[clang-tools-extra] [clangd] Retrieve documentation for member function instance from index (PR #153337)

[Younan Zhang via cfe-commits]

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@@ -1887,7 +1887,19 @@ 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->getInstantiatedFromMemberFunction()) {
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zyn0217 wrote:

I think so? getInstantiatedFromMemberFunction gives you the last pattern that this instantiation comes from, and that pattern could be a half-instantiated template declaration.

So if you're looking for the primary template, I think we should turn to getTemplateInstantiationPattern.

Also note that there are some edge cases where we lack a tracking of the pattern, e.g. a templated friend declaration

https://github.com/llvm/llvm-project/pull/153337