re PR c++/85108 (Incorrect warning for constexpr lambda with -Wunused-but-set-variable)

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

From-SVN: r258952

gcc/testsuite/ChangeLog

 2018-03-29  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c++/85108
+	* g++.dg/warn/Wunused-var-31.C: New test.
+
 	PR c/85094
 	* c-c++-common/Wduplicated-branches-14.c: New test.
 

gcc/testsuite/g++.dg/warn/Wunused-var-31.C

+// 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;
+}