From 907d3ed824d8b82525c687747662b358accded9c Mon Sep 17 00:00:00 2001
From: Brad King <brad.king@kitware.com>
Date: Wed, 4 Sep 2019 08:24:09 -0400
Subject: [PATCH] FindBoost: Tolerate future Boost INTERFACE libraries

Since commit 0dd6772a89 (FindBoost: Add legacy variables and targets for
compatibility, 2019-06-14, v3.15.0-rc2~3^2~1) we query imported targets
provided by `BoostConfig.cmake` for their `IMPORTED_LOCATION_<cfg>`.
Querying this property is not allowed on INTERFACE libraries, so add a
condition to avoid doing so in case Boost adds one in the future.

Suggested-by: Alexander Grund <alexander.grund@tu-dresden.de>
Issue: #19656, #19402
---
 Modules/FindBoost.cmake | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/Modules/FindBoost.cmake b/Modules/FindBoost.cmake
index 845a3731b7..078000f228 100644
--- a/Modules/FindBoost.cmake
+++ b/Modules/FindBoost.cmake
@@ -353,14 +353,20 @@ function(_boost_set_legacy_variables_from_config)
       _boost_set_if_unset(Boost_${uppercomp}_LIBRARY "${target}")
       _boost_set_if_unset(Boost_${uppercomp}_LIBRARIES "${target}") # Very old legacy variable
       list(APPEND libraries "${target}")
-      foreach(cfg RELEASE DEBUG)
-        get_target_property(lib ${target} IMPORTED_LOCATION_${cfg})
-        if(lib)
-          get_filename_component(lib_dir "${lib}" DIRECTORY)
-          list(APPEND library_dirs ${lib_dir})
-          _boost_set_cache_if_unset(Boost_${uppercomp}_LIBRARY_${cfg} "${lib}")
-        endif()
-      endforeach()
+      get_property(type TARGET "${target}" PROPERTY TYPE)
+      if(NOT type STREQUAL "INTERFACE_LIBRARY")
+        foreach(cfg RELEASE DEBUG)
+          get_target_property(lib ${target} IMPORTED_LOCATION_${cfg})
+          if(lib)
+            get_filename_component(lib_dir "${lib}" DIRECTORY)
+            list(APPEND library_dirs ${lib_dir})
+            _boost_set_cache_if_unset(Boost_${uppercomp}_LIBRARY_${cfg} "${lib}")
+          endif()
+        endforeach()
+      elseif(Boost_DEBUG OR Boost_VERBOSE)
+        # For projects using only the Boost::* targets this warning can be safely ignored.
+        message(WARNING "Imported target '${target}' for required component '${comp}' has no artifact. Legacy variables for this component might be missing. Refer to the documentation of your Boost installation for help on variables to use.")
+      endif()
       _boost_get_canonical_target_name("${comp}" canonical_target)
       if(NOT TARGET "${canonical_target}")
         add_library("${canonical_target}" INTERFACE IMPORTED)