c++: wrong error with static constexpr var in tmpl [PR109876]

Since r8-509, we'll no longer create a static temporary var for
the initializer '{ 1, 2 }' for num in the attached test because
the code in finish_compound_literal is now guarded by
'&& fcl_context == fcl_c99' but it's fcl_functional here.  This
causes us to reject num as non-constant when evaluating it in
a template.

Jason's idea was to treat num as value-dependent even though it
actually isn't.  This patch implements that suggestion.

We weren't marking objects whose type is an empty class type
constant.  This patch changes that so that v_d_e_p doesn't need
to check is_really_empty_class.

Co-authored-by: Jason Merrill <jason@redhat.com>

	PR c++/109876

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Set TREE_CONSTANT when initializing
	an object of empty class type.
	* pt.cc (value_dependent_expression_p) <case VAR_DECL>: Treat a
	constexpr-declared non-constant variable as value-dependent.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-template12.C: New test.
	* g++.dg/cpp1z/constexpr-template1.C: New test.
	* g++.dg/cpp1z/constexpr-template2.C: New test.

5 files changed, 105 insertions, 3 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 60f107d..792ab33 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -8200,7 +8200,6 @@ void
 cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 		tree asmspec_tree, int flags)
 {
-  tree type;
   vec<tree, va_gc> *cleanups = NULL;
   const char *asmspec = NULL;
   int was_readonly = 0;
@@ -8220,7 +8219,7 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
   /* Parameters are handled by store_parm_decls, not cp_finish_decl.  */
   gcc_assert (TREE_CODE (decl) != PARM_DECL);
 
-  type = TREE_TYPE (decl);
+  tree type = TREE_TYPE (decl);
   if (type == error_mark_node)
     return;
 
@@ -8410,7 +8409,7 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  if (decl_maybe_constant_var_p (decl)
 	      /* FIXME setting TREE_CONSTANT on refs breaks the back end.  */
 	      && !TYPE_REF_P (type))
-	    TREE_CONSTANT (decl) = 1;
+	    TREE_CONSTANT (decl) = true;
 	}
       /* This is handled mostly by gimplify.cc, but we have to deal with
 	 not warning about int x = x; as it is a GCC extension to turn off
@@ -8421,6 +8420,14 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  && !warning_enabled_at (DECL_SOURCE_LOCATION (decl), OPT_Winit_self))
 	suppress_warning (decl, OPT_Winit_self);
     }
+  else if (VAR_P (decl)
+	   && COMPLETE_TYPE_P (type)
+	   && !TYPE_REF_P (type)
+	   && !dependent_type_p (type)
+	   && is_really_empty_class (type, /*ignore_vptr*/false))
+    /* We have no initializer but there's nothing to initialize anyway.
+       Treat DECL as constant due to c++/109876.  */
+    TREE_CONSTANT (decl) = true;
 
   if (flag_openmp
       && TREE_CODE (decl) == FUNCTION_DECL
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index fa15b75..255d18b 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -27983,6 +27983,13 @@ value_dependent_expression_p (tree expression)
       else if (TYPE_REF_P (TREE_TYPE (expression)))
 	/* FIXME cp_finish_decl doesn't fold reference initializers.  */
 	return true;
+      /* We have a constexpr variable and we're processing a template.  When
+	 there's lifetime extension involved (for which finish_compound_literal
+	 used to create a temporary), we'll not be able to evaluate the
+	 variable until instantiating, so pretend it's value-dependent.  */
+      else if (DECL_DECLARED_CONSTEXPR_P (expression)
+	       && !TREE_CONSTANT (expression))
+	return true;
       return false;
 
     case DYNAMIC_CAST_EXPR:
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
new file mode 100644
index 0000000..a9e0653
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-template12.C
@@ -0,0 +1,38 @@
+// PR c++/109876
+// { dg-do compile { target c++11 } }
+
+using size_t = decltype(sizeof 0);
+
+namespace std {
+template <class> struct initializer_list {
+  const int *_M_array;
+  size_t _M_len;
+  constexpr size_t size() const { return _M_len; }
+};
+} // namespace std
+
+constexpr std::initializer_list<int> gnum{2};
+
+template <int> struct Array {};
+template <int> void g()
+{
+  static constexpr std::initializer_list<int> num{2};
+  static_assert(num.size(), "");
+  Array<num.size()> ctx;
+
+  constexpr Array<1> num1{};
+}
+
+template <int N>
+struct Foo
+{
+  static constexpr std::initializer_list<int> num = { 1, 2 };
+  static_assert(num.size(), "");
+  Array<num.size()> ctx;
+};
+
+void
+f (Foo<5>)
+{
+  g<0>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
new file mode 100644
index 0000000..58be046
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-template1.C
@@ -0,0 +1,25 @@
+// PR c++/109876
+// { dg-do compile { target c++17 } }
+
+struct Foo {};
+template <const Foo&> struct X {};
+
+void g()
+{
+  static constexpr Foo foo;
+  X<foo> x;
+}
+
+template<int>
+void f()
+{
+  static constexpr Foo foo;
+  X<foo> x;
+}
+
+void
+h ()
+{
+  f<0>();
+  f<1>();
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C
new file mode 100644
index 0000000..5a9a96b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-template2.C
@@ -0,0 +1,25 @@
+// PR c++/109876
+// { dg-do compile { target c++17 } }
+
+struct Foo {};
+template <const Foo&> struct X {};
+
+void g()
+{
+  static constexpr Foo foo{};
+  X<foo> x;
+}
+
+template<int>
+void f()
+{
+  static constexpr Foo foo{};
+  X<foo> x;
+}
+
+void
+h ()
+{
+  f<0>();
+  f<1>();
+}