[interop] do not warn about a template with too many specializations that can't be imported

include/swift/AST/DiagnosticsClangImporter.def

@@ -128,10 +128,6 @@ WARNING(libstdcxx_modulemap_not_found, none,
         "module map for 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))
-
 WARNING(api_pattern_attr_ignored, none,
         "'%0' swift attribute ignored on type '%1': type is not copyable or destructible",
         (StringRef, StringRef))

lib/ClangImporter/ImportDecl.cpp

@@ -2914,12 +2914,12 @@ namespace {
       if (size_t(
               llvm::size(decl->getSpecializedTemplate()->specializations())) >
           specializationLimit) {
-        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);
+        // Note: it would be nice to import a dummy unavailable struct,
+        // but we would then need to instantiate the template here,
+        // as we cannot import a struct without a definition. That would
+        // defeat the purpose. Also, we can't make the dummy
+        // struct simply unavailable, as that still makes the
+        // typelias that references it available.
         return nullptr;
       }
 

lib/ClangImporter/ImporterImpl.h

@@ -655,10 +655,6 @@ public:
   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;

test/Interop/Cxx/templates/Inputs/module.modulemap

@@ -152,3 +152,8 @@ module UnevaluatedContext {
   header "unevaluated-context.h"
   requires cplusplus
 }
+
+module ManySpecializations {
+  header "many-specializations.h"
+  requires cplusplus
+}

test/Interop/Cxx/templates/many-specializations.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swift-ide-test -print-module -module-to-print=ManySpecializations -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
+
+
+// CHECK: @available(*, unavailable, message: "Un-specialized class templates are not currently supported. Please use a specialization of this type.")
+// CHECK-NEXT: struct TemplateStruct<T> {
+// CHECK-NEXT: }
+
+// CHECK-NOT: typealias