re PR c++/65942 ([C++14] cannot use std::function as comparator in algorithms)

	PR c++/65942
	* decl2.c (mark_used): Don't always instantiate constexpr fns.
	* constexpr.c (cxx_eval_call_expression): Instantiate them here.

From-SVN: r224008

gcc/cp/ChangeLog

 2015-06-01  Jason Merrill  <jason@redhat.com>
 
+	PR c++/65942
+	* decl2.c (mark_used): Don't always instantiate constexpr fns.
+	* constexpr.c (cxx_eval_call_expression): Instantiate them here.
+
 	PR c++/44282
 	* mangle.c (attr_strcmp): New.
 	(write_CV_qualifiers_for_type): Also write out attributes that

gcc/cp/constexpr.c

@@ -1240,6 +1240,15 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
 	return build_zero_init (DECL_CONTEXT (fun), NULL_TREE, false);
     }
 
+  /* We can't defer instantiating the function any longer.  */
+  if (!DECL_INITIAL (fun)
+      && DECL_TEMPLOID_INSTANTIATION (fun))
+    {
+      ++function_depth;
+      instantiate_decl (fun, /*defer_ok*/false, /*expl_inst*/false);
+      --function_depth;
+    }
+
   /* If in direct recursive call, optimize definition search.  */
   if (ctx && ctx->call && ctx->call->fundef->decl == fun)
     new_call.fundef = ctx->call->fundef;

gcc/cp/decl2.c

@@ -5046,8 +5046,7 @@ mark_used (tree decl, tsubst_flags_t complain)
       && DECL_TEMPLATE_INFO (decl)
       && (decl_maybe_constant_var_p (decl)
 	  || (TREE_CODE (decl) == FUNCTION_DECL
-	      && (DECL_DECLARED_CONSTEXPR_P (decl)
-		  || DECL_OMP_DECLARE_REDUCTION_P (decl)))
+	      && DECL_OMP_DECLARE_REDUCTION_P (decl))
 	  || undeduced_auto_decl (decl))
       && !uses_template_parms (DECL_TI_ARGS (decl)))
     {

gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C

+// PR c++/65942
+// { dg-do compile { target c++11 } }
+
+template <typename T> constexpr int f(T t) { return t; }
+template <typename T, typename = decltype(f(T()))> void g(T) { }
+void g(...) { }
+int main() { g(""); }