C++ PATCH for c++/46472 (rejects-valid with constexpr constructor template and implicitly declared constructor)

[Jason Merrill <jason at redhat dot com>]

In this testcase, we were trusting the declaration of the constructor 
template and therefore decided that the B() constructor was constexpr; 
but then when we instantiate the A<int> constructor we see that it isn't 
actually constexpr for int, so we complain about using it in constexpr 
B().  Oops.  The fix is to require A<int>() to be instantiated before we 
use its constexprness in determining the constexprness of B().

Tested x86_64-pc-linux-gnu, applied to trunk.
commit 5bed32784ef8e874786de97c3c31b4bb94891b3e
Author: Jason Merrill <jason@redhat.com>
Date:   Sun Feb 20 17:13:49 2011 -0500

    	PR c++/46472
    	* method.c (process_subob_fn): Instantiate constexpr templates.
    	* optimize.c (maybe_clone_body): Propagate DECL_DECLARED_CONSTEXPR_P.

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index bfe8a06..47f1808 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -941,8 +941,17 @@ process_subob_fn (tree fn, bool move_p, tree *spec_p, bool *trivial_p,
       goto bad;
     }
 
-  if (constexpr_p && !DECL_DECLARED_CONSTEXPR_P (fn))
-    *constexpr_p = false;
+  if (constexpr_p)
+    {
+      /* If this is a specialization of a constexpr template, we need to
+	 force the instantiation now so that we know whether or not it's
+	 really constexpr.  */
+      if (DECL_DECLARED_CONSTEXPR_P (fn) && DECL_TEMPLATE_INSTANTIATION (fn)
+	  && !DECL_TEMPLATE_INSTANTIATED (fn))
+	instantiate_decl (fn, /*defer_ok*/false, /*expl_class*/false);
+      if (!DECL_DECLARED_CONSTEXPR_P (fn))
+	*constexpr_p = false;
+    }
 
   return;
 
diff --git a/gcc/cp/optimize.c b/gcc/cp/optimize.c
index b00bc28..ed59f91 100644
--- a/gcc/cp/optimize.c
+++ b/gcc/cp/optimize.c
@@ -256,6 +256,7 @@ maybe_clone_body (tree fn)
       /* Update CLONE's source position information to match FN's.  */
       DECL_SOURCE_LOCATION (clone) = DECL_SOURCE_LOCATION (fn);
       DECL_DECLARED_INLINE_P (clone) = DECL_DECLARED_INLINE_P (fn);
+      DECL_DECLARED_CONSTEXPR_P (clone) = DECL_DECLARED_CONSTEXPR_P (fn);
       DECL_COMDAT (clone) = DECL_COMDAT (fn);
       DECL_WEAK (clone) = DECL_WEAK (fn);
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-synth1.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-synth1.C
new file mode 100644
index 0000000..9830939
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-synth1.C
@@ -0,0 +1,14 @@
+// PR c++/46472
+// { dg-options -std=c++0x }
+
+template<class T> struct A {
+  T t;
+  constexpr A(){}
+};
+
+struct B
+{
+  A<int> a;
+};
+
+B b;