[cxx-interop] Avoid cycles in ClangDeclExplicitSafety requests

Author: j-hui

lib/ClangImporter/ClangImporter.cpp

@@ -8709,9 +8709,14 @@ ExplicitSafety ClangTypeExplicitSafety::evaluate(
   if (auto recordDecl = clangType->getAsTagDecl()) {
     // If we reached this point the types is not imported as a shared reference,
     // so we don't need to check the bases whether they are shared references.
-    return evaluateOrDefault(evaluator,
-                             ClangDeclExplicitSafety({recordDecl, false}),
-                             ExplicitSafety::Unspecified);
+    auto req = ClangDeclExplicitSafety({recordDecl, false});
+    if (evaluator.hasActiveRequest(req))
+      // Cycles are allowed in templates, e.g.:
+      //   template <typename>   class Foo { ... }; // throws away template arg
+      //   template <typename T> class Bar : Foo<Bar<T>> { ... };
+      // We need to avoid them here.
+      return ExplicitSafety::Unspecified;
+    return evaluateOrDefault(evaluator, req, ExplicitSafety::Unspecified);
   }
 
   // Everything else is safe.