re PR c++/71128 ([concepts] ICE on ill-formed explicit instantiation of a function concept)

/cp
2018-10-08  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/71128
	* pt.c (do_decl_instantiation): Per 12.6.8/5, a concept cannot be
	explicitly instantiated.

/testsuite
2018-10-08  Paolo Carlini  <paolo.carlini@oracle.com>

	PR c++/71128
	* g++.dg/concepts/pr71128.C: New.

From-SVN: r264914

gcc/cp/ChangeLog

+2018-10-08  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/71128
+	* pt.c (do_decl_instantiation): Per 12.6.8/5, a concept cannot be
+	explicitly instantiated.
+
 2018-10-05  David Malcolm  <dmalcolm@redhat.com>
 
 	PR c++/56856

gcc/cp/pt.c

@@ -23127,6 +23127,14 @@ do_decl_instantiation (tree decl, tree storage)
       error ("explicit instantiation of non-template %q#D", decl);
       return;
     }
+  else if (DECL_DECLARED_CONCEPT_P (decl))
+    {
+      if (VAR_P (decl))
+	error ("explicit instantiation of variable concept %q#D", decl);
+      else
+	error ("explicit instantiation of function concept %q#D", decl);
+      return;
+    }
 
   bool var_templ = (DECL_TEMPLATE_INFO (decl)
                     && variable_template_p (DECL_TI_TEMPLATE (decl)));

gcc/testsuite/ChangeLog

+2018-10-08  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	PR c++/71128
+	* g++.dg/concepts/pr71128.C: New.
+
 2018-10-08  Richard Sandiford  <richard.sandiford@arm.com>
 
 	PR c/87286

gcc/testsuite/g++.dg/concepts/pr71128.C

+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fconcepts" }
+
+template<typename T>
+concept bool C() { return true; }
+template bool C<int>();  // { dg-error "explicit instantiation of function concept" }
+
+template<typename T>
+concept bool D = true;
+template bool D<int>;  // { dg-error "explicit instantiation of variable concept" }