[committed] Add testcase for already fixed PR c++/82075

[Jakub Jelinek]

Hi!

This got fixed with r257057 aka PR84031 fix, but the committed testcase
doesn't resemble anything close to this one.

Tested on x86_64-linux, committed to trunk as obvious.

2019-03-08  Jakub Jelinek  <ja...@redhat.com>

        PR c++/82075
        * g++.dg/cpp1z/decomp49.C: New test.

--- gcc/testsuite/g++.dg/cpp1z/decomp49.C.jj    2019-03-08 19:25:32.680069446 
+0100
+++ gcc/testsuite/g++.dg/cpp1z/decomp49.C       2019-03-08 19:26:06.192521962 
+0100
@@ -0,0 +1,14 @@
+// PR c++/82075
+// { dg-do run { target c++11 } }
+// { dg-options "" }
+
+struct B { };
+struct D : B { int i; };
+
+int
+main ()
+{
+  auto [i] = D{};      // { dg-warning "only available with" "" { target 
c++14_down } }
+  if (i != 0)
+    __builtin_abort ();
+}

        Jakub