Fix false positive unsequenced access and modification warning in array subscript expression.

Summary: In the [expr.sub] p1, we can read that for a given E1[E2], E1 is sequenced before E2. 

Patch by Mateusz Janek.

Reviewers: rsmith, Rakete1111

Reviewed By: rsmith, Rakete1111

Subscribers: riccibruno, lebedev.ri, Rakete1111, hiraditya, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D50766

llvm-svn: 350874
diff --git a/clang/test/SemaCXX/warn-unsequenced-cxx17.cpp b/clang/test/SemaCXX/warn-unsequenced-cxx17.cpp
new file mode 100644
index 0000000..3c221fb
--- /dev/null
+++ b/clang/test/SemaCXX/warn-unsequenced-cxx17.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 -Wno-unused %s
+
+void test() {
+  int xs[10];
+  int *p = xs;
+  // expected-no-diagnostics
+  p[(long long unsigned)(p = 0)]; // ok
+}