Add `-DPYBIND11_WERROR=ON` to mingw cmake commands (#4073)

* Add `-DPYBIND11_WERROR=ON` to mingw cmake commands (and `-DCMAKE_VERBOSE_MAKEFILE:BOOL=ON`).

* Using no-destructor idiom to side-step overzealous MINGW warning.

* Add __MINGW32__ pragma GCC diagnostic ignored in eigen.h

* Add another no-destructor workaround.

* Temporarily add -k (keep-going) flags to hopefully speed up finding all warnings.

* Revert "Temporarily add -k (keep-going) flags to hopefully speed up finding all warnings."

This reverts commit f36b0af8f9.

* Very minor shuffle to avoid MSVC warnings.

* Remove all `:BOOL` as suggested by @henryiii

.github/workflows/ci.yml

@@ -911,7 +911,7 @@ jobs:
     - name: Configure C++11
       # LTO leads to many undefined reference like
       # `pybind11::detail::function_call::function_call(pybind11::detail::function_call&&)
-      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=11 -DDOWNLOAD_CATCH=ON -S . -B build
+      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=11 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build
 
     - name: Build C++11
       run: cmake --build build -j 2
@@ -929,7 +929,7 @@ jobs:
       run: git clean -fdx
 
     - name: Configure C++14
-      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DDOWNLOAD_CATCH=ON -S . -B build2
+      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build2
 
     - name: Build C++14
       run: cmake --build build2 -j 2
@@ -947,7 +947,7 @@ jobs:
       run: git clean -fdx
 
     - name: Configure C++17
-      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DDOWNLOAD_CATCH=ON -S . -B build3
+      run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build3
 
     - name: Build C++17
       run: cmake --build build3 -j 2

include/pybind11/eigen.h

@@ -27,6 +27,9 @@
 #    pragma warning(disable : 4127) // C4127: conditional expression is constant
 #    pragma warning(disable : 5054) // https://github.com/pybind/pybind11/pull/3741
 //       C5054: operator '&': deprecated between enumerations of different types
+#elif defined(__MINGW32__)
+#    pragma GCC diagnostic push
+#    pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
 #endif
 
 #include <Eigen/Core>
@@ -34,6 +37,8 @@
 
 #if defined(_MSC_VER)
 #    pragma warning(pop)
+#elif defined(__MINGW32__)
+#    pragma GCC diagnostic pop
 #endif
 
 // Eigen prior to 3.2.7 doesn't have proper move constructors--but worse, some classes get implicit

tests/test_builtin_casters.cpp

@@ -266,9 +266,14 @@ TEST_SUBMODULE(builtin_casters, m) {
     });
     m.def("lvalue_nested", []() -> const decltype(lvnested) & { return lvnested; });
 
-    static std::pair<int, std::string> int_string_pair{2, "items"};
     m.def(
-        "int_string_pair", []() { return &int_string_pair; }, py::return_value_policy::reference);
+        "int_string_pair",
+        []() {
+            // Using no-destructor idiom to side-step warnings from overzealous compilers.
+            static auto *int_string_pair = new std::pair<int, std::string>{2, "items"};
+            return int_string_pair;
+        },
+        py::return_value_policy::reference);
 
     // test_builtins_cast_return_none
     m.def("return_none_string", []() -> std::string * { return nullptr; });

tests/test_stl.cpp

@@ -176,9 +176,14 @@ TEST_SUBMODULE(stl, m) {
     m.def("load_bool_vector",
           [](const std::vector<bool> &v) { return v.at(0) == true && v.at(1) == false; });
     // Unnumbered regression (caused by #936): pointers to stl containers aren't castable
-    static std::vector<RValueCaster> lvv{2};
     m.def(
-        "cast_ptr_vector", []() { return &lvv; }, py::return_value_policy::reference);
+        "cast_ptr_vector",
+        []() {
+            // Using no-destructor idiom to side-step warnings from overzealous compilers.
+            static auto *v = new std::vector<RValueCaster>{2};
+            return v;
+        },
+        py::return_value_policy::reference);
 
     // test_deque
     m.def("cast_deque", []() { return std::deque<int>{1}; });
@@ -237,6 +242,7 @@ TEST_SUBMODULE(stl, m) {
     lvn["b"].emplace_back();        // add a list
     lvn["b"].back().emplace_back(); // add an array
     lvn["b"].back().emplace_back(); // add another array
+    static std::vector<RValueCaster> lvv{2};
     m.def("cast_lv_vector", []() -> const decltype(lvv) & { return lvv; });
     m.def("cast_lv_array", []() -> const decltype(lva) & { return lva; });
     m.def("cast_lv_map", []() -> const decltype(lvm) & { return lvm; });