CMake: install artifacts reflect the compiled ABI

Change `absl/base/options.h` at install time to reflect the ABI used
to compile the libraries, i.e., if Abseil was compiled with
C++ >= 17 then the `ABSL_OPTION_USE_STD_*` macros are set to `1`,
because then the C++ 17 types are embedded in the ABI of the installed
artifacts.

Likewise, set the `target_compile_features()` of Abseil libraries
to reflect the C++ version used to compile them. If they the Abseil
libraries are compiled with C++ >= 17, then all downstream
dependencies must also be compiled with C++ >= 17.

Fixes #696

PiperOrigin-RevId: 472538165
Change-Id: Ic9e346ebe277c882a18ad96e1918c9e3511c91c3

CMake/AbseilHelpers.cmake

@@ -40,6 +40,27 @@ function(_target_compile_features_if_available TARGET TYPE FEATURE)
   endif()
 endfunction()
 
+include(CheckCXXSourceCompiles)
+
+check_cxx_source_compiles(
+  [==[
+#ifdef _MSC_VER
+#  if _MSVC_LANG < 201700L
+#    error "The compiler defaults or is configured for C++ < 17"
+#  endif
+#elif __cplusplus < 201700L
+#  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)
+  set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_17)
+else()
+  set(ABSL_INTERNAL_CXX_STD_FEATURE cxx_std_14)
+endif()
+
 # absl_cc_library()
 #
 # CMake function to imitate Bazel's cc_library rule.
@@ -279,7 +300,7 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
       # Abseil libraries require C++14 as the current minimum standard.
       # Top-level application CMake projects should ensure a consistent C++
       # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
-      _target_compile_features_if_available(${_NAME} PUBLIC cxx_std_14)
+      _target_compile_features_if_available(${_NAME} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
     else()
       # Note: This is legacy (before CMake 3.8) behavior. Setting the
       # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is
@@ -327,7 +348,7 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
       # Abseil libraries require C++14 as the current minimum standard.
       # Top-level application CMake projects should ensure a consistent C++
       # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
-      _target_compile_features_if_available(${_NAME} INTERFACE cxx_std_14)
+      _target_compile_features_if_available(${_NAME} INTERFACE ${ABSL_INTERNAL_CXX_STD_FEATURE})
 
       # (INTERFACE libraries can't have the CXX_STANDARD property set, so there
       # is no legacy behavior else case).
@@ -439,7 +460,7 @@ function(absl_cc_test)
     # Abseil libraries require C++14 as the current minimum standard.
     # Top-level application CMake projects should ensure a consistent C++
     # standard for all compiled sources by setting CMAKE_CXX_STANDARD.
-    _target_compile_features_if_available(${_NAME} PUBLIC cxx_std_14)
+    _target_compile_features_if_available(${_NAME} PUBLIC ${ABSL_INTERNAL_CXX_STD_FEATURE})
   else()
     # Note: This is legacy (before CMake 3.8) behavior. Setting the
     # target-level CXX_STANDARD property to ABSL_CXX_STANDARD (which is

CMakeLists.txt

@@ -46,6 +46,11 @@ if (POLICY CMP0091)
   cmake_policy(SET CMP0091 NEW)
 endif (POLICY CMP0091)
 
+# try_compile() honors the CMAKE_CXX_STANDARD value
+if (POLICY CMP0067)
+  cmake_policy(SET CMP0067 NEW)
+endif (POLICY CMP0067)
+
 project(absl LANGUAGES CXX)
 include(CTest)
 
@@ -229,4 +234,25 @@ if(ABSL_ENABLE_INSTALL)
       PATTERN "copts" EXCLUDE
       PATTERN "testdata" EXCLUDE
     )
+
+  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"
+      ABSL_INTERNAL_OPTIONS_H_PINNED
+      "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+  else()
+    string(REGEX REPLACE
+      "#define ABSL_OPTION_USE_STD_([^ ]*) 2"
+      "#define ABSL_OPTION_USE_STD_\\1 0"
+      ABSL_INTERNAL_OPTIONS_H_PINNED
+      "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
+  endif()
+  file(WRITE "${CMAKE_BINARY_DIR}/options-pinned.h" "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+
+  install(FILES "${CMAKE_BINARY_DIR}/options-pinned.h"
+         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/absl/base
+         RENAME "options.h")
+
 endif()  # ABSL_ENABLE_INSTALL