[dead store] Dead store false positive caused by forgetting expression inside decltype

Summary: We do not want to export unnecessary information from clang plugin, hence, the solution to this false positive would be to annotate with `__unused__` attribute.

Reviewed By: jvillard

Differential Revision: D13861333

fbshipit-source-id: 774009f37

infer/tests/codetoanalyze/cpp/liveness/dead_stores.cpp

@@ -505,4 +505,15 @@ ChainedCalls chain_method_calls_ok() {
   return x.chained(5).chained(6);
 }
 
+struct A {
+  int f : 4;
+};
+
+int decltype_read_ok_FP(int x) {
+  A a; // reports here as frontend forgets the expression used in decltype below
+       // a solution would be to annotate with __unused__ (T26148700)
+  decltype(a.f) i;
+  return x + i;
+}
+
 } // namespace dead_stores

infer/tests/codetoanalyze/cpp/liveness/issues.exp

@@ -8,6 +8,7 @@ codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_store_before_captu
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_struct_no_destructor_bad, 0, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_struct_rvalue_ref_bad, 0, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::dead_then_live_bad, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
+codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::decltype_read_ok_FP, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::easy_bad, 0, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::init_capture_no_call_bad, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]
 codetoanalyze/cpp/liveness/dead_stores.cpp, dead_stores::init_capture_reassign_bad, 1, DEAD_STORE, no_bucket, ERROR, [Write of unused value]