[clangd] Retrieve documentation for member function instance from index

Fixes clangd/clangd#2290

Author: HighCommander4

clang-tools-extra/clangd/CodeComplete.cpp

@@ -1883,7 +1883,19 @@ class CodeCompleteFlow {
         for (auto &Cand : C.first) {
           if (Cand.SemaResult &&
               Cand.SemaResult->Kind == CodeCompletionResult::RK_Declaration) {
-            auto ID = clangd::getSymbolID(Cand.SemaResult->getDeclaration());
+            const NamedDecl *DeclToLookup = Cand.SemaResult->getDeclaration();
+            // For instantiations of members of class templates, the
+            // documentation will be stored at the member's original
+            // declaration.
+            // FIXME: We'd like to handle fields too but FieldDecl is missing a
+            // method equivalent to getInstantiatedFromMemberFunction().
+            if (const auto *FD = dyn_cast<FunctionDecl>(DeclToLookup)) {
+              if (const auto *InstantiatedFrom =
+                      FD->getTemplateInstantiationPattern()) {
+                DeclToLookup = InstantiatedFrom;
+              }
+            }
+            auto ID = clangd::getSymbolID(DeclToLookup);
             if (!ID)
               continue;
             Req.IDs.insert(ID);

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

@@ -1154,23 +1154,45 @@ TEST(CompletionTest, CommentsOnMembersFromHeader) {
       /// This is a member function.
       int delta();
     };
+
+    template <typename T>
+    struct beta {
+      /// This is a member field inside a template.
+      int omega;
+
+      /// This is a member function inside a template.
+      int epsilon();
+    };
   )cpp";
 
   auto File = testPath("foo.cpp");
   Annotations Test(R"cpp(
 #include "foo.h"
 alpha a;
-int x = a.^
+beta<int> b;
+int x = a.$p1^;
+int y = b.$p2^;
      )cpp");
   runAddDocument(Server, File, Test.code());
   auto CompletionList =
-      llvm::cantFail(runCodeComplete(Server, File, Test.point(), {}));
+      llvm::cantFail(runCodeComplete(Server, File, Test.point("p1"), {}));
 
   EXPECT_THAT(CompletionList.Completions,
               Contains(AllOf(named("gamma"), doc("This is a member field."))));
   EXPECT_THAT(
       CompletionList.Completions,
       Contains(AllOf(named("delta"), doc("This is a member function."))));
+
+  CompletionList =
+      llvm::cantFail(runCodeComplete(Server, File, Test.point("p2"), {}));
+
+  EXPECT_THAT(CompletionList.Completions,
+              Contains(AllOf(named("omega")
+                             /* FIXME: Doc retrieval does not work yet*/)));
+  EXPECT_THAT(
+      CompletionList.Completions,
+      Contains(AllOf(named("epsilon"),
+                     doc("This is a member function inside a template."))));
 }
 
 TEST(CompletionTest, CommentsOnMembersFromHeaderOverloadBundling) {