[flang][Parser] Convert applyMem to invoke non-void members

flang/docs/ParserCombinators.md

@@ -141,7 +141,7 @@ collect the values that they return.
 * `applyLambda([](&&x){}, p1, p2, ...)` is the same thing, but for lambdas
   and other function objects.
 * `applyMem(mf, p1, p2, ...)` is the same thing, but invokes a member
-  function of the result of the first parser for updates in place.
+  function of the result of the first parser.
 
 ### Token Parsers
 Last, we have these basic parsers on which the actual grammar of the Fortran

flang/lib/Parser/basic-parsers.h

@@ -580,11 +580,11 @@ template <typename PA> inline constexpr auto defaulted(PA p) {
 // applyLambda(f, ...) is the same concept extended to std::function<> functors.
 // It is not constexpr.
 //
-// Member function application is supported by applyMem(f, a).  If the
-// parser a succeeds and returns some value ax, the result is that returned
-// by ax.f().  Additional parser arguments can be specified to supply their
-// results to the member function call, so applyMem(f, a, b) succeeds if
-// both a and b do so and returns the result of calling ax.f(std::move(bx)).
+// Member function application is supported by applyMem(&C::f, a).  If the
+// parser a succeeds and returns some value ax of type C, the result is that
+// returned by ax.f().  Additional parser arguments can be specified to supply
+// their results to the member function call, so applyMem(&C::f, a, b) succeeds
+// if both a and b do so and returns the result of calling ax.f(std::move(bx)).
 
 // Runs a sequence of parsers until one fails or all have succeeded.
 // Collects their results in a std::tuple<std::optional<>...>.
@@ -654,39 +654,31 @@ inline /* not constexpr */ auto applyLambda(
 }
 
 // Member function application
-template <typename OBJPARSER, typename... PARSER> class AMFPHelper {
-  using resultType = typename OBJPARSER::resultType;
-
-public:
-  using type = void (resultType::*)(typename PARSER::resultType &&...);
-};
-template <typename OBJPARSER, typename... PARSER>
-using ApplicableMemberFunctionPointer =
-    typename AMFPHelper<OBJPARSER, PARSER...>::type;
-
-template <typename OBJPARSER, typename... PARSER, std::size_t... J>
-inline auto ApplyHelperMember(
-    ApplicableMemberFunctionPointer<OBJPARSER, PARSER...> mfp,
-    ApplyArgs<OBJPARSER, PARSER...> &&args, std::index_sequence<J...>) ->
-    typename OBJPARSER::resultType {
-  ((*std::get<0>(args)).*mfp)(std::move(*std::get<J + 1>(args))...);
-  return std::get<0>(std::move(args));
+template <typename MEMFUNC, typename OBJPARSER, typename... PARSER,
+    std::size_t... J>
+inline auto ApplyHelperMember(MEMFUNC mfp,
+    ApplyArgs<OBJPARSER, PARSER...> &&args, std::index_sequence<J...>) {
+  return ((*std::get<0>(args)).*mfp)(std::move(*std::get<J + 1>(args))...);
 }
 
-template <typename OBJPARSER, typename... PARSER> class ApplyMemberFunction {
-  using funcType = ApplicableMemberFunctionPointer<OBJPARSER, PARSER...>;
+template <typename MEMFUNC, typename OBJPARSER, typename... PARSER>
+class ApplyMemberFunction {
+  static_assert(std::is_member_function_pointer_v<MEMFUNC>);
+  using funcType = MEMFUNC;
 
 public:
-  using resultType = typename OBJPARSER::resultType;
+  using resultType =
+      std::invoke_result_t<MEMFUNC, typename OBJPARSER::resultType, PARSER...>;
+
   constexpr ApplyMemberFunction(const ApplyMemberFunction &) = default;
-  constexpr ApplyMemberFunction(funcType f, OBJPARSER o, PARSER... p)
+  constexpr ApplyMemberFunction(MEMFUNC f, OBJPARSER o, PARSER... p)
       : function_{f}, parsers_{o, p...} {}
   std::optional<resultType> Parse(ParseState &state) const {
     ApplyArgs<OBJPARSER, PARSER...> results;
     using Sequence1 = std::index_sequence_for<OBJPARSER, PARSER...>;
     using Sequence2 = std::index_sequence_for<PARSER...>;
     if (ApplyHelperArgs(parsers_, results, state, Sequence1{})) {
-      return ApplyHelperMember<OBJPARSER, PARSER...>(
+      return ApplyHelperMember<MEMFUNC, OBJPARSER, PARSER...>(
           function_, std::move(results), Sequence2{});
     } else {
       return std::nullopt;
@@ -698,11 +690,11 @@ template <typename OBJPARSER, typename... PARSER> class ApplyMemberFunction {
   const std::tuple<OBJPARSER, PARSER...> parsers_;
 };
 
-template <typename OBJPARSER, typename... PARSER>
+template <typename MEMFUNC, typename OBJPARSER, typename... PARSER>
 inline constexpr auto applyMem(
-    ApplicableMemberFunctionPointer<OBJPARSER, PARSER...> mfp,
-    const OBJPARSER &objParser, PARSER... parser) {
-  return ApplyMemberFunction<OBJPARSER, PARSER...>{mfp, objParser, parser...};
+    MEMFUNC memfn, const OBJPARSER &objParser, PARSER... parser) {
+  return ApplyMemberFunction<MEMFUNC, OBJPARSER, PARSER...>{
+      memfn, objParser, parser...};
 }
 
 // As is done with function application via applyFunction() above, class