[clangd] Fix DocumentOutline for concepts

[kadircet]

Summary

Fixes https://github.com/clangd/clangd/issues/256

[merge_guards_bot]

Unit tests: pass. 62012 tests passed, 0 failed and 783 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[kbobyrev]

LGTM

clang-tools-extra/clangd/FindSymbols.cpp

for (auto &TopLevel : AST.getLocalTopLevelDecls())
       traverseDecl(TopLevel, Results);
     return Results;
   }
 
 private:
   enum class VisitKind { No, OnlyDecl, DeclAndChildren };
 
   void traverseDecl(Decl *D, std::vector<DocumentSymbol> &Results) {
-    if (auto *Templ = llvm::dyn_cast<TemplateDecl>(D))
-      D = Templ->getTemplatedDecl();
+    if (auto *Templ = llvm::dyn_cast<TemplateDecl>(D)) {
+      // TemplatedDecl might be null, e.g. concepts.
+      if (auto *TD = Templ->getTemplatedDecl())
+        D = TD;
+    }
     auto *ND = llvm::dyn_cast<NamedDecl>(D);
     if (!ND)
       return;
     VisitKind Visit = shouldVisit(ND);
     if (Visit == VisitKind::No)
       return;
     llvm::Optional<DocumentSymbol> Sym = declToSym(AST.getASTContext(), *ND);
     if (!Sym)

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

EXPECT_THAT(
       ElementsAre(
           AllOf(WithName("Foo"), WithKind(SymbolKind::Class),
                 Children(AllOf(WithName("f"), WithKind(SymbolKind::Method),
                                SymNameRange(Main.range("decl"))))),
           AllOf(WithName("Foo::f"), WithKind(SymbolKind::Method),
                 SymNameRange(Main.range("def")))));
 }
 
+TEST_F(DocumentSymbolsTest, Concepts) {
+  CDB.ExtraClangFlags = {"-std=c++2a"};
+  std::string FilePath = testPath("foo.cpp");
+  addFile(FilePath,
+          "template <typename T> concept C = requires(T t) { t.foo(); };");
+
+  EXPECT_THAT(getSymbols(FilePath), ElementsAre(WithName("C")));
+}
+
 TEST_F(DocumentSymbolsTest, ExternSymbol) {
   std::string FilePath = testPath("foo.cpp");
   addFile(testPath("foo.h"), R"cpp(
       extern int var;
       )cpp");
   addFile(FilePath, R"cpp(
       #include "foo.h"
       )cpp");