From a8ff78d8b90d6f7a45c9586449986dadb65809c8 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Mon, 6 Nov 2023 13:53:00 +0200
Subject: Fix basic issues with C++20 named modules support in Clang

Note that modules support now require Clang 16 or later.
---
 libbuild2/cc/compile-rule.cxx | 17 +++++------------
 libbuild2/cxx/init.cxx        | 12 +++++++++++-
 2 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/libbuild2/cc/compile-rule.cxx b/libbuild2/cc/compile-rule.cxx
index 4f94167..ee97b39 100644
--- a/libbuild2/cc/compile-rule.cxx
+++ b/libbuild2/cc/compile-rule.cxx
@@ -6893,7 +6893,8 @@ namespace build2
           //
 #if 0
           // In Clang the module implementation's unit .pcm is special and
-          // must be "loaded".
+          // must be "loaded". Note: not anymore, not from Clang 16 and is
+          // deprecated in 17.
           //
           if (ut == unit_type::module_impl)
           {
@@ -6926,17 +6927,9 @@ namespace build2
             const file& f (pt->as<file> ());
             string s (relative (f.path ()).string ());
 
-            // In Clang the module implementation's unit .pcm is special and
-            // must be "loaded".
-            //
-            if (ut == unit_type::module_impl && i == ms.start)
-              s.insert (0, "-fmodule-file=");
-            else
-            {
-              s.insert (0, 1, '=');
-              s.insert (0, cast<string> (f.state[a].vars[c_module_name]));
-              s.insert (0, "-fmodule-file=");
-            }
+            s.insert (0, 1, '=');
+            s.insert (0, cast<string> (f.state[a].vars[c_module_name]));
+            s.insert (0, "-fmodule-file=");
 
             stor.push_back (move (s));
           }
diff --git a/libbuild2/cxx/init.cxx b/libbuild2/cxx/init.cxx
index 0d4faf2..3023ad5 100644
--- a/libbuild2/cxx/init.cxx
+++ b/libbuild2/cxx/init.cxx
@@ -433,6 +433,8 @@ namespace build2
             }
           case compiler_type::clang:
             {
+              // @@ TMP: revise
+              //
               // At the time of this writing, support for C++20 modules in
               // Clang is incomplete. And starting with Clang 9 (Apple Clang
               // 11.0.3), they are enabled by default in the C++2a mode which
@@ -448,8 +450,16 @@ namespace build2
               //
               if (modules.value)
               {
+                if (mj < 16)
+                {
+                  fail << "support for C++ modules requires Clang 16 or later" <<
+                    info << "C++ compiler is " << ci.signature <<
+                    info << "required by " << project (rs) << '@' << rs;
+                }
+
+                // See https://github.com/llvm/llvm-project/issues/71364
+                //
                 prepend ("-D__cpp_modules=201704"); // p0629r0
-                mode.push_back ("-fmodules-ts"); // For the hack to work.
                 modules = true;
               }
 
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