[C++] Add Command-Line Flag to Suppress MIN and MAX Enums (#7705)

Add the --no-minmax-values flag to prevent flatc from generating C++
enums with MIN and MAX enumerated values that otherwise would be set
to the inclusive lower and upper bound respectively of the enum.

This command-line flag is needed to avoid collisions when an enum that
is being ported to FlatBuffers already has a MIN or MAX enumerated
value.

It is also needed to work around a long-standing problem with
magic_enum that causes magic_enum to not see enumerated values that
are not unique.  For example, if FlatBuffers sets MIN = FOO and MAX =
BAR, MIN and FOO share the same underlying value so they are not
unique.  The same is true of MAX and BAR.  This prevents magic_enum
from converting FOO and BAR to and from strings as well as causing
magic_enum to return a count of enumerated values that is two fewer
than it should be.

Co-authored-by: Paul Serice <paul@serice.net>

docs/source/Compiler.md

@@ -90,7 +90,10 @@ Additional options:
 
 -   `--scoped-enums` : Use C++11 style scoped and strongly typed enums in
     generated C++. This also implies `--no-prefix`.
-
+    
+-   `--no-emit-min-max-enum-values` : Disable generation of MIN and MAX
+    enumerated values for scoped enums and prefixed enums.
+    
 -   `--gen-includes` : (deprecated), this is the default behavior.
                        If the original behavior is required (no include
 	                   statements) use `--no-includes.`

include/flatbuffers/idl.h

@@ -605,6 +605,7 @@ struct IDLOptions {
   bool output_enum_identifiers;
   bool prefixed_enums;
   bool scoped_enums;
+  bool emit_min_max_enum_values;
   bool swift_implementation_only;
   bool include_dependence_headers;
   bool mutable_buffer;
@@ -718,6 +719,7 @@ struct IDLOptions {
         output_enum_identifiers(true),
         prefixed_enums(true),
         scoped_enums(false),
+        emit_min_max_enum_values(true),
         swift_implementation_only(false),
         include_dependence_headers(true),
         mutable_buffer(false),

src/flatc.cpp

@@ -88,6 +88,9 @@ const static FlatCOption options[] = {
   { "", "scoped-enums", "",
     "Use C++11 style scoped and strongly typed enums. Also implies "
     "--no-prefix." },
+  { "", "no-emit-min-max-enum-values", "",
+    "Disable generation of MIN and MAX enumerated values for scoped enums "
+    "and prefixed enums." },
   { "", "swift-implementation-only", "",
     "Adds a @_implementationOnly to swift imports" },
   { "", "gen-includes", "",
@@ -464,6 +467,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
       } else if (arg == "--scoped-enums") {
         opts.prefixed_enums = false;
         opts.scoped_enums = true;
+      } else if (arg == "--no-emit-min-max-enum-values") {
+        opts.emit_min_max_enum_values = false;
       } else if (arg == "--no-union-value-namespacing") {
         opts.union_value_namespacing = false;
       } else if (arg == "--gen-mutable") {

src/idl_gen_cpp.cpp

@@ -1223,6 +1223,8 @@ class CppGenerator : public BaseGenerator {
       FLATBUFFERS_ASSERT(minv && maxv);
 
       code_.SetValue("SEP", ",\n");
+
+      // MIN & MAX are useless for bit_flags
       if (enum_def.attributes.Lookup("bit_flags")) {
         code_.SetValue("KEY", GenEnumValDecl(enum_def, "NONE"));
         code_.SetValue("VALUE", "0");
@@ -1233,7 +1235,7 @@ class CppGenerator : public BaseGenerator {
                        NumToStringCpp(enum_def.AllFlags(),
                                       enum_def.underlying_type.base_type));
         code_ += "{{SEP}}  {{KEY}} = {{VALUE}}\\";
-      } else {  // MIN & MAX are useless for bit_flags
+      } else if (opts_.emit_min_max_enum_values) {
         code_.SetValue("KEY", GenEnumValDecl(enum_def, "MIN"));
         code_.SetValue("VALUE", GenEnumValDecl(enum_def, Name(*minv)));
         code_ += "{{SEP}}  {{KEY}} = {{VALUE}}\\";