find_package: Use PackageName_ROOT variables as search prefixes

This feature was originally added by commit v3.9.0-rc1~71^2~2 (find_*:
Add a new PackageRoot search path group, 2017-05-03) and documented by
commit v3.9.0-rc1~71^2 (find_*: Add docs for PackageRoot search path
group, 2017-05-03).  However, we had to disable the feature and remove
the documentation in commit v3.9.1~2^2 (find_*: Disable the PACKAGE_ROOT
search path group for CMake 3.9, 2017-08-08) due to breaking projects
that used `PackageName_ROOT` variables themselves.

Add policy `CMP0074` to restore the `PackageName_ROOT` variable behavior
in a compatible way.  Also revise the stack of root paths to store the
paths themselves rather than the package names.  This way the policy can
be considered at the `find_package` call site instead of individual
`find_` calls inside a find module.

Co-Author: Chuck Atkins <chuck.atkins@kitware.com>
Issue: #17144

1 files changed, 6 insertions, 3 deletions

diff --git a/Source/cmMakefile.h b/Source/cmMakefile.h
index 95ba53a..aafe60b 100644
--- a/Source/cmMakefile.h
+++ b/Source/cmMakefile.h
@@ -832,9 +832,11 @@ public:
   void RemoveExportBuildFileGeneratorCMP0024(cmExportBuildFileGenerator* gen);
   void AddExportBuildFileGenerator(cmExportBuildFileGenerator* gen);
 
-  // Maintain a stack of package names to determine the depth of find modules
-  // we are currently being called with
-  std::deque<std::string> FindPackageModuleStack;
+  // Maintain a stack of package roots to allow nested PACKAGE_ROOT_PATH
+  // searches
+  std::deque<std::vector<std::string>> FindPackageRootPathStack;
+
+  void MaybeWarnCMP0074(std::string const& pkg);
 
 protected:
   // add link libraries and directories to the target
@@ -1001,6 +1003,7 @@ private:
   bool WarnUnused;
   bool CheckSystemVars;
   bool CheckCMP0000;
+  std::set<std::string> WarnedCMP0074;
   bool IsSourceFileTryCompile;
   mutable bool SuppressWatches;
 };