build: Enable -Wsuggest-override

configure.ac

@@ -368,6 +368,10 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=return-type],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=conditional-uninitialized],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=conditional-uninitialized"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=sign-compare],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=sign-compare"],,[[$CXXFLAG_WERROR]])
+  dnl -Wsuggest-override is broken with GCC before 9.2
+  dnl https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78010
+  AX_CHECK_COMPILE_FLAG([-Werror=suggest-override],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=suggest-override"],,[[$CXXFLAG_WERROR]],
+                        [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then
@@ -385,6 +389,8 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wdate-time],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wconditional-uninitialized],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wconditional-uninitialized"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wsign-compare],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsign-compare"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wsuggest-override],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"],,[[$CXXFLAG_WERROR]],
+                        [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
 
   dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   dnl unknown options if any other warning is produced. Test the -Wfoo case, and

src/Makefile.leveldb.include

@@ -36,7 +36,7 @@ LEVELDB_CPPFLAGS_INT += -DLEVELDB_PLATFORM_POSIX
 endif
 
 leveldb_libleveldb_a_CPPFLAGS = $(AM_CPPFLAGS) $(LEVELDB_CPPFLAGS_INT) $(LEVELDB_CPPFLAGS)
-leveldb_libleveldb_a_CXXFLAGS = $(filter-out -Wconditional-uninitialized -Werror=conditional-uninitialized, $(AM_CXXFLAGS)) $(PIE_FLAGS)
+leveldb_libleveldb_a_CXXFLAGS = $(filter-out -Wconditional-uninitialized -Werror=conditional-uninitialized -Wsuggest-override -Werror=suggest-override, $(AM_CXXFLAGS)) $(PIE_FLAGS)
 
 leveldb_libleveldb_a_SOURCES=
 leveldb_libleveldb_a_SOURCES += leveldb/port/port_stdcxx.h

src/wallet/db.h

@@ -19,7 +19,14 @@
 #include <unordered_map>
 #include <vector>
 
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wsuggest-override"
+#endif
 #include <db_cxx.h>
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
 
 struct bilingual_str;