[clangd] Reland b5135a86e0476: Fix a crash in expected types

Reverted in 6de45772e0910bf7fa626e5493a2798b071eb26c.
With a fix to Windows tests.

Differential Revision: https://reviews.llvm.org/D70481

2 files changed, 14 insertions, 4 deletions

diff --git a/clang-tools-extra/clangd/ExpectedTypes.cpp b/clang-tools-extra/clangd/ExpectedTypes.cpp
index 3b0779ea66b..a82a64cf14e 100644
--- a/clang-tools-extra/clangd/ExpectedTypes.cpp
+++ b/clang-tools-extra/clangd/ExpectedTypes.cpp
@@ -44,12 +44,10 @@ static const Type *toEquivClass(ASTContext &Ctx, QualType T) {
 static llvm::Optional<QualType>
 typeOfCompletion(const CodeCompletionResult &R) {
   const NamedDecl *D = R.Declaration;
-  if (!D)
-    return llvm::None;
   // Templates do not have a type on their own, look at the templated decl.
-  if (auto *Template = dyn_cast<TemplateDecl>(D))
+  if (auto *Template = dyn_cast_or_null<TemplateDecl>(D))
     D = Template->getTemplatedDecl();
-  auto *VD = dyn_cast<ValueDecl>(D);
+  auto *VD = dyn_cast_or_null<ValueDecl>(D);
   if (!VD)
     return llvm::None; // We handle only variables and functions below.
   auto T = VD->getType();
diff --git a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
index 5b50b9fe9f8..6757a8c686a 100644
--- a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
@@ -141,6 +141,8 @@ CodeCompleteResult completions(llvm::StringRef Text,
                                PathRef FilePath = "foo.cpp") {
   MockFSProvider FS;
   MockCompilationDatabase CDB;
+  // To make sure our tests for completiopns inside templates work on Windows.
+  CDB.ExtraClangFlags = {"-fno-delayed-template-parsing"};
   IgnoreDiagnostics DiagConsumer;
   ClangdServer Server(CDB, FS, DiagConsumer, ClangdServer::optsForTest());
   return completions(Server, Text, std::move(IndexSymbols), std::move(Opts),
@@ -1030,6 +1032,16 @@ TEST(CompletionTest, DefaultArgs) {
                         SnippetSuffix("(${1:int A})"))));
 }
 
+TEST(CompletionTest, NoCrashWithTemplateParamsAndPreferredTypes) {
+  auto Completions = completions(R"cpp(
+template <template <class> class TT> int foo() {
+  int a = ^
+}
+)cpp")
+                         .Completions;
+  EXPECT_THAT(Completions, Contains(Named("TT")));
+}
+
 SignatureHelp signatures(llvm::StringRef Text, Position Point,
                          std::vector<Symbol> IndexSymbols = {}) {
   std::unique_ptr<SymbolIndex> Index;