From dee4686227842aa0e8380c7925049a5df9c4f781 Mon Sep 17 00:00:00 2001
From: Michael Kruse <llvm-project@meinersbur.de>
Date: Wed, 16 Sep 2020 14:58:29 -0500
Subject: [PATCH]  [flang][msvc] Work around if constexpr (false) evaluation.
 NFC.

MSVC tries to expand templates that are in the false-branch of a `if constexpr` construct. In this case, the condition checks whether a tuple has at least one element and then is trying to access it using `std::get<0>`, which fails when the tuple has 0 elements.

The workaround is to extract that case into a separate method.

This patch is part of the series to make flang compilable with MS Visual Studio <http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html>.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D87728
---
 flang/lib/Parser/basic-parsers.h | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/flang/lib/Parser/basic-parsers.h b/flang/lib/Parser/basic-parsers.h
index 56d9ff1..c92ece0 100644
--- a/flang/lib/Parser/basic-parsers.h
+++ b/flang/lib/Parser/basic-parsers.h
@@ -729,13 +729,7 @@ public:
       return RESULT{};
     } else {
       if constexpr (sizeof...(PARSER) == 1) {
-        if constexpr (std::is_same_v<Success, typename PARSER::resultType...>) {
-          if (std::get<0>(parsers_).Parse(state)) {
-            return RESULT{};
-          }
-        } else if (auto arg{std::get<0>(parsers_).Parse(state)}) {
-          return RESULT{std::move(*arg)};
-        }
+        return ParseOne(state);
       } else {
         ApplyArgs<PARSER...> results;
         using Sequence = std::index_sequence_for<PARSER...>;
@@ -749,6 +743,17 @@ public:
   }
 
 private:
+  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{};
+      }
+    } else if (auto arg{std::get<0>(parsers_).Parse(state)}) {
+      return RESULT{std::move(*arg)};
+    }
+    return std::nullopt;
+  }
+
   const std::tuple<PARSER...> parsers_;
 };
 
-- 
2.7.4