find_package: Add option to prefer Config mode

Add a `CMAKE_FIND_PACKAGE_PREFER_CONFIG` variable to tell
`find_package` calls to look for a package configuration file
first even if a find module is available.

Fixes: #16805, #19236

1 files changed, 53 insertions, 42 deletions

diff --git a/Source/cmFindPackageCommand.cxx b/Source/cmFindPackageCommand.cxx
index 5310a1b84d..8eefaa7617 100644
--- a/Source/cmFindPackageCommand.cxx
+++ b/Source/cmFindPackageCommand.cxx
@@ -499,50 +499,61 @@ bool cmFindPackageCommand::InitialPass(std::vector<std::string> const& args,
 
   // See if there is a Find<PackageName>.cmake module.
   bool loadedPackage = false;
-  if (this->UseFindModules && this->FindPackageUsingModuleMode()) {
-    loadedPackage = true;
-  } else {
-    // Handle CMAKE_FIND_PACKAGE_WARN_NO_MODULE (warn when CONFIG mode is
-    // implicitly assumed)
-    if (this->UseFindModules && this->UseConfigFiles &&
-        this->Makefile->IsOn("CMAKE_FIND_PACKAGE_WARN_NO_MODULE")) {
-      std::ostringstream aw;
-      if (this->RequiredCMakeVersion >= CMake_VERSION_ENCODE(2, 8, 8)) {
-        aw << "find_package called without either MODULE or CONFIG option and "
-              "no Find"
-           << this->Name
-           << ".cmake module is in CMAKE_MODULE_PATH.  "
-              "Add MODULE to exclusively request Module mode and fail if "
-              "Find"
-           << this->Name
-           << ".cmake is missing.  "
-              "Add CONFIG to exclusively request Config mode and search for a "
-              "package configuration file provided by "
-           << this->Name << " (" << this->Name << "Config.cmake or "
-           << cmSystemTools::LowerCase(this->Name) << "-config.cmake).  ";
-      } else {
-        aw << "find_package called without NO_MODULE option and no "
-              "Find"
-           << this->Name
-           << ".cmake module is in CMAKE_MODULE_PATH.  "
-              "Add NO_MODULE to exclusively request Config mode and search "
-              "for a "
-              "package configuration file provided by "
-           << this->Name << " (" << this->Name << "Config.cmake or "
-           << cmSystemTools::LowerCase(this->Name)
-           << "-config.cmake).  "
-              "Otherwise make Find"
-           << this->Name
-           << ".cmake available in "
-              "CMAKE_MODULE_PATH.";
-      }
-      aw << "\n"
-            "(Variable CMAKE_FIND_PACKAGE_WARN_NO_MODULE enabled this "
-            "warning.)";
-      this->Makefile->IssueMessage(MessageType::AUTHOR_WARNING, aw.str());
+  if (this->Makefile->IsOn("CMAKE_FIND_PACKAGE_PREFER_CONFIG")) {
+    if (this->UseConfigFiles && this->FindPackageUsingConfigMode()) {
+      loadedPackage = true;
+    } else if (this->FindPackageUsingModuleMode()) {
+      loadedPackage = true;
     }
-    if (this->FindPackageUsingConfigMode()) {
+  } else {
+    if (this->UseFindModules && this->FindPackageUsingModuleMode()) {
       loadedPackage = true;
+    } else {
+      // Handle CMAKE_FIND_PACKAGE_WARN_NO_MODULE (warn when CONFIG mode is
+      // implicitly assumed)
+      if (this->UseFindModules && this->UseConfigFiles &&
+          this->Makefile->IsOn("CMAKE_FIND_PACKAGE_WARN_NO_MODULE")) {
+        std::ostringstream aw;
+        if (this->RequiredCMakeVersion >= CMake_VERSION_ENCODE(2, 8, 8)) {
+          aw << "find_package called without either MODULE or CONFIG option "
+                "and "
+                "no Find"
+             << this->Name
+             << ".cmake module is in CMAKE_MODULE_PATH.  "
+                "Add MODULE to exclusively request Module mode and fail if "
+                "Find"
+             << this->Name
+             << ".cmake is missing.  "
+                "Add CONFIG to exclusively request Config mode and search for "
+                "a "
+                "package configuration file provided by "
+             << this->Name << " (" << this->Name << "Config.cmake or "
+             << cmSystemTools::LowerCase(this->Name) << "-config.cmake).  ";
+        } else {
+          aw << "find_package called without NO_MODULE option and no "
+                "Find"
+             << this->Name
+             << ".cmake module is in CMAKE_MODULE_PATH.  "
+                "Add NO_MODULE to exclusively request Config mode and search "
+                "for a "
+                "package configuration file provided by "
+             << this->Name << " (" << this->Name << "Config.cmake or "
+             << cmSystemTools::LowerCase(this->Name)
+             << "-config.cmake).  "
+                "Otherwise make Find"
+             << this->Name
+             << ".cmake available in "
+                "CMAKE_MODULE_PATH.";
+        }
+        aw << "\n"
+              "(Variable CMAKE_FIND_PACKAGE_WARN_NO_MODULE enabled this "
+              "warning.)";
+        this->Makefile->IssueMessage(MessageType::AUTHOR_WARNING, aw.str());
+      }
+
+      if (this->FindPackageUsingConfigMode()) {
+        loadedPackage = true;
+      }
     }
   }