[clangd] Report xref for base methods.

See: https://github.com/clangd/clangd/issues/668

```
struct A { virtual void foo() = 0; };
struct B : A { void foo() override; };
```

Find refs on `B::foo()` will show:
- decls of `A::foo()` (new)
- decls of `B::foo()`
- refs to `A::foo()` (new)
- refs to `B::foo()`.

Differential Revision: https://reviews.llvm.org/D95852

clang-tools-extra/clangd/XRefs.cpp

@@ -1281,6 +1281,21 @@ std::vector<LocatedSymbol> findImplementations(ParsedAST &AST, Position Pos,
   return findImplementors(std::move(IDs), QueryKind, Index, *MainFilePath);
 }
 
+namespace {
+// Recursively finds all the overridden methods of `CMD` in complete type
+// hierarchy.
+void getOverriddenMethods(const CXXMethodDecl *CMD,
+                          llvm::DenseSet<SymbolID> &OverriddenMethods) {
+  if (!CMD)
+    return;
+  for (const CXXMethodDecl *Base : CMD->overridden_methods()) {
+    if (auto ID = getSymbolID(Base))
+      OverriddenMethods.insert(ID);
+    getOverriddenMethods(Base, OverriddenMethods);
+  }
+}
+} // namespace
+
 ReferencesResult findReferences(ParsedAST &AST, Position Pos, uint32_t Limit,
                                 const SymbolIndex *Index) {
   if (!Limit)
@@ -1300,7 +1315,7 @@ ReferencesResult findReferences(ParsedAST &AST, Position Pos, uint32_t Limit,
     return {};
   }
 
-  llvm::DenseSet<SymbolID> IDs;
+  llvm::DenseSet<SymbolID> IDs, OverriddenMethods;
 
   const auto *IdentifierAtCursor =
       syntax::spelledIdentifierTouching(*CurLoc, AST.getTokens());
@@ -1343,13 +1358,16 @@ ReferencesResult findReferences(ParsedAST &AST, Position Pos, uint32_t Limit,
     if (Index) {
       OverriddenBy.Predicate = RelationKind::OverriddenBy;
       for (const NamedDecl *ND : Decls) {
-        // Special case: Inlcude declaration of overridding methods.
+        // Special case: For virtual methods, report decl/def of overrides and
+        // references to all overridden methods in complete type hierarchy.
         if (const auto *CMD = llvm::dyn_cast<CXXMethodDecl>(ND)) {
           if (CMD->isVirtual())
             if (IdentifierAtCursor && SM.getSpellingLoc(CMD->getLocation()) ==
-                                          IdentifierAtCursor->location())
+                                          IdentifierAtCursor->location()) {
               if (auto ID = getSymbolID(CMD))
                 OverriddenBy.Subjects.insert(ID);
+              getOverriddenMethods(CMD, OverriddenMethods);
+            }
         }
       }
     }
@@ -1415,7 +1433,8 @@ ReferencesResult findReferences(ParsedAST &AST, Position Pos, uint32_t Limit,
     }
   }
   // Now query the index for references from other files.
-  auto QueryIndex = [&](llvm::DenseSet<SymbolID> IDs, bool AllowAttributes) {
+  auto QueryIndex = [&](llvm::DenseSet<SymbolID> IDs, bool AllowAttributes,
+                        bool AllowMainFileSymbols) {
     RefsRequest Req;
     Req.IDs = std::move(IDs);
     Req.Limit = Limit;
@@ -1427,7 +1446,8 @@ ReferencesResult findReferences(ParsedAST &AST, Position Pos, uint32_t Limit,
         return;
       auto LSPLoc = toLSPLocation(R.Location, *MainFilePath);
       // Avoid indexed results for the main file - the AST is authoritative.
-      if (!LSPLoc || LSPLoc->uri.file() == *MainFilePath)
+      if (!LSPLoc ||
+          (!AllowMainFileSymbols && LSPLoc->uri.file() == *MainFilePath))
         return;
       ReferencesResult::Reference Result;
       Result.Loc = std::move(*LSPLoc);
@@ -1442,12 +1462,17 @@ ReferencesResult findReferences(ParsedAST &AST, Position Pos, uint32_t Limit,
       Results.References.push_back(std::move(Result));
     });
   };
-  QueryIndex(std::move(IDs), /*AllowAttributes=*/true);
+  QueryIndex(std::move(IDs), /*AllowAttributes=*/true,
+             /*AllowMainFileSymbols=*/false);
+  // For a virtual method: Occurrences of BaseMethod should be treated as refs
+  // and not as decl/def. Allow symbols from main file since AST does not report
+  // these.
+  QueryIndex(std::move(OverriddenMethods), /*AllowAttributes=*/false,
+             /*AllowMainFileSymbols=*/true);
   if (Results.References.size() > Limit) {
     Results.HasMore = true;
     Results.References.resize(Limit);
   }
-  // FIXME: Report refs of base methods.
   return Results;
 }
 

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

@@ -1889,6 +1889,30 @@ TEST(FindReferences, IncludeOverrides) {
   checkFindRefs(Test, /*UseIndex=*/true);
 }
 
+TEST(FindReferences, RefsToBaseMethod) {
+  llvm::StringRef Test =
+      R"cpp(
+        class BaseBase {
+        public:
+          virtual void [[func]]();
+        };
+        class Base : public BaseBase {
+        public:
+          void [[func]]() override;
+        };
+        class Derived : public Base {
+        public:
+          void $decl[[fu^nc]]() override;
+        };
+        void test(BaseBase* BB, Base* B, Derived* D) {
+          // refs to overridden methods in complete type hierarchy are reported.
+          BB->[[func]]();
+          B->[[func]]();
+          D->[[func]]();
+        })cpp";
+  checkFindRefs(Test, /*UseIndex=*/true);
+}
+
 TEST(FindReferences, MainFileReferencesOnly) {
   llvm::StringRef Test =
       R"cpp(