[clangd] Don't offer to expand auto in structured binding declarations.

[sammccall]

Summary

auto must be used for the code to parse.

[usaxena95]

LGTM! Thanks.

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

private:
   /// Cache the AutoTypeLoc, so that we do not need to search twice.
   llvm::Optional<clang::AutoTypeLoc> CachedLocation;
 };
 
 REGISTER_TWEAK(ExpandAutoType)
 
 std::string ExpandAutoType::title() const { return "Expand auto type"; }
 
+// Structured bindings must use auto, e.g. `const auto& [a,b,c] = ...;`.
+// Return whether N (an AutoTypeLoc) is such an auto that must not be expanded.
+bool isStructuredBindingType(const SelectionTree::Node *N) {
+  // Walk up the TypeLoc chain, because auto may be qualified.
+  while (N && N->ASTNode.get<TypeLoc>())
+    N = N->Parent;
+  // The relevant type is the only direct type child of a Decomposition.
+  return N && N->ASTNode.get<DecompositionDecl>();
+}
+
 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())
+        if (!Result.getTypePtr()->isDecltypeAuto() &&
+            !isStructuredBindingType(Node))
           CachedLocation = Result;
       }
     }
   }
   return (bool) CachedLocation;
 }
 
 Expected<Tweak::Effect> ExpandAutoType::apply(const Selection& Inputs) {

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

TEST_F(ExpandAutoTypeTest, Test) {
   // local class
   EXPECT_EQ(apply("namespace x { void y() { struct S{}; ^auto z = S(); } }"),
             "namespace x { void y() { struct S{}; S z = S(); } }");
   // replace array types
   EXPECT_EQ(apply(R"cpp(au^to x = "test";)cpp"),
             R"cpp(const char * x = "test";)cpp");
 
   EXPECT_UNAVAILABLE("dec^ltype(au^to) x = 10;");
+  // expanding types in structured bindings is syntactically invalid.
+  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"));
 }