PR c++/85842 - -Wreturn-type, constexpr if and generic lambda.

	* pt.c (tsubst_lambda_expr): Copy current_function_returns_* to
	generic lambda.

From-SVN: r260685

gcc/cp/ChangeLog

+2018-05-24  Jason Merrill  <jason@redhat.com>
+
+	PR c++/85842 - -Wreturn-type, constexpr if and generic lambda.
+	* pt.c (tsubst_lambda_expr): Copy current_function_returns_* to
+	generic lambda.
+
 2018-05-24  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
 	Pedwarn on a non-standard position of a C++ attribute.

gcc/cp/pt.c

@@ -17636,6 +17636,17 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
       register_parameter_specializations (oldfn, fn);
 
+      if (oldtmpl)
+	{
+	  /* We might not partially instantiate some parts of the function, so
+	     copy these flags from the original template.  */
+	  language_function *ol = DECL_STRUCT_FUNCTION (oldfn)->language;
+	  current_function_returns_value = ol->returns_value;
+	  current_function_returns_null = ol->returns_null;
+	  current_function_returns_abnormally = ol->returns_abnormally;
+	  current_function_infinite_loop = ol->infinite_loop;
+	}
+
       tsubst_expr (DECL_SAVED_TREE (oldfn), args, complain, r,
 		   /*constexpr*/false);
 

gcc/testsuite/g++.dg/cpp1z/constexpr-if23.C

+// PR c++/85842
+// { dg-additional-options -std=c++17 }
+
+template<class T>
+auto f = [](auto&& arg) -> T* {
+    if constexpr (sizeof(arg) == 1) {
+        return nullptr;
+    } else {
+        return static_cast<T*>(&arg);
+    }
+};
+
+auto p = f<int>(0);