Add --warnings-as-errors to flatc compiler. (#7034)

* Add --warnings-as-errors to flatc compiler.

With this option set, flatc will return an error on parsing if
any warnings occurred.

* Add unit test for opts.warnings_as_errors.

* Change explicit option setting to default.

src/flatc.cpp

@@ -199,6 +199,7 @@ const static FlatCOption options[] = {
     "Used with \"binary\" and \"json\" options, it generates data using "
     "schema-less FlexBuffers." },
   { "", "no-warnings", "", "Inhibit all warnings messages." },
+  { "", "warning-as-errors", "", "Treat all warnings as errors." },
   { "", "cs-global-alias", "",
     "Prepend \"global::\" to all user generated csharp classes and "
     "structs." },
@@ -496,6 +497,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.gen_jvmstatic = true;
       } else if (arg == "--no-warnings") {
         opts.no_warnings = true;
+      } else if (arg == "--warnings-as-errors") {
+        opts.warnings_as_errors = true;
       } else if (arg == "--cpp-std") {
         if (++argi >= argc)
           Error("missing C++ standard specification" + arg, true);

src/idl_parser.cpp

@@ -144,7 +144,10 @@ void Parser::Message(const std::string &msg) {
 }
 
 void Parser::Warning(const std::string &msg) {
-  if (!opts.no_warnings) Message("warning: " + msg);
+  if (!opts.no_warnings) {
+    Message("warning: " + msg);
+    has_warning_ = true;  // for opts.warnings_as_errors
+  }
 }
 
 CheckedError Parser::Error(const std::string &msg) {
@@ -3444,6 +3447,9 @@ CheckedError Parser::DoParse(const char *source, const char **include_paths,
       ECHECK(ParseDecl(source_filename));
     }
   }
+  if (opts.warnings_as_errors && has_warning_) {
+    return Error("treating warnings as errors, failed due to above warnings");
+  }
   return NoError();
 }