Audit and fixups for GCC and Clang (#7212)

Added (for compiler versions that support it):
-Wmissing-declarations
-Wzero-as-null-pointer-constant

Then, fixes to problems identified by the extra warnings
Tested only on GCC 9.4.0

Adjusted the CPP code generator to output nullptr where appropriate,
to satisfy -Wzero-as-null-pointer-constant

Added a lot of 'static' declarations in front of functions,
to satisfy -Wmissing-declarations,
and wrap static function defs in anonymous namespaces.

There are advantages to both anonymous namespaces and static,
it seems that marking a function as static will not publish the name in
the symbol table at all, thus giving the linker less work to do.

tests/cpp17/test_cpp17.cpp

@@ -47,6 +47,7 @@ using ::cpp17::MyGame::Example::Vec3;
 /*******************************************************************************
 ** Build some FB objects.
 *******************************************************************************/
+namespace {
 const Monster *BuildMonster(flatbuffers::FlatBufferBuilder &fbb) {
   using ::cpp17::MyGame::Example::Color;
   using ::cpp17::MyGame::Example::MonsterBuilder;
@@ -250,6 +251,7 @@ int FlatBufferCpp17Tests() {
   StringifyAnyFlatbuffersTypeTest();
   return 0;
 }
+} // namespace
 
 int main(int /*argc*/, const char * /*argv*/[]) {
   InitTestEngine();

tests/fuzzer/test_init.h

@@ -11,7 +11,7 @@ struct OneTimeTestInit {
   // This hook terminate program both in Debug and Release.
   static bool TestFailListener(const char *expval, const char *val,
                                const char *exp, const char *file, int line,
-                               const char *func = 0) {
+                               const char *func = nullptr) {
     (void)expval;
     (void)val;
     (void)exp;

tests/test.cpp

@@ -42,6 +42,10 @@
 #include "native_type_test_generated.h"
 #include "test_assert.h"
 
+void FlatBufferBuilderTest();
+
+namespace {
+
 // clang-format off
 // Check that char* and uint8_t* are interoperable types.
 // The reinterpret_cast<> between the pointers are used to simplify data loading.
@@ -63,8 +67,6 @@ static const auto infinity_d = std::numeric_limits<double>::infinity();
 
 using namespace MyGame::Example;
 
-void FlatBufferBuilderTest();
-
 // Include simple random number generator to ensure results will be the
 // same cross platform.
 // http://en.wikipedia.org/wiki/Park%E2%80%93Miller_random_number_generator
@@ -776,19 +778,19 @@ template<typename T, typename U, U qnan_base> bool is_quiet_nan_impl(T v) {
   return ((b & qnan_base) == qnan_base);
 }
 #  if defined(__mips__) || defined(__hppa__)
-static bool is_quiet_nan(float v) {
+bool is_quiet_nan(float v) {
   return is_quiet_nan_impl<float, uint32_t, 0x7FC00000u>(v) ||
          is_quiet_nan_impl<float, uint32_t, 0x7FBFFFFFu>(v);
 }
-static bool is_quiet_nan(double v) {
+bool is_quiet_nan(double v) {
   return is_quiet_nan_impl<double, uint64_t, 0x7FF8000000000000ul>(v) ||
          is_quiet_nan_impl<double, uint64_t, 0x7FF7FFFFFFFFFFFFu>(v);
 }
 #  else
-static bool is_quiet_nan(float v) {
+bool is_quiet_nan(float v) {
   return is_quiet_nan_impl<float, uint32_t, 0x7FC00000u>(v);
 }
-static bool is_quiet_nan(double v) {
+bool is_quiet_nan(double v) {
   return is_quiet_nan_impl<double, uint64_t, 0x7FF8000000000000ul>(v);
 }
 #  endif
@@ -4722,6 +4724,7 @@ int FlatBufferTests() {
   ErrorTest();
   ValueTest();
   EnumValueTest();
+  NestedListTest();
   EnumStringsTest();
   EnumNamesTest();
   EnumOutOfRangeTest();
@@ -4775,6 +4778,7 @@ int FlatBufferTests() {
   VectorSpanTest();
   return 0;
 }
+} // namespace
 
 int main(int argc, const char *argv[]) {
   for (int argi = 1; argi < argc; argi++) {

tests/test_builder.cpp

@@ -117,6 +117,9 @@ bool release_n_verify(flatbuffers::FlatBufferBuilder &fbb,
   return verify(buf, expected_name, color);
 }
 
+// forward-declared in test.cpp
+void FlatBufferBuilderTest();
+
 void FlatBufferBuilderTest() {
   using flatbuffers::FlatBufferBuilder;
 
@@ -138,5 +141,8 @@ void FlatBufferBuilderTest() {
       TestSelector(tests, tests + 4));
 }
 
+// forward-declared in test_builder.h
+void CheckTestGeneratedIsValid(const MyGame::Example::Color&);
+
 // Link-time check using pointer type.
 void CheckTestGeneratedIsValid(const MyGame::Example::Color &) {}