Do not produce legacy _LIB_DEPENDS cache entries

Introduce policy `CMP0073` to avoid producing these cache entries.

Fixes: #16364

Help/manual/cmake-policies.7.rst

@@ -51,6 +51,14 @@ The :variable:`CMAKE_MINIMUM_REQUIRED_VERSION` variable may also be used
 to determine whether to report an error on use of deprecated macros or
 functions.
 
+Policies Introduced by CMake 3.12
+=================================
+
+.. toctree::
+   :maxdepth: 1
+
+   CMP0073: Do not produce legacy _LIB_DEPENDS cache entries. </policy/CMP0073>
+
 Policies Introduced by CMake 3.11
 =================================
 

Help/policy/CMP0073.rst

@@ -0,0 +1,25 @@
+CMP0073
+-------
+
+Do not produce legacy ``_LIB_DEPENDS`` cache entries.
+
+Ancient CMake versions once used ``<tgt>_LIB_DEPENDS`` cache entries to
+propagate library link dependencies.  This has long been done by other
+means, leaving the :command:`export_library_dependencies` command as the
+only user of these values.  That command has long been disallowed by
+policy :policy:`CMP0033`, but the ``<tgt>_LIB_DEPENDS`` cache entries
+were left for compatibility with possible non-standard uses by projects.
+
+CMake 3.12 and above now prefer to not produce these cache entries
+at all.  This policy provides compatibility with projects that have
+not been updated to avoid using them.
+
+The ``OLD`` behavior for this policy is to set ``<tgt>_LIB_DEPENDS`` cache
+entries.  The ``NEW`` behavior for this policy is to not set them.
+
+This policy was introduced in CMake version 3.12.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike most policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt

Help/release/dev/avoid-LIB_DEPENDS.rst

@@ -0,0 +1,5 @@
+avoid-LIB_DEPENDS
+-----------------
+
+* CMake no longer produces ``<tgt>_LIB_DEPENDS`` cache entries
+  for library targets.  See policy :policy:`CMP0073`.

Source/cmPolicies.h

@@ -214,6 +214,9 @@ class cmMakefile;
          3, 10, 0, cmPolicies::WARN)                                          \
   SELECT(POLICY, CMP0072,                                                     \
          "FindOpenGL prefers GLVND by default when available.", 3, 11, 0,     \
+         cmPolicies::WARN)                                                    \
+  SELECT(POLICY, CMP0073,                                                     \
+         "Do not produce legacy _LIB_DEPENDS cache entries.", 3, 12, 0,       \
          cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
@@ -238,7 +241,8 @@ class cmMakefile;
   F(CMP0063)                                                                  \
   F(CMP0065)                                                                  \
   F(CMP0068)                                                                  \
-  F(CMP0069)
+  F(CMP0069)                                                                  \
+  F(CMP0073)
 
 /** \class cmPolicies
  * \brief Handles changes in CMake behavior and policies

Source/cmTarget.cxx

@@ -737,7 +737,9 @@ void cmTarget::AddLinkLibrary(cmMakefile& mf, const std::string& lib,
   // and we removing one instance will break the link line. Duplicates
   // will be appropriately eliminated at emit time.
   if (this->TargetTypeValue >= cmStateEnums::STATIC_LIBRARY &&
-      this->TargetTypeValue <= cmStateEnums::MODULE_LIBRARY) {
+      this->TargetTypeValue <= cmStateEnums::MODULE_LIBRARY &&
+      (this->GetPolicyStatusCMP0073() == cmPolicies::OLD ||
+       this->GetPolicyStatusCMP0073() == cmPolicies::WARN)) {
     std::string targetEntry = this->Name;
     targetEntry += "_LIB_DEPENDS";
     std::string dependencies;

Tests/RunCMake/TargetPolicies/PolicyList-stderr.txt

@@ -23,6 +23,7 @@
    \* CMP0065
    \* CMP0068
    \* CMP0069
+   \* CMP0073
 
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)

Tests/RunCMake/add_library/CMP0073-stdout.txt

@@ -0,0 +1,3 @@
+-- warn_LIB_DEPENDS='general;bar;'
+-- old_LIB_DEPENDS='general;bar;'
+-- new_LIB_DEPENDS=''

Tests/RunCMake/add_library/CMP0073.cmake

@@ -0,0 +1,18 @@
+enable_language(C)
+
+add_library(warn empty.c)
+target_link_libraries(warn bar)
+message(STATUS "warn_LIB_DEPENDS='${warn_LIB_DEPENDS}'")
+
+cmake_policy(SET CMP0073 OLD)
+add_library(old empty.c)
+target_link_libraries(old bar)
+message(STATUS "old_LIB_DEPENDS='${old_LIB_DEPENDS}'")
+
+cmake_policy(SET CMP0073 NEW)
+add_library(new empty.c)
+target_link_libraries(new bar)
+message(STATUS "new_LIB_DEPENDS='${new_LIB_DEPENDS}'")
+if(DEFINED new_LIB_DEPENDS)
+  message(FATAL_ERROR "new_LIB_DEPENDS set but should not be")
+endif()

Tests/RunCMake/add_library/RunCMakeTest.cmake

@@ -1,5 +1,7 @@
 include(RunCMake)
 
+run_cmake(CMP0073)
+
 run_cmake(INTERFACEwithNoSources)
 run_cmake(OBJECTwithNoSources)
 run_cmake(STATICwithNoSources)