On Tue, Apr 11, 2017 at 03:41:39PM +0200, Jakub Jelinek wrote:
> So like this?  Or just type_dependent_expression_p check rather than
> both?

To answer myself, testing there both type_dependent_expression_p
and value_dependent_expression_p for value_expr breaks
the lambda-generic-const2.C testcase.  So I'll test this instead:

2017-04-11  Jakub Jelinek  <ja...@redhat.com>

        PR c++/80370
        * decl.c (cp_finish_decomp): If processing_template_decl on
        non-dependent decl, only set TREE_TYPE on the v[i] decls, but don't
        change their DECL_VALUE_EXPR nor cp_finish_decl them.  Instead make
        sure DECL_VALUE_EXPR is the canonical NULL type ARRAY_REF for tsubst
        processing.
        * pt.c (value_dependent_expression_p) <case VAR_DECL>: For variables
        with DECL_VALUE_EXPR, return true if DECL_VALUE_EXPR is type
        dependent.

        * g++.dg/cpp1z/decomp28.C: New test.

--- gcc/cp/decl.c.jj    2017-04-10 22:26:56.441388051 +0200
+++ gcc/cp/decl.c       2017-04-11 15:21:24.473163169 +0200
@@ -7473,6 +7473,8 @@ cp_finish_decomp (tree decl, tree first,
        {
          TREE_TYPE (v[i]) = eltype;
          layout_decl (v[i], 0);
+         if (processing_template_decl)
+           continue;
          tree t = unshare_expr (dexp);
          t = build4_loc (DECL_SOURCE_LOCATION (v[i]), ARRAY_REF,
                          eltype, t, size_int (i), NULL_TREE,
@@ -7492,6 +7494,8 @@ cp_finish_decomp (tree decl, tree first,
        {
          TREE_TYPE (v[i]) = eltype;
          layout_decl (v[i], 0);
+         if (processing_template_decl)
+           continue;
          tree t = unshare_expr (dexp);
          t = build1_loc (DECL_SOURCE_LOCATION (v[i]),
                          i ? IMAGPART_EXPR : REALPART_EXPR, eltype,
@@ -7510,6 +7514,8 @@ cp_finish_decomp (tree decl, tree first,
        {
          TREE_TYPE (v[i]) = eltype;
          layout_decl (v[i], 0);
+         if (processing_template_decl)
+           continue;
          tree t = unshare_expr (dexp);
          convert_vector_to_array_for_subscript (DECL_SOURCE_LOCATION (v[i]),
                                                 &t, size_int (i));
@@ -7559,8 +7565,9 @@ cp_finish_decomp (tree decl, tree first,
              SET_DECL_VALUE_EXPR (v[i], NULL_TREE);
              DECL_HAS_VALUE_EXPR_P (v[i]) = 0;
            }
-         cp_finish_decl (v[i], init, /*constexpr*/false,
-                         /*asm*/NULL_TREE, LOOKUP_NORMAL);
+         if (!processing_template_decl)
+           cp_finish_decl (v[i], init, /*constexpr*/false,
+                           /*asm*/NULL_TREE, LOOKUP_NORMAL);
        }
     }
   else if (TREE_CODE (type) == UNION_TYPE)
@@ -7615,12 +7622,26 @@ cp_finish_decomp (tree decl, tree first,
              tt = TREE_OPERAND (tt, 0);
            TREE_TYPE (v[i]) = TREE_TYPE (tt);
            layout_decl (v[i], 0);
-           SET_DECL_VALUE_EXPR (v[i], tt);
-           DECL_HAS_VALUE_EXPR_P (v[i]) = 1;
+           if (!processing_template_decl)
+             {
+               SET_DECL_VALUE_EXPR (v[i], tt);
+               DECL_HAS_VALUE_EXPR_P (v[i]) = 1;
+             }
            i++;
          }
     }
-  if (DECL_NAMESPACE_SCOPE_P (decl))
+  if (processing_template_decl)
+    {
+      for (unsigned int i = 0; i < count; i++)
+       if (!DECL_HAS_VALUE_EXPR_P (v[i]))
+         {
+           tree a = build_nt (ARRAY_REF, decl, size_int (i),
+                              NULL_TREE, NULL_TREE);
+           SET_DECL_VALUE_EXPR (v[i], a);
+           DECL_HAS_VALUE_EXPR_P (v[i]) = 1;
+         }
+    }
+  else if (DECL_NAMESPACE_SCOPE_P (decl))
     SET_DECL_ASSEMBLER_NAME (decl, mangle_decomp (decl, v));
 }
 
--- gcc/cp/pt.c.jj      2017-04-11 19:06:52.871624849 +0200
+++ gcc/cp/pt.c 2017-04-11 19:10:39.795803274 +0200
@@ -23576,6 +23576,12 @@ value_dependent_expression_p (tree expre
              || type_dependent_expression_p (DECL_INITIAL (expression))
              || value_dependent_expression_p (DECL_INITIAL (expression))))
        return true;
+      if (DECL_HAS_VALUE_EXPR_P (expression))
+       {
+         tree value_expr = DECL_VALUE_EXPR (expression);
+         if (type_dependent_expression_p (value_expr))
+           return true;
+       }
       return false;
 
     case DYNAMIC_CAST_EXPR:
--- gcc/testsuite/g++.dg/cpp1z/decomp28.C.jj    2017-04-11 15:21:24.473163169 
+0200
+++ gcc/testsuite/g++.dg/cpp1z/decomp28.C       2017-04-11 15:21:24.473163169 
+0200
@@ -0,0 +1,39 @@
+// PR c++/80370
+// { dg-do compile { target c++14 } }
+// { dg-options "" }
+
+namespace std {
+  template <typename> struct tuple_size;
+  template <long, typename> struct tuple_element;
+  template <typename...> struct tuple {};
+  template <typename... T> struct tuple_size<tuple<T...>> { static constexpr 
int value = 1; };
+  template <typename T, typename... U> struct tuple_element<0, tuple<T, U...>> 
{ typedef T type; };
+  template <int, typename... T> int& get (tuple<T...>);
+}
+
+template <int N>
+void
+foo (std::tuple<int> b)
+{
+  auto [c] = b;                // { dg-warning "decomposition declaration only 
available with" "" { target c++14_down } }
+}
+
+template <typename T>
+void
+bar (std::tuple<T> b)
+{
+  auto [c] = b;                // { dg-warning "decomposition declaration only 
available with" "" { target c++14_down } }
+}
+
+void
+baz (std::tuple<int> b)
+{
+  foo <5> (b);
+  bar (b);
+}
+
+int
+main ()
+{
+  [](auto) { [](std::tuple<int> b) { auto[c] = b; }; } (0); // { dg-warning 
"decomposition declaration only available with" "" { target c++14_down } }
+}


        Jakub

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