FindBoost: Fix and modernize config mode with FPHSA

Fixes: #19186

Help/release/dev/FindBoost-fphsa.rst

@@ -12,3 +12,17 @@ FindBoost-fphsa
     ``Boost_VERSION_MAJOR``, ``Boost_VERSION_MINOR``,
     ``Boost_VERSION_PATCH``, and ``Boost_VERSION_COUNT``
     were added.
+
+  * The internal logic for determining the value for
+    ``Boost_FOUND``, for version and component checks, and
+    for reporting the result to the user was replaced with
+    the :module:`FindPackageHandleStandardArgs` module. (This
+    fixed a bug that sometimes printed wrong status
+    messages in config mode.)
+
+  * The ``QUIET`` argument passed to :command:`find_package` is no
+    longer ignored in config mode.
+
+  * *Known issue*: The CMake package shipped with Boost ``1.70.0``
+    ignores the ``QUIET`` argument passed to :command:`find_package`.
+    This is fixed in the next Boost release.

Modules/FindBoost.cmake

@@ -233,9 +233,9 @@ Example to find Boost headers and some *static* (release only) libraries::
 Boost CMake
 ^^^^^^^^^^^
 
-If Boost was built using the boost-cmake project it provides a package
-configuration file for use with find_package's Config mode.  This
-module looks for the package configuration file called
+If Boost was built using the boost-cmake project or from Boost 1.70.0 on
+it provides a package configuration file for use with find_package's config mode.
+This module looks for the package configuration file called
 ``BoostConfig.cmake`` or ``boost-config.cmake`` and stores the result in
 ``CACHE`` entry "Boost_DIR".  If found, the package configuration file is loaded
 and this module returns with no further action.  See documentation of
@@ -244,13 +244,17 @@ the Boost CMake package configuration for details on what it provides.
 Set ``Boost_NO_BOOST_CMAKE`` to ``ON``, to disable the search for boost-cmake.
 #]=======================================================================]
 
+# The FPHSA helper provides standard way of reporting final search results to
+# the user including the version and component checks.
+include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
+
 # Save project's policies
 cmake_policy(PUSH)
 cmake_policy(SET CMP0057 NEW) # if IN_LIST
 
 #-------------------------------------------------------------------------------
-# Before we go searching, check whether boost-cmake is available, unless the
-# user specifically asked NOT to search for boost-cmake.
+# Before we go searching, check whether a boost cmake package is available, unless
+# the user specifically asked NOT to search for one.
 #
 # If Boost_DIR is set, this behaves as any find_package call would. If not,
 # it looks at BOOST_ROOT and BOOSTROOT to find Boost.
@@ -272,13 +276,25 @@ if (NOT Boost_NO_BOOST_CMAKE)
   find_package(Boost QUIET NO_MODULE)
   mark_as_advanced(Boost_DIR)
 
-  # If we found boost-cmake, then we're done.  Print out what we found.
+  # If we found a boost cmake package, then we're done. Print out what we found.
   # Otherwise let the rest of the module try to find it.
-  if (Boost_FOUND)
-    message(STATUS "Boost ${Boost_FIND_VERSION} found.")
-    if (Boost_FIND_COMPONENTS)
-      message(STATUS "Found Boost components:\n   ${Boost_FIND_COMPONENTS}")
+  if(Boost_FOUND)
+    # Neither the legacy boost-cmake nor the new builtin BoostConfig (as in 1.70)
+    # report the found components in the standard variables, so we need to convert
+    # them here
+    if(Boost_FIND_COMPONENTS)
+      foreach(_comp IN LISTS Boost_FIND_COMPONENTS)
+        string(TOUPPER ${_comp} _uppercomp)
+        if(DEFINED Boost${_comp}_FOUND)
+          set(Boost_${_comp}_FOUND ${Boost${_comp}_FOUND})
+        elseif(DEFINED Boost_${_uppercomp}_FOUND)
+          set(Boost_${_comp}_FOUND ${Boost_${_uppercomp}_FOUND})
+        endif()
+      endforeach()
     endif()
+
+    find_package_handle_standard_args(Boost HANDLE_COMPONENTS CONFIG_MODE)
+
     # Restore project's policies
     cmake_policy(POP)
     return()

Tests/RunCMake/FindBoost/CMakePackage-stdout.txt

@@ -1,3 +1,2 @@
--- Boost 1\.12345 found\.
--- Found Boost components:
-   date_time
+-- Found Boost: [^
+]* \(found suitable version "1\.12345", minimum required is "1\.12345"\) found components:  date_time

Tests/RunCMake/FindBoost/CMakePackage/BoostConfig.cmake

@@ -0,0 +1 @@
+set(Boost_DATE_TIME_FOUND 1)