Fix Windows build with --enable-werror on Ubuntu Focal

configure.ac

@@ -430,7 +430,13 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=range-loop-analysis],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=range-loop-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=unused-variable],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unused-variable"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=date-time],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=date-time"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Werror=return-type],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"],,[[$CXXFLAG_WERROR]])
+
+  dnl -Wreturn-type is broken in GCC for MinGW-w64.
+  dnl https://sourceforge.net/p/mingw-w64/bugs/306/
+  AX_CHECK_COMPILE_FLAG([-Werror=return-type], [ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"], [], [$CXXFLAG_WERROR],
+                        [AC_LANG_SOURCE([[#include <cassert>
+                                          int f(){ assert(false); }]])])
+
   AX_CHECK_COMPILE_FLAG([-Werror=conditional-uninitialized],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=conditional-uninitialized"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=sign-compare],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=sign-compare"],,[[$CXXFLAG_WERROR]])
   dnl -Wsuggest-override is broken with GCC before 9.2

src/fs.cpp

@@ -154,7 +154,10 @@ std::string get_filesystem_error_message(const fs::filesystem_error& e)
 #ifdef __GLIBCXX__
 
 // reference: https://github.com/gcc-mirror/gcc/blob/gcc-7_3_0-release/libstdc%2B%2B-v3/include/std/fstream#L270
-
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wswitch"
+#endif
 static std::string openmodeToStr(std::ios_base::openmode mode)
 {
     switch (mode & ~std::ios_base::ate) {
@@ -192,6 +195,9 @@ static std::string openmodeToStr(std::ios_base::openmode mode)
         return std::string();
     }
 }
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 
 void ifstream::open(const fs::path& p, std::ios_base::openmode mode)
 {

src/qt/winshutdownmonitor.h

@@ -17,7 +17,7 @@ class WinShutdownMonitor : public QAbstractNativeEventFilter
 {
 public:
     /** Implements QAbstractNativeEventFilter interface for processing Windows messages */
-    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult);
+    bool nativeEventFilter(const QByteArray &eventType, void *pMessage, long *pnResult) override;
 
     /** Register the reason for blocking shutdown on Windows to allow clean client exit */
     static void registerShutdownBlockReason(const QString& strReason, const HWND& mainWinId);