c++: CTAD and constexpr ctor [PR115207]

Here we failed to constant-evaluate the A<int> constructor because DECL_SIZE
wasn't set on 'a' yet, so compare_address thinks we can't be sure it isn't
at the same address as 'i'.

Normally DECL_SIZE is set by build_decl calling layout_decl, but that
doesn't happen here because we don't have a type yet.  So we need to
layout_decl again after deduction.

	PR c++/115207

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Call layout_decl after CTAD.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction118.C: New test.

2 files changed, 14 insertions, 0 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 84398e5..03e8c98 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -8899,6 +8899,9 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  TREE_TYPE (decl) = error_mark_node;
 	  return;
 	}
+
+      /* Now that we have a type, try these again.  */
+      layout_decl (decl, 0);
       cp_apply_type_quals_to_decl (cp_type_quals (type), decl);
 
       /* Update the type of the corresponding TEMPLATE_DECL to match.  */
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction118.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction118.C
new file mode 100644
index 0000000..64d814b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction118.C
@@ -0,0 +1,11 @@
+// PR c++/115207
+// { dg-do compile { target c++17 } }
+
+template <class T>
+struct A {
+  T t;
+  constexpr A(T* p): t (p != &t) { }
+};
+
+const int i = 42;
+constexpr A a = &i;