From d228d6e33b460be975ac0a0325a94a7f3307c0e2 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Tue, 14 May 2024 09:16:23 +0200
Subject: Map C23/C2X to /std:clatest starting from MSVC 17.9 (19.39)

In particular, this option enables C23 typeof support.
---
 libbuild2/c/init.cxx   | 26 +++++++++++++++++---------
 libbuild2/cxx/init.cxx |  6 +++---
 2 files changed, 20 insertions(+), 12 deletions(-)

diff --git a/libbuild2/c/init.cxx b/libbuild2/c/init.cxx
index 8bc2f7d..f100abc 100644
--- a/libbuild2/c/init.cxx
+++ b/libbuild2/c/init.cxx
@@ -111,23 +111,31 @@ namespace build2
           // From version 16.8 VC now supports /std:c11 and /std:c17 options
           // which enable C11/17 conformance. However, as of version 16.10,
           // neither SDK nor CRT can be compiled in these modes (see the /std
-          // option documentation for details/updates).
+          // option documentation for details/updates). There is also now
+          // /std:clatest which can be used to enable C23 typeof as of MSVC
+          // 17.9. So let's map C23 to that.
           //
           if (v == nullptr)
             ;
           else if (!stdcmp ("90"))
           {
-            uint64_t cver (ci.version.major);
-
-            if ((stdcmp ("99")   && cver < 16) ||  // Since VS2010/10.0.
-                ((stdcmp ("11") ||
-                  stdcmp ("17") ||
-                  stdcmp ("18")) && cver < 18) ||  // Since VS????/11.0.
-                (stdcmp ("23", "2x")           ))
+            uint64_t mj (ci.version.major);
+            uint64_t mi (ci.version.minor);
+
+            if      (stdcmp ("99") && mj >= 16) // Since VS2010/10.0.
+              ;
+            else if ((stdcmp ("11") ||
+                      stdcmp ("17") ||
+                      stdcmp ("18")) && mj >= 18) // Since VS????/11.0.
+              ;
+            else if (stdcmp ("23", "2x") &&
+                     (mj > 19 || (mj == 19 && mi >= 39))) // Since 17.9.
             {
+              mode.insert (mode.begin (), "/std:clatest");
+            }
+            else
               fail << "C " << *v << " is not supported by " << ci.signature <<
                 info << "required by " << project (rs) << '@' << rs;
-            }
           }
           break;
         }
diff --git a/libbuild2/cxx/init.cxx b/libbuild2/cxx/init.cxx
index 8159d18..0c29d41 100644
--- a/libbuild2/cxx/init.cxx
+++ b/libbuild2/cxx/init.cxx
@@ -498,8 +498,8 @@ namespace build2
 
       // Unless disabled by the user, try to enable C++ modules.
       //
-      // NOTE: see also diagnostics about modules support required in compile
-      //       rule.
+      // NOTE: see also diagnostics about modules support required (if
+      //       attempting to use) in compile rule.
       //
       if (!modules.value || *modules.value)
       {
@@ -580,7 +580,7 @@ namespace build2
             // around Clang 16 so we don't support anything earlier than
             // that (it's not practically usable anyway).
             //
-            // Clang enable modules by default in c++20 or later but they
+            // Clang enables modules by default in c++20 or later but they
             // don't yet (as of Clang 18) define __cpp_modules. When they
             // do, we can consider enabling modules by default on our side.
             // For now, we only enable modules if forced with explicit
-- 
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