[flang][NFC] Add a no-arg constructor for `Verbatim`.

[rupprecht]

Summary

In C++20, types that declare or delete any constructors are no longer aggregates, breaking compilation of many existing uses of aggregate initialization.

Although Verbatim declares itself to not have a no-arg default constructor, this is circumvented in basic-parsers.h which returns a RESULT{} a.k.a. Verbatim{}. Adding the no-arg constructor while still deleting the copy/assignment constructors maintains the current state and also supports eventually building this in c++20 mode.

Fix suggested in https://discourse.llvm.org/t/build-failure-when-attempting-to-build-flang-with-c-20/66953.

[klausler]

Where in basic-parsers.h is a Verbatim constructed with no argument? That would be somewhat dangerous since Verbatim::source really should be valid.

[rupprecht]

https://discourse.llvm.org/t/build-failure-when-attempting-to-build-flang-with-c-20/66953 has the full build error, but: the RESULT{} is here: https://github.com/llvm/llvm-project/blob/1294fa697176c244e92e72c0b01fd3b5e3a06477/flang/lib/Parser/basic-parsers.h#L754
I think the Verbatim type is being inferred from here: https://github.com/llvm/llvm-project/blob/1294fa697176c244e92e72c0b01fd3b5e3a06477/flang/lib/Parser/openmp-parsers.cpp#L354

[klausler]

In D139228#3967643, @rupprecht wrote:

https://discourse.llvm.org/t/build-failure-when-attempting-to-build-flang-with-c-20/66953 has the full build error, but: the RESULT{} is here: https://github.com/llvm/llvm-project/blob/1294fa697176c244e92e72c0b01fd3b5e3a06477/flang/lib/Parser/basic-parsers.h#L754
I think the Verbatim type is being inferred from here: https://github.com/llvm/llvm-project/blob/1294fa697176c244e92e72c0b01fd3b5e3a06477/flang/lib/Parser/openmp-parsers.cpp#L354

I doubt it; the template function "verbatim()" defined near the end of token-parsers.h is going to construct a Verbatim on whatever the result type of the argument is -- and in the case of a "..."_tok token, that's a Success indicator.

[rupprecht]

In D139228#3967681, @klausler wrote:

In D139228#3967643, @rupprecht wrote:

https://discourse.llvm.org/t/build-failure-when-attempting-to-build-flang-with-c-20/66953 has the full build error, but: the RESULT{} is here: https://github.com/llvm/llvm-project/blob/1294fa697176c244e92e72c0b01fd3b5e3a06477/flang/lib/Parser/basic-parsers.h#L754
I think the Verbatim type is being inferred from here: https://github.com/llvm/llvm-project/blob/1294fa697176c244e92e72c0b01fd3b5e3a06477/flang/lib/Parser/openmp-parsers.cpp#L354

I doubt it; the template function "verbatim()" defined near the end of token-parsers.h is going to construct a Verbatim on whatever the result type of the argument is -- and in the case of a "..."_tok token, that's a Success indicator.

I agree that *that* verbatim instance is valid, but the type of it gets interred by a template method, and some machinery attempts to construct a no-arg version.

The source is this parser:

TYPE_PARSER(sourced(construct<OpenMPCancellationPointConstruct>(
    verbatim("CANCELLATION POINT"_tok), Parser<OmpCancelType>{})))

The fully expanded type of that: Fortran::parser::SourcedParser<Fortran::parser::ApplyConstructor<Fortran::parser::OpenMPCancellationPointConstruct, Fortran::parser::SourcedParser<Fortran::parser::ApplyConstructor<Fortran::parser::Verbatim, Fortran::parser::TokenStringMatch<false, false>>>, Fortran::parser::Parser<Fortran::parser::OmpCancelType>>>

The case of a`Success` verbatim token in ApplyConstructor returns a no-arg object here: https://github.com/llvm/llvm-project/blob/a74c5707be279bdddb51fe49aa9383c1ddc99fbe/flang/lib/Parser/basic-parsers.h#L751

std::optional<resultType> ParseOne(ParseState &state) const {
  if constexpr (std::is_same_v<Success, typename PARSER::resultType...>) {
    if (std::get<0>(parsers_).Parse(state)) {
      return RESULT{};
    }

flang/include/flang/Parser/parse-tree.h

 struct OpenMPDeclarativeConstruct;
 struct OmpEndLoopDirective;
 
 // Cooked character stream locations
 using Location = const char *;
 
 // A parse tree node with provenance only
 struct Verbatim {
-  BOILERPLATE(Verbatim);
+  // Allow a no-arg constructor for Verbatim so parsers can return `RESULT{}`.
+  constexpr Verbatim() {}
+  COPY_AND_ASSIGN_BOILERPLATE(Verbatim);
   using EmptyTrait = std::true_type;
   CharBlock source;
 };
 
 // Implicit definitions of the Standard
 
 // R403 scalar-xyz -> xyz
 // These template class wrappers correspond to the Standard's modifiers