[clangd] Fix a crash in expected types

[merge_guards_bot]

Build result: pass - 60077 tests passed, 0 failed and 729 were skipped.
Log files: console-log.txt, CMakeCache.txt

[kadircet]

LGTM

[thakis]

This broke clangd tests on Windows: http://lab.llvm.org:8011/builders/clang-x64-windows-msvc/builds/12324

We now have clangd tests running on main llvm bots (namely, on http://lab.llvm.org:8011/builders/clang-x64-windows-msvc/). Do they not send mail?

[thakis]

Reverted for now in 6de45772e0

[ilya-biryukov]

Relanded in 339502cc8abb375e3dc8b5d3e9ef35c2541ccbbd.

clang-tools-extra/clangd/ExpectedTypes.cpp

static const Type *toEquivClass(ASTContext &Ctx, QualType T) {
   // Drop the qualifiers and return the resulting type.
   // FIXME: also drop qualifiers from pointer types, e.g. 'const T* => T*'
   return T.getTypePtr();
 }
 
 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();
   if (T.isNull())
     return llvm::None;
   if (auto FuncT = T->getAs<FunctionType>()) {
     // Functions are a special case. They are completed as 'foo()' and we want
     // to match their return type rather than the function type itself.

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

EXPECT_THAT(completions(Context + "int y = Y^", {}, Opts).Completions,
               UnorderedElementsAre(AllOf(Labeled("Y(int A, int B = 0)"),
                                          SnippetSuffix("(${1:int A})"))));
   EXPECT_THAT(completions(Context + "int y = Z^", {}, Opts).Completions,
               UnorderedElementsAre(
                   AllOf(Labeled("Z(int A, int B = 0, int C = 0, int D = 0)"),
                         SnippetSuffix("(${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;
   if (!IndexSymbols.empty())
     Index = memIndex(IndexSymbols);
 
   MockFSProvider FS;
   MockCompilationDatabase CDB;