In the discussion of 65942 it was pointed out that there's really no
reason to instantiate f in this testcase; we (and Clang) had been
working from the idea that you instantiate a constexpr function when the
function is mentioned even in unevaluated context, but we might as well
wait until we actually need the definition.
Tested x86_64-pc-linux-gnu, applying to trunk. I might apply it to the
5 branch later.
commit 95f0a88be340698e6fed34bb7ac086291a995dd9
Author: Jason Merrill <ja...@redhat.com>
Date: Mon Jun 1 17:24:57 2015 -0400
PR c++/65942
* decl2.c (mark_used): Don't always instantiate constexpr fns.
* constexpr.c (cxx_eval_call_expression): Instantiate them here.
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 6c0d1fe..5278876 100644
--- a/gcc/cp/constexpr.c
+++ b/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;
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index f1b3d0c..8ba19cf 100644
--- a/gcc/cp/decl2.c
+++ b/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)))
{
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C
new file mode 100644
index 0000000..15ca995
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-decltype2.C
@@ -0,0 +1,7 @@
+// 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(""); }
