[cxx-interop] Elaborate comment for use of hasActiveRequest()

Author: j-hui

lib/ClangImporter/ClangImporter.cpp

@@ -8795,10 +8795,18 @@ static bool hasUnsafeType(Evaluator &evaluator, clang::QualType clangType) {
     // so we don't need to check the bases whether they are shared references.
     auto req = ClangDeclExplicitSafety({recordDecl, false});
     if (evaluator.hasActiveRequest(req))
-      // Cycles are allowed in templates, e.g.:
+      // Normally, using hasActiveRequest() to avoid cycles is an anti-pattern
+      // because cycles should be treated as errors. However, cycles are allowed
+      // in C++ template, e.g.:
+      //
       //   template <typename>   class Foo { ... }; // throws away template arg
       //   template <typename T> class Bar : Foo<Bar<T>> { ... };
-      // We need to avoid them here.
+      //
+      // A common use case of cyclic templates is the CRTP pattern.
+      //
+      // We need to avoid request cycles, so if there is already an active
+      // request for this type, just assume it is not explicitly unsafe for now
+      // (i.e., as if it were unspecified).
       return false;
     return evaluateOrDefault(evaluator, req, ExplicitSafety::Unspecified) ==
            ExplicitSafety::Unsafe;