[cxx-interop] Avoid importing too complex specializations

This reduces the specialization limit from 10000 to 1000 to prevent Swift from failing to import libstdc++ due to `std::_Index_tuple` being defined recursively.

This also adds a diagnostic to let the user know why a template instantiation wasn't imported.

rdar://96324175

Author: egorzhdan

include/swift/AST/DiagnosticsClangImporter.def

@@ -124,6 +124,10 @@ WARNING(libstdcxx_not_found, none,
         "libstdc++ not found for '%0'; C++ stdlib may be unavailable",
         (StringRef))
 
+WARNING(too_many_class_template_instantiations, none,
+        "template instantiation for '%0' not imported: too many instantiations",
+        (StringRef))
+
 NOTE(macro_not_imported_unsupported_operator, none, "operator not supported in macro arithmetic", ())
 NOTE(macro_not_imported_unsupported_named_operator, none, "operator '%0' not supported in macro arithmetic", (StringRef))
 NOTE(macro_not_imported_invalid_string_literal, none, "invalid string literal", ())

lib/ClangImporter/ImportDecl.cpp

@@ -2486,8 +2486,16 @@ namespace {
       // deep/complex template, or we've run into an infinite loop. In either
       // case, its not worth the compile time, so bail.
       // TODO: this could be configurable at some point.
-      if (llvm::size(decl->getSpecializedTemplate()->specializations()) > 10000)
+      if (llvm::size(decl->getSpecializedTemplate()->specializations()) >
+          1000) {
+        std::string name;
+        llvm::raw_string_ostream os(name);
+        decl->printQualifiedName(os);
+        // Emit a warning if we haven't warned about this decl yet.
+        if (Impl.tooDeepTemplateSpecializations.insert(name).second)
+          Impl.diagnose({}, diag::too_many_class_template_instantiations, name);
         return nullptr;
+      }
 
       // `Sema::isCompleteType` will try to instantiate the class template as a
       // side-effect and we rely on this here. `decl->getDefinition()` can

lib/ClangImporter/ImporterImpl.h

@@ -626,6 +626,10 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   llvm::DenseMap<clang::FunctionDecl *, ValueDecl *>
       specializedFunctionTemplates;
 
+  /// Stores qualified names of C++ template specializations that were too deep
+  /// to import into Swift.
+  llvm::StringSet<> tooDeepTemplateSpecializations;
+
   /// Keeps track of the Clang functions that have been turned into
   /// properties.
   llvm::DenseMap<const clang::FunctionDecl *, VarDecl *> FunctionsAsProperties;