Fix Clang -Wno-undefined-var-template in dynamic multi-database support code

Note that after this change we can no longer support multi-file circular
dependencies in this mode since we need to generate extern template
declarations involving classes that may participate in the cycle (see now
excluded odb-tests/common/circular/multiple/).

4 files changed, 50 insertions, 27 deletions

diff --git a/odb-tests/build/root.build b/odb-tests/build/root.build
index 27552ac..41f0556 100644
--- a/odb-tests/build/root.build
+++ b/odb-tests/build/root.build
@@ -176,24 +176,6 @@ if! $skeleton
     }
   }
 
-  # @@ BUILD2 Clang issues the following warnings while compile the
-  #           odb-generated headers:
-  #
-  #    In file included from odb-tests/common/view/olv/test1-odb-sqlite.cxx:10:
-  #    odb-tests/common/view/olv/test1-odb-sqlite.hxx:68:80: warning: instantiation of variable 'odb::query_columns<test1::object1, odb::id_common, odb::access::object_traits_impl< ::test1::object1, id_common>>::id' required here, but no definition is available [-Wundefined-var-template]
-  #       68 |   id (query_columns< ::test1::object1, id_common, typename A::common_traits >::id,
-  #          |                                                                                ^
-  #    odb-tests/common/view/olv/test1-odb.hxx:91:21: note: forward declaration of template entity is here
-  #       91 |     static id_type_ id;
-  #          |                     ^
-  #    odb-tests/common/view/olv/test1-odb-sqlite.hxx:68:80: note: add an explicit instantiation declaration to suppress this warning if 'odb::query_columns<test1::object1, odb::id_common, odb::access::object_traits_impl< ::test1::object1, id_common>>::id' is explicitly instantiated in another translation unit
-  #       68 |   id (query_columns< ::test1::object1, id_common, typename A::common_traits >::id,
-  #
-  #    Note: -Wno-undefined-var-template is temporarily added to suppress them.
-  #
-  if ($cxx.id.type == 'clang')
-    cxx.coptions += -Wno-undefined-var-template
-
   # Import odb that we are testing.
   #
   import! [metadata] odb = odb%exe{odb}
diff --git a/odb-tests/common/circular/buildfile b/odb-tests/common/circular/buildfile
new file mode 100644
index 0000000..2e793b9
--- /dev/null
+++ b/odb-tests/common/circular/buildfile
@@ -0,0 +1,9 @@
+# file      : common/circular/buildfile
+# license   : GNU GPL v2; see accompanying LICENSE file
+
+./: {*/ -multiple/}
+
+# We cannot support this case in multi-database support since we need to
+# generate extern template involving classes that participate in the cycle.
+#
+./: multiple/: include = (!$multi)
diff --git a/odb/odb/common-query.cxx b/odb/odb/common-query.cxx
index 517c92c..0b5d063 100644
--- a/odb/odb/common-query.cxx
+++ b/odb/odb/common-query.cxx
@@ -1031,14 +1031,30 @@ traverse (type& c)
   {
     // If we have the extern symbol, generate extern template declarations.
     //
-    if (!ext.empty ())
+    // Without a declaration of explicit template instantiation Clang issues
+    // -Wundefined-var-template. Note that extern template is only available
+    // since C++11 and this only appears to be an issue in dynamic multi-
+    // database support for id_common.
+    //
+    // Note also that this break our support for multi-file circular
+    // dependencies (see odb-tests/common/circule/multiple/).
+    //
+    if (!ext.empty () ||
+        (multi_dynamic &&
+         db == database::common &&
+         options.std () >= cxx_version::cxx11))
     {
       bool has_ptr (has_a (c, test_pointer | exclude_base));
       bool reuse_abst (abstract (c) && !polymorphic (c));
 
       if (has_ptr || !reuse_abst)
       {
-        os << "#ifdef " << ext << endl
+        const string& guard (
+          !ext.empty ()
+          ? ext
+          : make_guard ("ODB_" + db.string () + "_QUERY_COLUMNS_DEF"));
+
+        os << (!ext.empty () ? "#ifdef " : "#ifndef ") << guard << endl
            << endl;
 
         if (has_ptr)
@@ -1055,7 +1071,7 @@ traverse (type& c)
         if (!reuse_abst)
           generate_inst (c);
 
-        os << "#endif // " << ext << endl
+        os << "#endif // " << guard << endl
            << endl;
       }
     }
@@ -1128,7 +1144,7 @@ generate_inst (type& c)
   string const& type (class_fq_name (c));
 
   // Explicit template instantiations. Here is what we need to
-  // instantiate
+  // instantiate:
   //
   // 1. Reuse inheritance bases all the way to the ultimate base.
   //    Unlike poly inheritance, reuse inheritance uses the table
@@ -1216,14 +1232,25 @@ generate_decl (type& c)
   // Do it before query_columns since the inheritance will trigger
   // instantiation and we won't be able to change visibility (GCC).
   //
-  if (obj_count != 0 && multi_dynamic && !ext.empty ())
+  // See query_columns_type::traverse() for background.
+  //
+  if (obj_count != 0 && multi_dynamic &&
+      (!ext.empty () ||
+       (multi_dynamic &&
+        db == database::common &&
+        options.std () >= cxx_version::cxx11)))
   {
-    os << "#ifdef " << ext << endl
+    const string& guard (
+      !ext.empty ()
+      ? ext
+      : make_guard ("ODB_" + db.string () + "_QUERY_COLUMNS_DEF"));
+
+    os << (!ext.empty () ? "#ifdef " : "#ifndef ") << guard << endl
        << endl;
 
     generate_inst (c);
 
-    os << "#endif // " << ext << endl
+    os << "#endif // " << guard << endl
        << endl;
   }
 
diff --git a/odb/odb/context.cxx b/odb/odb/context.cxx
index 13fc1b3..f678e64 100644
--- a/odb/odb/context.cxx
+++ b/odb/odb/context.cxx
@@ -2961,8 +2961,13 @@ strlit (string const& str)
 void context::
 inst_header (bool decl, bool omit_exp)
 {
-  if (decl && !ext.empty ())
-    os << ext << " ";
+  if (decl)
+  {
+    if (!ext.empty ())
+      os << ext << " ";
+    else
+      os << "extern ";
+  }
 
   os << "template struct";