Also add LLVM 20 availability

Author: ldionne

libcxx/include/__configuration/availability.h

@@ -118,9 +118,23 @@
 #  define _LIBCPP_INTRODUCED_IN_LLVM_21_ATTRIBUTE __attribute__((unavailable))
 
 // LLVM 20
-// TODO: Fill this in
-#  define _LIBCPP_INTRODUCED_IN_LLVM_20 0
-#  define _LIBCPP_INTRODUCED_IN_LLVM_20_ATTRIBUTE __attribute__((unavailable))
+//
+// Note that versions for most Apple OSes were bumped forward and aligned in that release.
+#  if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 260000) ||       \
+      (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 260000) ||     \
+      (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 260000) ||             \
+      (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ < 260000) ||       \
+      (defined(__ENVIRONMENT_BRIDGE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_BRIDGE_OS_VERSION_MIN_REQUIRED__ < 100000)
+#    define _LIBCPP_INTRODUCED_IN_LLVM_20 0
+#  else
+#    define _LIBCPP_INTRODUCED_IN_LLVM_20 1
+#  endif
+#  define _LIBCPP_INTRODUCED_IN_LLVM_20_ATTRIBUTE                                                                 \
+    __attribute__((availability(macos, strict, introduced = 26.0)))                                               \
+    __attribute__((availability(ios, strict, introduced = 26.0)))                                                 \
+    __attribute__((availability(tvos, strict, introduced = 26.0)))                                                \
+    __attribute__((availability(watchos, strict, introduced = 26.0)))                                             \
+    __attribute__((availability(bridgeos, strict, introduced = 10.0)))
 
 // LLVM 19
 #  if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 150400) ||       \

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

@@ -8,12 +8,6 @@
 
 // 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

libcxx/utils/libcxx/test/features.py

@@ -733,6 +733,13 @@ def check_gdb(cfg):
 # Helpers to define correspondances between LLVM versions and vendor system versions.
 # Those are used for backdeployment features below, do not use directly in tests.
 DEFAULT_FEATURES += [
+    Feature(
+        name="_target-has-llvm-20",
+        when=lambda cfg: BooleanExpression.evaluate(
+            "_target-has-llvm-21 || target={{.+}}-apple-macosx26.{{\d+(.\d+)?}}",
+            cfg.available_features,
+        ),
+    ),
     Feature(
         name="_target-has-llvm-19",
         when=lambda cfg: BooleanExpression.evaluate(

libcxxabi/test/test_demangle.pass.cpp

@@ -13,6 +13,10 @@
 // dd8b266ef.
 // UNSUPPORTED: using-built-library-before-llvm-20
 
+// This test exercises support for BitInt demangling introduced in
+// 20f56d140909a01c74e9981835373eaab6021af9.
+// UNSUPPORTED: using-built-library-before-llvm-21
+
 // XFAIL: win32-broken-printf-a-precision
 
 #include "support/timer.h"