Use C++17 [[fallthrough]] attribute, and drop -Wno-implicit-fallthrough

configure.ac

@@ -463,6 +463,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wsuggest-override],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"],,[[$CXXFLAG_WERROR]],
                         [AC_LANG_SOURCE([[struct A { virtual void f(); }; struct B : A { void f() final; };]])])
   AX_CHECK_COMPILE_FLAG([-Wunreachable-code-loop-increment],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code-loop-increment"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wimplicit-fallthrough"], [], [$CXXFLAG_WERROR])
 
   if test x$suppress_external_warnings != xno ; then
     AX_CHECK_COMPILE_FLAG([-Wdocumentation],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"],,[[$CXXFLAG_WERROR]])
@@ -474,7 +475,6 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wunused-parameter],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-parameter"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wself-assign],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-self-assign"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
-  AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-implicit-fallthrough"],,[[$CXXFLAG_WERROR]])
   if test x$suppress_external_warnings != xyes ; then
     AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],[NOWARN_CXXFLAGS="$NOWARN_CXXFLAGS -Wno-deprecated-copy"],,[[$CXXFLAG_WERROR]])
   fi

src/Makefile.leveldb.include

@@ -22,6 +22,7 @@ LEVELDB_CPPFLAGS_INT += -DHAVE_SNAPPY=0 -DHAVE_CRC32C=1
 LEVELDB_CPPFLAGS_INT += -DHAVE_FDATASYNC=@HAVE_FDATASYNC@
 LEVELDB_CPPFLAGS_INT += -DHAVE_FULLFSYNC=@HAVE_FULLFSYNC@
 LEVELDB_CPPFLAGS_INT += -DHAVE_O_CLOEXEC=@HAVE_O_CLOEXEC@
+LEVELDB_CPPFLAGS_INT += -DFALLTHROUGH_INTENDED=[[fallthrough]]
 
 if WORDS_BIGENDIAN
 LEVELDB_CPPFLAGS_INT += -DLEVELDB_IS_BIG_ENDIAN=1

src/hash.cpp

@@ -47,8 +47,10 @@ unsigned int MurmurHash3(unsigned int nHashSeed, Span<const unsigned char> vData
     switch (vDataToHash.size() & 3) {
         case 3:
             k1 ^= tail[2] << 16;
+            [[fallthrough]];
         case 2:
             k1 ^= tail[1] << 8;
+            [[fallthrough]];
         case 1:
             k1 ^= tail[0];
             k1 *= c1;

src/qt/rpcconsole.cpp

@@ -287,6 +287,7 @@ bool RPCConsole::RPCParseCommandLine(interfaces::Node* node, std::string &strRes
                 }
                 if (breakParsing)
                     break;
+                [[fallthrough]];
             }
             case STATE_ARGUMENT: // In or after argument
             case STATE_EATING_SPACES_IN_ARG:
@@ -400,6 +401,7 @@ bool RPCConsole::RPCParseCommandLine(interfaces::Node* node, std::string &strRes
                 strResult = lastResult.get_str();
             else
                 strResult = lastResult.write(2);
+            [[fallthrough]];
         case STATE_ARGUMENT:
         case STATE_EATING_SPACES:
             return true;

src/rest.cpp

@@ -524,6 +524,7 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
         // convert hex to bin, continue then with bin part
         std::vector<unsigned char> strRequestV = ParseHex(strRequestMutable);
         strRequestMutable.assign(strRequestV.begin(), strRequestV.end());
+        [[fallthrough]];
     }
 
     case RetFormat::BINARY: {

src/rpc/mining.cpp

@@ -850,7 +850,7 @@ static RPCHelpMan getblocktemplate()
             case ThresholdState::LOCKED_IN:
                 // Ensure bit is set in block version
                 pblock->nVersion |= g_versionbitscache.Mask(consensusParams, pos);
-                // FALL THROUGH to get vbavailable set...
+                [[fallthrough]];
             case ThresholdState::STARTED:
             {
                 const struct VBDeploymentInfo& vbinfo = VersionBitsDeploymentInfo[pos];

src/tinyformat.h

@@ -797,27 +797,27 @@ inline const char* streamStateFromFormat(std::ostream& out, bool& positionalMode
             break;
         case 'X':
             out.setf(std::ios::uppercase);
-            // Falls through
+            [[fallthrough]];
         case 'x': case 'p':
             out.setf(std::ios::hex, std::ios::basefield);
             intConversion = true;
             break;
         case 'E':
             out.setf(std::ios::uppercase);
-            // Falls through
+            [[fallthrough]];
         case 'e':
             out.setf(std::ios::scientific, std::ios::floatfield);
             out.setf(std::ios::dec, std::ios::basefield);
             break;
         case 'F':
             out.setf(std::ios::uppercase);
-            // Falls through
+            [[fallthrough]];
         case 'f':
             out.setf(std::ios::fixed, std::ios::floatfield);
             break;
         case 'A':
             out.setf(std::ios::uppercase);
-            // Falls through
+            [[fallthrough]];
         case 'a':
 #           ifdef _MSC_VER
             // Workaround https://developercommunity.visualstudio.com/content/problem/520472/hexfloat-stream-output-does-not-ignore-precision-a.html
@@ -829,7 +829,7 @@ inline const char* streamStateFromFormat(std::ostream& out, bool& positionalMode
             break;
         case 'G':
             out.setf(std::ios::uppercase);
-            // Falls through
+            [[fallthrough]];
         case 'g':
             out.setf(std::ios::dec, std::ios::basefield);
             // As in boost::format, let stream decide float format.