size() const; // constexpr since C++20

libcxx/test/std/strings/basic.string/string.capacity/over_max_size.pass.cpp

@@ -8,6 +8,12 @@
 
 // UNSUPPORTED: no-exceptions
 
+// After changing the alignment of the allocated pointer from 16 to 8, the exception
+// thrown is no longer `bad_alloc` but instead length_error on systems using new
+// headers but a dylib that doesn't contain 04ce0ba.
+//
+// XFAIL: using-built-library-before-llvm-19
+
 // <string>
 
 // size_type max_size() const; // constexpr since C++20