[libc++] Add availability mapping for LLVM 18 on Apple platforms (#110158)

Author: ldionne

libcxx/include/__configuration/availability.h

@@ -138,9 +138,19 @@
 #  define _LIBCPP_INTRODUCED_IN_LLVM_19_ATTRIBUTE __attribute__((unavailable))
 
 // LLVM 18
-// TODO: Fill this in
-#  define _LIBCPP_INTRODUCED_IN_LLVM_18 0
-#  define _LIBCPP_INTRODUCED_IN_LLVM_18_ATTRIBUTE __attribute__((unavailable))
+#  if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 150000) ||       \
+      (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ < 180000) ||     \
+      (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ < 180000) ||             \
+      (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ < 110000)
+#    define _LIBCPP_INTRODUCED_IN_LLVM_18 0
+#  else
+#    define _LIBCPP_INTRODUCED_IN_LLVM_18 1
+#  endif
+#  define _LIBCPP_INTRODUCED_IN_LLVM_18_ATTRIBUTE                                                                 \
+    __attribute__((availability(macos, strict, introduced = 15.0)))                                               \
+    __attribute__((availability(ios, strict, introduced = 18.0)))                                                 \
+    __attribute__((availability(tvos, strict, introduced = 18.0)))                                                \
+    __attribute__((availability(watchos, strict, introduced = 11.0)))
 
 // LLVM 17
 #  if (defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) && __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 140400) ||       \

libcxx/utils/libcxx/test/features.py

@@ -610,10 +610,17 @@ 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-18",
+        when=lambda cfg: BooleanExpression.evaluate(
+            "target={{.+}}-apple-macosx{{15(.[0-9]+)?(.[0-9]+)?}}",
+            cfg.available_features,
+        ),
+    ),
     Feature(
         name="_target-has-llvm-17",
         when=lambda cfg: BooleanExpression.evaluate(
-            "target={{.+}}-apple-macosx{{14.[4-9](.0)?}} || target={{.+}}-apple-macosx{{1[5-9]([.].+)?}}",
+            "_target-has-llvm-18 || target={{.+}}-apple-macosx{{14.[4-9](.0)?}} || target={{.+}}-apple-macosx{{1[5-9]([.].+)?}}",
             cfg.available_features,
         ),
     ),