[llvm] [Support]Look up in top-level subcommand as a fallback when looking options for a custom subcommand. (PR #71776)

[Snehasish Kumar via llvm-commits]

================
@@ -525,6 +525,74 @@ TEST(CommandLineTest, LookupFailsInWrongSubCommand) {
   EXPECT_FALSE(Errs.empty());
 }
 
+TEST(CommandLineTest, SubcommandOptions) {
+  enum LiteralOptionEnum {
+    foo,
+    bar,
+    baz,
+  };
+
+  cl::ResetCommandLineParser();
+
+  // This is a top-level option and not associated with a subcommand.
+  // A command line using subcommand should parse both subcommand options and
+  // top-level options.  A valid use case is that users of llvm command line
+  // tools should be able to specify top-level options defined in any library.
+  cl::opt<std::string> TopLevelOpt("str", cl::init("txt"),
+                                   cl::desc("A top-level option."));
+
+  StackSubCommand SC("sc", "Subcommand");
+
+  // The positional argument.
+  StackOption<std::string> PositionalOpt(
+      cl::Positional, cl::desc("positional argument test coverage"),
+      cl::sub(SC));
+  // Thel literal argument.
+  StackOption<LiteralOptionEnum> LiteralOpt(
+      cl::desc("literal argument test coverage"), cl::sub(SC), cl::init(bar),
+      cl::values(clEnumVal(foo, "foo"), clEnumVal(bar, "bar"),
+                 clEnumVal(baz, "baz")));
+  StackOption<bool> BoolOpt("enable", cl::sub(SC), cl::init(false));
+
+  std::string Errs;
+  raw_string_ostream OS(Errs);
+
+  for (const char *literalArg : {"--bar", "--foo", "--baz"}) {
----------------
snehasish wrote:

If you structure this loop to iterate over a list of {input, want} pairs then you can avoid the switch case below and the conversion for checking.

```
for (auto& test : {std::pair{"--bar", bar}, {"--foo", foo}, {"--baz", baz}) {
  ...
  EXPECT_EQ(LiteralOpt, test.second);
}
```

https://github.com/llvm/llvm-project/pull/71776