PR c++/81525 - wrong constant value with generic lambda

	* pt.c (tsubst_decl) [VAR_DECL]: Avoid clobbering auto.
	(tsubst_copy) [VAR_DECL]: Handle auto.

From-SVN: r250999

gcc/cp/ChangeLog

 2017-08-09  Jason Merrill  <jason@redhat.com>
 
+	PR c++/81525 - wrong constant value with generic lambda
+	* pt.c (tsubst_decl) [VAR_DECL]: Avoid clobbering auto.
+	(tsubst_copy) [VAR_DECL]: Handle auto.
+
 	PR c++/81359 - Unparsed NSDMI error from SFINAE context.
 	* init.c (get_nsdmi): Add complain parm.
 	* typeck2.c (digest_nsdmi_init): Add complain parm.

gcc/cp/pt.c

@@ -12875,7 +12875,15 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 		&& VAR_HAD_UNKNOWN_BOUND (t)
 		&& type != error_mark_node)
 	      type = strip_array_domain (type);
+	    tree auto_node = type_uses_auto (type);
+	    int len = TREE_VEC_LENGTH (args);
+	    if (auto_node)
+	      /* Mask off any template args past the variable's context so we
+		 don't replace the auto with an unrelated argument.  */
+	      TREE_VEC_LENGTH (args) = TEMPLATE_TYPE_LEVEL (auto_node) - 1;
 	    type = tsubst (type, args, complain, in_decl);
+	    if (auto_node)
+	      TREE_VEC_LENGTH (args) = len;
 	  }
 	if (VAR_P (r))
 	  {
@@ -14656,6 +14664,10 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 			DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (r)
 			  = TREE_CONSTANT (r) = true;
 		      DECL_INITIAL (r) = init;
+		      if (tree auto_node = type_uses_auto (TREE_TYPE (r)))
+			TREE_TYPE (r)
+			  = do_auto_deduction (TREE_TYPE (r), init, auto_node,
+					       complain, adc_variable_type);
 		    }
 		  gcc_assert (cp_unevaluated_operand || TREE_STATIC (r)
 			      || decl_constant_var_p (r)

gcc/testsuite/g++.dg/cpp1y/lambda-generic-const4.C

+// PR c++/81525
+// { dg-do compile { target c++14 } }
+
+template <int i> struct A {
+  constexpr operator int () const { return i; }
+};
+template <int i> constexpr A<i> a = {};
+
+template <typename F> void foo (F f) {
+  f (A<0>{});
+}
+template <typename T>
+void bar (T) {
+  constexpr auto N = a<1>;
+  auto f = [&] (auto i) {
+    static_assert (static_cast<int>(N) == 1, "");
+  };
+  foo (f);
+}
+int main () { bar (0); }