[clangd] Don't show 'auto' type hint when type deduction fails

[nridge]

Summary

Fixes https://github.com/clangd/clangd/issues/1475

[kadircet]

thanks!

clang-tools-extra/clangd/InlayHints.cpp

308

nit

[nridge]

address nit

clang-tools-extra/clangd/InlayHints.cpp

bool VisitVarDecl(VarDecl *D) {
     if (auto *DD = dyn_cast<DecompositionDecl>(D)) {
       for (auto *Binding : DD->bindings()) {
         addTypeHint(Binding->getLocation(), Binding->getType(), /*Prefix=*/": ",
                     StructuredBindingPolicy);
       }
       return true;
     }
 
-    if (D->getType()->getContainedAutoType()) {
-      if (!D->getType()->isDependentType()) {
+    if (auto *AT = D->getType()->getContainedAutoType()) {
+      if (AT->isDeduced() && !D->getType()->isDependentType()) {
         // Our current approach is to place the hint on the variable
         // and accordingly print the full type
         // (e.g. for `const auto& x = 42`, print `const int&`).
         // Alternatively, we could place the hint on the `auto`
         // (and then just print the type deduced for the `auto`).
         addTypeHint(D->getLocation(), D->getType(), /*Prefix=*/": ");
       }
     }

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

 TEST(TypeHints, StructuredBindings_NoInitializer) {
   assertTypeHints(R"cpp(
     // No initializer (ill-formed).
     // Do not show useless "NULL TYPE" hint.    
     auto [x, y];  /*error-ok*/
   )cpp");
 }
 
+TEST(TypeHints, InvalidType) {
+  assertTypeHints(R"cpp(
+    auto x = (unknown_type)42; /*error-ok*/
+    auto *y = (unknown_ptr)nullptr;
+  )cpp");
+}
+
 TEST(TypeHints, ReturnTypeDeduction) {
   assertTypeHints(
       R"cpp(
     auto f1(int x$ret1a[[)]];  // Hint forward declaration too
     auto f1(int x$ret1b[[)]] { return x + 1; }
 
     // Include pointer operators in hint
     int s;