AutoGen: Retrieve Qt version from moc as fallback

Consider the case where the find_package call for QtCore is wrapped in a
function call. Then AutoGen cannot determine the Qt version, because it
only looks at variables and directory properties. The former don't leave
the function scope and the latter are not set by default.

As a fallback, locate the moc executable via its target and call it with
the --version argument to determine the Qt version.

Issue: #22028

6 files changed, 114 insertions, 5 deletions

diff --git a/Source/cmQtAutoGenGlobalInitializer.cxx b/Source/cmQtAutoGenGlobalInitializer.cxx
index f79ffd4a0f..5896bb2c4b 100644
--- a/Source/cmQtAutoGenGlobalInitializer.cxx
+++ b/Source/cmQtAutoGenGlobalInitializer.cxx
@@ -114,7 +114,8 @@ cmQtAutoGenGlobalInitializer::cmQtAutoGenGlobalInitializer(
           target->GetSafeProperty(this->kw().AUTORCC_EXECUTABLE);
 
         // We support Qt4, Qt5 and Qt6
-        auto qtVersion = cmQtAutoGenInitializer::GetQtVersion(target.get());
+        auto qtVersion =
+          cmQtAutoGenInitializer::GetQtVersion(target.get(), mocExec);
         bool const validQt = (qtVersion.first.Major == 4) ||
           (qtVersion.first.Major == 5) || (qtVersion.first.Major == 6);
 
diff --git a/Source/cmQtAutoGenInitializer.cxx b/Source/cmQtAutoGenInitializer.cxx
index 3adeb1aee0..148591ce27 100644
--- a/Source/cmQtAutoGenInitializer.cxx
+++ b/Source/cmQtAutoGenInitializer.cxx
@@ -6,8 +6,8 @@
 #include <deque>
 #include <initializer_list>
 #include <map>
-#include <ostream>
 #include <set>
+#include <sstream> // for basic_ios, istringstream
 #include <string>
 #include <unordered_set>
 #include <utility>
@@ -1833,8 +1833,63 @@ void cmQtAutoGenInitializer::ConfigFileClean(ConfigString& configString)
   }
 }
 
+static cmQtAutoGen::IntegerVersion parseMocVersion(std::string str)
+{
+  cmQtAutoGen::IntegerVersion result;
+
+  static const std::string prelude = "moc ";
+  size_t pos = str.find(prelude);
+  if (pos == std::string::npos) {
+    return result;
+  }
+
+  str.erase(0, prelude.size() + pos);
+  std::istringstream iss(str);
+  std::string major;
+  std::string minor;
+  if (!std::getline(iss, major, '.') || !std::getline(iss, minor, '.')) {
+    return result;
+  }
+
+  result.Major = static_cast<unsigned int>(std::stoi(major));
+  result.Minor = static_cast<unsigned int>(std::stoi(minor));
+  return result;
+}
+
+static cmQtAutoGen::IntegerVersion GetMocVersion(
+  const std::string& mocExecutablePath)
+{
+  std::string capturedStdOut;
+  int exitCode;
+  if (!cmSystemTools::RunSingleCommand({ mocExecutablePath, "--version" },
+                                       &capturedStdOut, nullptr, &exitCode,
+                                       nullptr, cmSystemTools::OUTPUT_NONE)) {
+    return {};
+  }
+
+  if (exitCode != 0) {
+    return {};
+  }
+
+  return parseMocVersion(capturedStdOut);
+}
+
+static std::string FindMocExecutableFromMocTarget(cmMakefile* makefile,
+                                                  unsigned int qtMajorVersion)
+{
+  std::string result;
+  const std::string mocTargetName =
+    "Qt" + std::to_string(qtMajorVersion) + "::moc";
+  cmTarget* mocTarget = makefile->FindTargetToUse(mocTargetName);
+  if (mocTarget) {
+    result = mocTarget->GetSafeProperty("IMPORTED_LOCATION");
+  }
+  return result;
+}
+
 std::pair<cmQtAutoGen::IntegerVersion, unsigned int>
-cmQtAutoGenInitializer::GetQtVersion(cmGeneratorTarget const* target)
+cmQtAutoGenInitializer::GetQtVersion(cmGeneratorTarget const* target,
+                                     std::string mocExecutable)
 {
   // Converts a char ptr to an unsigned int value
   auto toUInt = [](const char* const input) -> unsigned int {
@@ -1909,6 +1964,21 @@ cmQtAutoGenInitializer::GetQtVersion(cmGeneratorTarget const* target)
       }
     }
   }
+
+  if (res.first.Major == 0) {
+    // We could not get the version number from variables or directory
+    // properties. This might happen if the find_package call for Qt is wrapped
+    // in a function. Try to find the moc executable path from the available
+    // targets and call "moc --version" to get the Qt version.
+    if (mocExecutable.empty()) {
+      mocExecutable =
+        FindMocExecutableFromMocTarget(target->Makefile, res.second);
+    }
+    if (!mocExecutable.empty()) {
+      res.first = GetMocVersion(mocExecutable);
+    }
+  }
+
   return res;
 }
 
