Add `ENABLE_WERROR` option for use on CI (#4381)

Defaults to `OFF`. Downstream packagers are advised against enabling it
as we can't guarantee it working on every possible configuration.

Added the actual compile flag in the GTK client subdirectory and not
the top-level CMakeLists.txt file due to other subprojects not being
ready.

Fixed the only GTK client warning reported by Clang on Mac.

.github/workflows/actions.yml

@@ -210,6 +210,7 @@ jobs:
             -DENABLE_TESTS=OFF \
             -DENABLE_UTILS=${{ (needs.what-to-make.outputs.make-utils == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_WEB=OFF \
+            -DENABLE_WERROR=ON \
             -DRUN_CLANG_TIDY=OFF
       - name: Make
         run: cmake --build obj --config RelWithDebInfo
@@ -284,6 +285,7 @@ jobs:
             -DENABLE_TESTS=OFF \
             -DENABLE_UTILS=${{ (needs.what-to-make.outputs.make-utils == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_WEB=OFF \
+            -DENABLE_WERROR=ON \
             -DRUN_CLANG_TIDY=OFF
       - name: Make
         run: cmake --build obj --config RelWithDebInfo
@@ -375,6 +377,7 @@ jobs:
             -DENABLE_TESTS=ON `
             -DENABLE_UTILS=${{ (needs.what-to-make.outputs.make-dist == 'true' || needs.what-to-make.outputs.make-utils == 'true') && 'ON' || 'OFF' }} `
             -DENABLE_WEB=OFF `
+            -DENABLE_WERROR=ON `
             -DRUN_CLANG_TIDY=OFF
       - name: Make
         run: |
@@ -478,6 +481,7 @@ jobs:
             -DENABLE_TESTS=${{ (needs.what-to-make.outputs.make-tests == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_UTILS=${{ (needs.what-to-make.outputs.make-utils == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_WEB=OFF \
+            -DENABLE_WERROR=ON \
             -DRUN_CLANG_TIDY=OFF
       - name: Make
         run: cmake --build obj --config RelWithDebInfo
@@ -553,6 +557,7 @@ jobs:
             -DENABLE_TESTS=${{ (needs.what-to-make.outputs.make-tests == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_UTILS=${{ (needs.what-to-make.outputs.make-utils == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_WEB=OFF \
+            -DENABLE_WERROR=ON \
             -DRUN_CLANG_TIDY=OFF
       - name: Build
         run: cmake --build obj --config RelWithDebInfo
@@ -625,6 +630,7 @@ jobs:
             -DENABLE_TESTS=${{ (needs.what-to-make.outputs.make-tests == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_UTILS=${{ (needs.what-to-make.outputs.make-utils == 'true') && 'ON' || 'OFF' }} \
             -DENABLE_WEB=OFF \
+            -DENABLE_WERROR=ON \
             -DRUN_CLANG_TIDY=OFF
       - name: Build
         run: cmake --build obj --config RelWithDebInfo

CMakeLists.txt

@@ -50,6 +50,7 @@ tr_auto_option(ENABLE_MAC           "Build Mac client" AUTO)
         option(ENABLE_TESTS         "Build unit tests" ON)
         option(ENABLE_LIGHTWEIGHT   "Optimize libtransmission for low-resource systems: smaller cache size, prefer unencrypted peer connections, etc." OFF)
         option(ENABLE_UTP           "Build µTP support" ON)
+        option(ENABLE_WERROR        "Treat warnings as errors" OFF)
         option(ENABLE_NLS           "Enable native language support" ON)
         option(INSTALL_DOC          "Build/install documentation" ON)
         option(INSTALL_LIB          "Install the library" OFF)

gtk/CMakeLists.txt

@@ -1,6 +1,9 @@
 project(trgtk)
 
 add_compile_options(${C_WARNING_FLAGS})
+if(ENABLE_WERROR)
+    add_compile_options($<IF:$<CXX_COMPILER_ID:MSVC>,/WX,-Werror>)
+endif()
 
 execute_process(
     COMMAND

gtk/PathButton.cc

@@ -41,9 +41,9 @@ private:
 #endif
 
 private:
+#if GTKMM_CHECK_VERSION(4, 0, 0)
     PathButton& widget_;
 
-#if GTKMM_CHECK_VERSION(4, 0, 0)
     Glib::Property<Gtk::FileChooser::Action> action_;
     Glib::Property<Glib::ustring> title_;
 
@@ -59,9 +59,9 @@ private:
 #endif
 };
 
-PathButton::Impl::Impl(PathButton& widget)
-    : widget_(widget)
+PathButton::Impl::Impl([[maybe_unused]] PathButton& widget)
 #if GTKMM_CHECK_VERSION(4, 0, 0)
+    : widget_(widget)
     , action_(widget, "action", Gtk::FileChooser::Action::OPEN)
     , title_(widget, "title", {})
     , image_(Gtk::make_managed<Gtk::Image>())