From f749aab34924a61710aa28a33ad223e866aa5843 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Mon, 24 Feb 2020 14:56:33 +0200
Subject: Work around bug in Clang 10 targeting MSVC in c++2a (LLVM bug #44956)

---
 libbuild2/c/init.cxx    |  6 +++++-
 libbuild2/cc/module.cxx |  2 +-
 libbuild2/cc/module.hxx |  5 ++++-
 libbuild2/cxx/init.cxx  | 29 ++++++++++++++++++++++++-----
 4 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/libbuild2/c/init.cxx b/libbuild2/c/init.cxx
index d792f0d..ffea0ca 100644
--- a/libbuild2/c/init.cxx
+++ b/libbuild2/c/init.cxx
@@ -38,6 +38,7 @@ namespace build2
 
       virtual strings
       translate_std (const compiler_info&,
+                     const target_triplet&,
                      scope&,
                      const string*) const override;
     };
@@ -45,7 +46,10 @@ namespace build2
     using cc::module;
 
     strings config_module::
-    translate_std (const compiler_info& ci, scope& rs, const string* v) const
+    translate_std (const compiler_info& ci,
+                   const target_triplet&,
+                   scope& rs,
+                   const string* v) const
     {
       strings r;
 
diff --git a/libbuild2/cc/module.cxx b/libbuild2/cc/module.cxx
index c74f26d..d8365e4 100644
--- a/libbuild2/cc/module.cxx
+++ b/libbuild2/cc/module.cxx
@@ -384,7 +384,7 @@ namespace build2
 
         // Translate x_std value (if any) to the compiler option(s) (if any).
         //
-        tstd = translate_std (xi, rs, v);
+        tstd = translate_std (xi, tt, rs, v);
       }
 
       // config.x.translatable_header
diff --git a/libbuild2/cc/module.hxx b/libbuild2/cc/module.hxx
index d496779..4eca976 100644
--- a/libbuild2/cc/module.hxx
+++ b/libbuild2/cc/module.hxx
@@ -48,7 +48,10 @@ namespace build2
       // root scope.
       //
       virtual strings
-      translate_std (const compiler_info&, scope&, const string*) const = 0;
+      translate_std (const compiler_info&,
+                     const target_triplet&,
+                     scope&,
+                     const string*) const = 0;
 
       strings tstd;
 
diff --git a/libbuild2/cxx/init.cxx b/libbuild2/cxx/init.cxx
index a886cbc..4c4f6ad 100644
--- a/libbuild2/cxx/init.cxx
+++ b/libbuild2/cxx/init.cxx
@@ -39,6 +39,7 @@ namespace build2
 
       virtual strings
       translate_std (const compiler_info&,
+                     const target_triplet&,
                      scope&,
                      const string*) const override;
     };
@@ -46,7 +47,10 @@ namespace build2
     using cc::module;
 
     strings config_module::
-    translate_std (const compiler_info& ci, scope& rs, const string* v) const
+    translate_std (const compiler_info& ci,
+                   const target_triplet& tt,
+                   scope& rs,
+                   const string* v) const
     {
       strings r;
 
@@ -63,9 +67,9 @@ namespace build2
       // that is not necessarily complete or final but is practically usable.
       // In other words, a project that uses this value and does not rely on
       // any unstable/bleeding edge parts of the standard (or takes care to
-      // deal with them, for example, using feature test macros), can
-      // reasonably expect to work. In particular, this is the value we use by
-      // default in projects created by bdep-new(1) as well as to build the
+      // deal with them, for example, using feature test macros), can be
+      // reasonably expected to work. In particular, this is the value we use
+      // by default in projects created by bdep-new(1) as well as to build the
       // build2 toolchain itself.
       //
       // The `experimental` value, as the name suggests, is the latest
@@ -199,7 +203,22 @@ namespace build2
               }
             case compiler_type::clang:
               {
-                if       (mj >= 5)                         o = "-std=c++2a";
+                // Clang 10.0.0 targeting MSVC 16.4 and 16.5 (preview) in the
+                // c++2a mode uncovers some Concepts-related bugs in MSVC
+                // (LLVM bug #44956). So in this case we for now map `latest`
+                // to c++17. @@ TMP
+                //
+                // Note that if 10.0.0 is released without a workaround, then
+                // we will need to carry this forward at least for 16.4 since
+                // this version is unlikely to ever be fixed. Which means we
+                // will somehow need to pass the version of MSVC Clang is
+                // targeting.
+                //
+                if (latest && mj == 10 && tt.system == "win32-msvc")
+                {
+                  o = "-std=c++17";
+                }
+                else if  (mj >= 5)                         o = "-std=c++2a";
                 else if  (mj >  3 || (mj == 3 && mi >= 5)) o = "-std=c++1z";
                 else if  (mj == 3 && mi >= 4)              o = "-std=c++1y";
                 else     /* ??? */                         o = "-std=c++0x";
-- 
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