[clangd] Make ExpandAutoType not available on template params.

[adamcz]

Summary

We cannot expand auto when used inside a template param (C++17 feature),
so do not offer it there.

clang-tools-extra/clangd/refactor/tweaks/ExpandAutoType.cpp

if (const auto *DD = It->ASTNode.get<DeclaratorDecl>()) {
         if (auto *RD = DD->getType()->getAsRecordDecl())
           return RD->isLambda();
       }
     }
   }
   return false;
 }
 
+// Returns true iff "auto" in Node is really part of the template parameter,
+// which we cannot expand.
+bool isTemplateParam(const SelectionTree::Node *Node) {
+  if (Node->Parent)
+    if (Node->Parent->ASTNode.get<NonTypeTemplateParmDecl>())
+      return true;
+  return false;
+}
+
 bool ExpandAutoType::prepare(const Selection& Inputs) {
   CachedLocation = llvm::None;
   if (auto *Node = Inputs.ASTSelection.commonAncestor()) {
     if (auto *TypeNode = Node->ASTNode.get<TypeLoc>()) {
       if (const AutoTypeLoc Result = TypeNode->getAs<AutoTypeLoc>()) {
         // Code in apply() does handle 'decltype(auto)' yet.
         if (!Result.getTypePtr()->isDecltypeAuto() &&
             !isStructuredBindingType(Node) &&
-            !isDeducedAsLambda(Node, Result.getBeginLoc()))
+            !isDeducedAsLambda(Node, Result.getBeginLoc()) &&
+            !isTemplateParam(Node))
           CachedLocation = Result;
       }
     }
   }
 
   return (bool) CachedLocation;
 }
 

clang-tools-extra/clangd/unittests/tweaks/ExpandAutoTypeTests.cpp

TEST_F(ExpandAutoTypeTest, Test) {
   EXPECT_UNAVAILABLE("const ^auto &[x,y] = (int[]){1,2};");
 
   // FIXME: Auto-completion in a template requires disabling delayed template
   // parsing.
   ExtraArgs.push_back("-fno-delayed-template-parsing");
   // unknown types in a template should not be replaced
   EXPECT_THAT(apply("template <typename T> void x() { ^auto y = T::z(); }"),
               StartsWith("fail: Could not deduce type for 'auto' type"));
+
+  ExtraArgs.push_back("-std=c++17");
+  EXPECT_UNAVAILABLE("template <au^to X> class Y;");
 }
 
 } // namespace
 } // namespace clangd
 } // namespace clang