diff --git a/Source/cmQtAutoGenInitializer.h b/Source/cmQtAutoGenInitializer.h
index 8a6d8f924f..e76817b9c4 100644
--- a/Source/cmQtAutoGenInitializer.h
+++ b/Source/cmQtAutoGenInitializer.h
@@ -98,9 +98,11 @@ public:
       , GenNameUpper(cmQtAutoGen::GeneratorNameUpper(gen)){};
   };
 
-  /** @return The detected Qt version and the required Qt major version.  */
+  /** @param mocExecutable The file path to the moc executable. Will be used as
+     fallback to query the version
+      @return The detected Qt version and the required Qt major version. */
   static std::pair<IntegerVersion, unsigned int> GetQtVersion(
-    cmGeneratorTarget const* genTarget);
+    cmGeneratorTarget const* genTarget, std::string mocExecutable);
 
   cmQtAutoGenInitializer(cmQtAutoGenGlobalInitializer* globalInitializer,
                          cmGeneratorTarget* genTarget,
diff --git a/Tests/QtAutogen/Tests.cmake b/Tests/QtAutogen/Tests.cmake
index ab5686af65..a3c57a5871 100644
--- a/Tests/QtAutogen/Tests.cmake
+++ b/Tests/QtAutogen/Tests.cmake
@@ -49,6 +49,7 @@ if(QT_TEST_VERSION GREATER 4)
   ADD_AUTOGEN_TEST(MocMacroName mocMacroName)
   ADD_AUTOGEN_TEST(MocOsMacros)
   ADD_AUTOGEN_TEST(RerunMocPlugin)
+  ADD_AUTOGEN_TEST(WrappedFindPackage)
   if(APPLE)
     ADD_AUTOGEN_TEST(MacOsFW)
   endif()
diff --git a/Tests/QtAutogen/WrappedFindPackage/CMakeLists.txt b/Tests/QtAutogen/WrappedFindPackage/CMakeLists.txt
new file mode 100644
index 0000000000..e8f50d56e7
--- /dev/null
+++ b/Tests/QtAutogen/WrappedFindPackage/CMakeLists.txt
@@ -0,0 +1,16 @@
+cmake_minimum_required(VERSION 3.20)
+project(WrappedFindPackage)
+
+# Wrap the find_package call in a function.
+# Test whether AutoMoc can retrieve the Qt version from the moc executable.
+function(find_qt_package)
+  include("../AutogenCoreTest.cmake")
+  set(QT_LIBRARIES "${QT_LIBRARIES}" PARENT_SCOPE)
+endfunction()
+
+find_qt_package()
+
+set(CMAKE_AUTOMOC ON)
+
+add_executable(wrappedFindPackage main.cpp)
+target_link_libraries(wrappedFindPackage PRIVATE ${QT_LIBRARIES})
diff --git a/Tests/QtAutogen/WrappedFindPackage/main.cpp b/Tests/QtAutogen/WrappedFindPackage/main.cpp
new file mode 100644
index 0000000000..1e2c63fab3
--- /dev/null
+++ b/Tests/QtAutogen/WrappedFindPackage/main.cpp
@@ -0,0 +1,19 @@
+#include <qobject.h>
+
+class MyObject : public QObject
+{
+  Q_OBJECT
+public:
+  MyObject(QObject* parent = 0)
+    : QObject(parent)
+  {
+  }
+};
+
+int main()
+{
+  MyObject obj;
+  return 0;
+}
+
+#include "main.moc"