[Wunsafe-buffer-usage] Fix false positives in handling enums

Do not warn if the index is an enum and we an determine statically that
it's within bounds.

Author: mxms

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -463,6 +463,13 @@ AST_MATCHER(ArraySubscriptExpr, isSafeArraySubscript) {
       return true;
   }
 
+  // Array index wasn't an integer literal, let's see if it was an enum or
+  // something similar
+  const auto IntConst = Node.getIdx()->getIntegerConstantExpr(Finder->getASTContext());
+  if (IntConst && *IntConst > 0 && *IntConst < size) {
+    return true;
+  }
+
   return false;
 }
 

clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp

@@ -39,6 +39,23 @@ void constant_idx_unsafe(unsigned idx) {
   buffer[10] = 0;       // expected-note{{used in buffer access here}}
 }
 
+enum FooEnum {
+  A = 0,
+  B = 1,
+  C = 2,
+  D
+};
+
+void constant_enum_safe() {
+  int buffer[FooEnum::D] = { 0, 1, 2 };
+  buffer[C] = 0; // no-warning
+}
+
+void constant_enum_unsafe(FooEnum e) {
+  int buffer[FooEnum::D] = { 0, 1, 2 };
+  buffer[e] = 0; // expected-warning{{unsafe buffer access}}
+}
+
 void constant_id_string(unsigned idx) {
   char safe_char = "abc"[1]; // no-warning
   safe_char = ""[0];