XFAIL tuple deduction guide test on GCC

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304969 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.cnstr/implicit_deduction_guides.pass.cpp

@@ -10,6 +10,11 @@
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 // UNSUPPORTED: libcpp-no-deduction-guides
 
+// GCC's implementation of class template deduction is still immature and runs
+// into issues with libc++. However GCC accepts this code when compiling
+// against libstdc++.
+// XFAIL: gcc
+
 // <string>
 
 // Test that the constructors offered by std::basic_string are formulated