[clangd] Remove duplicates from code completion

[ilya-biryukov]

Summary

This patch removes hidden items from code completion.
Hidden items are the ones that are hidden by other items (e.g., by
other items in the child scopes).
This patch addresses a particular problem of a duplicate completion
item for the class in the following example:

struct Adapter { void method(); };
void Adapter::method() {
   Adapter^
}

We should probably investigate if there are other duplicates in
completion and remove them, possibly adding assertions that it never
happens.

clang-tools-extra/trunk/clangd/CodeComplete.cpp

void ProcessCodeCompleteResults(Sema &S, CodeCompletionContext Context,
                                   unsigned NumResults) override final {
     if (auto SS = Context.getCXXScopeSpecifier())
       CompletedName.SSInfo = extraCompletionScope(S, **SS);
 
     CompletedName.Filter = S.getPreprocessor().getCodeCompletionFilter();
     std::priority_queue<CompletionCandidate> Candidates;
     for (unsigned I = 0; I < NumResults; ++I) {
       auto &Result = Results[I];
+      // We drop hidden items, as they cannot be found by the lookup after
+      // inserting the corresponding completion item and only produce noise and
+      // duplicates in the completion list. However, there is one exception. If
+      // Result has a Qualifier which is non-informative, we can refer to an
+      // item by adding that qualifier, so we don't filter out this item.
+      if (Result.Hidden && (!Result.Qualifier || Result.QualifierIsInformative))
+        continue;
       if (!ClangdOpts.IncludeIneligibleResults &&
           (Result.Availability == CXAvailability_NotAvailable ||
            Result.Availability == CXAvailability_NotAccessible))
         continue;
       if (!CompletedName.Filter.empty() &&
           !fuzzyMatch(S, Context, CompletedName.Filter, Result))
         continue;
       Candidates.emplace(Result);

clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp

TEST(CompletionTest, ASTIndexMultiFile) {
   EXPECT_THAT(Results.items, Has("XYZ", CompletionItemKind::Class));
   EXPECT_THAT(Results.items, Has("foo", CompletionItemKind::Function));
   EXPECT_THAT(Results.items, Has("XXX", CompletionItemKind::Class));
   EXPECT_THAT(Results.items, Contains(AllOf(Named("fooooo"), Filter("fooooo"),
                                             Kind(CompletionItemKind::Function),
                                             Doc("Doooc"), Detail("void"))));
 }
 
+TEST(CompletionTest, NoDuplicates) {
+  auto Items = completions(R"cpp(
+struct Adapter {
+  void method();
+};
+
+void Adapter::method() {
+  Adapter^
+}
+  )cpp")
+                   .items;
+
+  // Make sure there are no duplicate entries of 'Adapter'.
+  EXPECT_THAT(Items, ElementsAre(Named("Adapter"), Named("~Adapter")));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang