[flang][driver] Fix -fdefault* family bug

[arnamoy10]

Summary

This patch provides a fix for the fdefault-* family in f18

(Please consult D96344 for details)

[awarzynski]

This patch will allow us to share the tests for -fdefault-{double-8|integer-8|real-8} and -flarge-sizes (introduced in https://reviews.llvm.org/D96344) between f18 and flang-new. This is a very much welcomed change, thank you!

I don't think that we need any changes for -module though. f18 already supports -module-dir and there are no tests using -module that would require this change in the future. Please, could you revert it?

Ideally this patch would be accompanied with tests, but since D96344 is practically ready to land (and which contains tests for this), I suggest that we merge this as is. Could you mention e.g. in the commit message that tests will be added in a forthcoming patch? Thank you!

[arnamoy10]

Taken out -module related changes, updated the commit message as well.

[awarzynski]

LGTM

flang/tools/f18/f18.cpp

 };
 
 struct DriverOptions {
   DriverOptions() {}
   bool verbose{false}; // -v
   bool compileOnly{false}; // -c
   std::string outputPath; // -o path
   std::vector<std::string> searchDirectories; // -I dir
-  std::string moduleDirectory{"."s}; // -module dir
+  std::string moduleDirectory{"."s}; // -module/-module-dir dir
   std::string moduleFileSuffix{".mod"}; // -moduleSuffix suff
   bool forcedForm{false}; // -Mfixed or -Mfree appeared
   bool warnOnNonstandardUsage{false}; // -Mstandard
   bool warningsAreErrors{false}; // -Werror
   bool byteswapio{false}; // -byteswapio
   Fortran::parser::Encoding encoding{Fortran::parser::Encoding::UTF_8};
   bool syntaxOnly{false};
   bool dumpProvenance{false};
   bool dumpCookedChars{false};
   bool dumpUnparse{false};
   bool dumpUnparseWithSymbols{false};
   bool dumpParseTree{false};
   bool dumpPreFirTree{false};
   bool dumpSymbols{false};
   bool debugNoSemantics{false};
   bool debugModuleWriter{false};
+  bool defaultReal8{false};
   bool measureTree{false};
   bool unparseTypedExprsToF18_FC{false};
   std::vector<std::string> F18_FCArgs;
   const char *prefix{nullptr};
   bool getDefinition{false};
   GetDefinitionArgs getDefinitionArgs{0, 0, 0};
   bool getSymbolsSources{false};
   std::optional<bool> forcePreprocessing; // -cpp & -nocpp
if (arg.empty()) {
       auto eq{arg.find('=')};
       if (eq == std::string::npos) {
         predefinitions.emplace_back(arg.substr(2), "1");
       } else {
         predefinitions.emplace_back(arg.substr(2, eq - 2), arg.substr(eq + 1));
       }
     } else if (arg.substr(0, 2) == "-U") {
       predefinitions.emplace_back(arg.substr(2), std::optional<std::string>{});
-    } else if (arg == "-fdefault-double-8") {
-      defaultKinds.set_defaultRealKind(4);
     } else if (arg == "-r8" || arg == "-fdefault-real-8") {
+      driver.defaultReal8 = true;
       defaultKinds.set_defaultRealKind(8);
+      defaultKinds.set_doublePrecisionKind(16);
+    } else if (arg == "-fdefault-double-8") {
+      if (!driver.defaultReal8) {
+        // -fdefault-double-8 has to be used with -fdefault-real-8
+        // to be compatible with gfortran
+        llvm::errs() << "Use of `-fdefault-double-8` requires `-fdefault-real-8`\n";
+        return EXIT_FAILURE;
+      }
+      // For -fdefault-double-8 + -fdefault-real-8, only the size of
+      // DOUBLE PRECISION type changes, the size of default real type stays
+      // the same
+      // https://gcc.gnu.org/onlinedocs/gfortran/Fortran-Dialect-Options.html
+      defaultKinds.set_doublePrecisionKind(8);
     } else if (arg == "-i8" || arg == "-fdefault-integer-8") {
       defaultKinds.set_defaultIntegerKind(8);
       defaultKinds.set_subscriptIntegerKind(8);
       defaultKinds.set_sizeIntegerKind(8);
       if (isPGF90) {
         driver.F18_FCArgs.push_back("-i8");
       } else {
         driver.F18_FCArgs.push_back("-fdefault-integer-8");
       }
     } else if (arg == "-flarge-sizes") {
       defaultKinds.set_sizeIntegerKind(8);
     } else if (arg == "-fno-large-sizes") {
       defaultKinds.set_sizeIntegerKind(4);
     } else if (arg == "-module") {
       driver.moduleDirectory = args.front();
+      driver.searchDirectories.push_back(driver.moduleDirectory);
       args.pop_front();
     } else if (arg == "-module-dir") {
       driver.moduleDirectory = args.front();
       driver.searchDirectories.push_back(driver.moduleDirectory);
       args.pop_front();
     } else if (arg == "-module-suffix") {
       driver.moduleFileSuffix = args.front();
       args.pop_front();
if (arg.empty()) {
           << "  -M[no]backslash      disable[enable] \\escapes in literals\n"
           << "  -Mstandard           enable conformance warnings\n"
           << "  -std=<standard>      enable conformance warnings\n"
           << "  -r8 | -fdefault-real-8 | -i8 | -fdefault-integer-8 | "
              "-fdefault-double-8   change default kinds of intrinsic types\n"
           << "  -Werror              treat warnings as errors\n"
           << "  -ed                  enable fixed form D lines\n"
           << "  -E                   prescan & preprocess only\n"
-          << "  -module dir          module output directory (default .)\n"
+          << "  -module/-module-dir <dir> module output directory (default .)\n"
           << "  -flatin              interpret source as Latin-1 (ISO 8859-1) "
              "rather than UTF-8\n"
           << "  -fsyntax-only        parsing and semantics only, no output "
              "except messages\n"
           << "  -funparse            parse & reformat only, no code "
              "generation\n"
           << "  -funparse-with-symbols  parse, resolve symbols, and unparse\n"
           << "  -fdebug-measure-parse-tree\n"