[committed] Add testcase for already fixed -Wunused-but-set-variable PR c++/85108

[Jakub Jelinek]

Hi!

This got fixed with r253266, but I don't see it covered with a testcase.
Tested on x86_64-linux and i686-linux, committed to trunk as obvious.

2018-03-29  Jakub Jelinek  <ja...@redhat.com>

        PR c++/85108
        * g++.dg/warn/Wunused-var-31.C: New test.

--- gcc/testsuite/g++.dg/warn/Wunused-var-31.C.jj       2018-03-29 
13:15:26.433012694 +0200
+++ gcc/testsuite/g++.dg/warn/Wunused-var-31.C  2018-03-29 13:09:48.184877285 
+0200
@@ -0,0 +1,11 @@
+// PR c++/85108
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wunused-but-set-variable" }
+
+int
+main ()
+{
+  auto constexpr add = [] (auto a, auto b) { return a + b; };  // { dg-bogus 
"set but not used" }
+  auto test_lambda = [&] () { return add (2, 2); };
+  return test_lambda () - 4;
+}

        Jakub