Fix CMake compiled ABI options after f845e60acd880dbf07788a5a2c0dbad0f9c57231.

f845e60acd880dbf07788a5a2c0dbad0f9c57231 added an option to use C++20's
<ordering> header but the CMake install configured this as an alias
in C++17 when it is not available.

Fixes #1597

PiperOrigin-RevId: 597258569
Change-Id: I40277d55702601e1686370bee6af9b7491fd2000

3 files changed, 51 insertions, 15 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index bf273ceb..47f3beeb 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -627,17 +627,32 @@ include(CheckCXXSourceCompiles)
 check_cxx_source_compiles(
   [==[
 #ifdef _MSC_VER
-#  if _MSVC_LANG < 201700L
+#  if _MSVC_LANG < 201703L
 #    error "The compiler defaults or is configured for C++ < 17"
 #  endif
-#elif __cplusplus < 201700L
+#elif __cplusplus < 201703L
 #  error "The compiler defaults or is configured for C++ < 17"
 #endif
 int main() { return 0; }
 ]==]
   ABSL_INTERNAL_AT_LEAST_CXX17)
 
-if(ABSL_INTERNAL_AT_LEAST_CXX17)
+check_cxx_source_compiles(
+  [==[
+#ifdef _MSC_VER
+#  if _MSVC_LANG < 202002L
+#    error "The compiler defaults or is configured for C++ < 20"
+#  endif
+#elif __cplusplus < 202002L
+#  error "The compiler defaults or is configured for C++ < 20"
+#endif
+int main() { return 0; }
+]==]
+  ABSL_INTERNAL_AT_LEAST_CXX20)
+
+if(ABSL_INTERNAL_AT_LEAST_CXX20)
+  set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_20)
+elseif(ABSL_INTERNAL_AT_LEAST_CXX17)
   set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_17)
 else()
   set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_14)
@@ -807,8 +822,8 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
 
   if(ABSL_PROPAGATE_CXX_STD)
     # Abseil libraries require C++14 as the current minimum standard. When
-    # compiled with C++17 (either because it is the compiler's default or
-    # explicitly requested), then Abseil requires C++17.
+    # compiled with a higher minimum (either because it is the compiler's
+    # default or explicitly requested), then Abseil requires that standard.
     target_compile_features(${_dll} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
   endif()
 
diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
index 5a6c57fd..18fb75ff 100644
--- a/CMake/AbseilHelpers.cmake
+++ b/CMake/AbseilHelpers.cmake
@@ -287,8 +287,8 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
 
     if(ABSL_PROPAGATE_CXX_STD)
       # Abseil libraries require C++14 as the current minimum standard. When
-      # compiled with C++17 (either because it is the compiler's default or
-      # explicitly requested), then Abseil requires C++17.
+      # compiled with a higher standard (either because it is the compiler's
+      # default or explicitly requested), then Abseil requires that standard.
       target_compile_features(${_NAME} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
     endif()
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ca74b80d..87f8ae82 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -236,20 +236,41 @@ if(ABSL_ENABLE_INSTALL)
       PATTERN "testdata" EXCLUDE
     )
 
+  # Rewrite options.h to use the compiled ABI.
   file(READ "absl/base/options.h" ABSL_INTERNAL_OPTIONS_H_CONTENTS)
-  if (ABSL_INTERNAL_AT_LEAST_CXX17)
-    string(REGEX REPLACE
-      "#define ABSL_OPTION_USE_STD_([^ ]*) 2"
-      "#define ABSL_OPTION_USE_STD_\\1 1"
+
+  # Handle features that require at least C++20.
+  if (ABSL_INTERNAL_AT_LEAST_CXX20)
+    foreach(FEATURE "ORDERING")
+      string(REPLACE
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 2"
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 1"
       ABSL_INTERNAL_OPTIONS_H_PINNED
       "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
-  else()
-    string(REGEX REPLACE
-      "#define ABSL_OPTION_USE_STD_([^ ]*) 2"
-      "#define ABSL_OPTION_USE_STD_\\1 0"
+      set(ABSL_INTERNAL_OPTIONS_H_CONTENTS "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+    endforeach()
+  endif()
+
+  # Handle features that require at least C++17.
+  if (ABSL_INTERNAL_AT_LEAST_CXX17)
+    foreach(FEATURE "ANY" "OPTIONAL" "STRING_VIEW" "VARIANT")
+      string(REPLACE
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 2"
+      "#define ABSL_OPTION_USE_STD_${FEATURE} 1"
       ABSL_INTERNAL_OPTIONS_H_PINNED
       "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+      set(ABSL_INTERNAL_OPTIONS_H_CONTENTS "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+    endforeach()
   endif()
+
+  # Any feature that still has the value of 2 (because it was not handled above)
+  # should be set to 0.
+  string(REGEX REPLACE
+    "#define ABSL_OPTION_USE_STD_([^ ]*) 2"
+    "#define ABSL_OPTION_USE_STD_\\1 0"
+    ABSL_INTERNAL_OPTIONS_H_PINNED
+    "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+
   file(WRITE "${CMAKE_BINARY_DIR}/options-pinned.h" "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
 
   install(FILES "${CMAKE_BINARY_DIR}/options-pinned.h"