[clangd] Allow renaming class templates in cross-file rename.

[hokein]

Summary

It was disabled because we don't handle explicit template
specialization well (due to the index limitation).

renaming templates is normal in practic, rather than disabling it, this patch
allows to rename them though it is not perfect (just a known limitation).

Context: https://github.com/clangd/clangd/issues/280

[kbobyrev]

I think we might still give a warning of some sort that this is known to produce results that are not always correct. Otherwise it might feel like a bug/unknown behavior.

[hokein]

In D74709#1882451, @kbobyrev wrote:

I think we might still give a warning of some sort that this is known to produce results that are not always correct. Otherwise it might feel like a bug/unknown behavior.

I agree that it would be nice to give a warning. Unfortunately, LSP specification doesn't have such things, returning either a workspaceEdits or an error message, but not both. I think it is still make sense to enable it, I'd say this is a known limitation (this will be mentioned in the user-facing document which I'm working on).

I also test it with llvm::Optional, it works (except the same issue of the tablegen files .inc).

[kbobyrev]

Okay, that is unfortunate, but I think trying to rename these classes with some inaccuracy is better than failing right away.

clang-tools-extra/clangd/refactor/Rename.cpp

if (!CrossFile) {
     // we disallow rename.
     return ReasonToReject::UsedOutsideFile;
   }
 
   assert(CrossFile);
   if (!Index)
     return ReasonToReject::NoIndexProvided;
 
-  // Blacklist symbols that are not supported yet in cross-file mode due to the
-  // limitations of our index.
-  // FIXME: Renaming templates requires to rename all related specializations,
-  // our index doesn't have this information.
-  if (RenameDecl.getDescribedTemplate())
-    return ReasonToReject::UnsupportedSymbol;
-
   // FIXME: Renaming virtual methods requires to rename all overridens in
   // subclasses, our index doesn't have this information.
   // Note: Within-file rename does support this through the AST.
   if (const auto *S = llvm::dyn_cast<CXXMethodDecl>(&RenameDecl)) {
     if (S->isVirtual())
       return ReasonToReject::UnsupportedSymbol;
   }
   return None;

clang-tools-extra/clangd/unittests/RenameTests.cpp

llvm::StringRef FooCC;
         [[Foo]]::~[[Foo]]() {}
 
         void func() {
           [[Foo]] foo;
         }
       )cpp",
       },
       {
+          // class templates.
+          R"cpp(
+        template <typename T>
+        class [[Foo]] {};
+        // FIXME: explicit template specilizations are not supported due the
+        // clangd index limitations.
+        template <>
+        class Foo<double> {};
+      )cpp",
+          R"cpp(
+        #include "foo.h"
+        void func() {
+          [[F^oo]]<int> foo;
+        }
+      )cpp",
+      },
+      {
           // class methods.
           R"cpp(
         class Foo {
           void [[f^oo]]();
         };
       )cpp",
           R"cpp(
         #include "foo.h"