[clangd] Add a test for SignatureHelp on dynamic index.

[hokein]

Summary

This would catch regressions caused by future changes of the index.

[ilya-biryukov]

Many thanks! Should've been there from the start, sorry about the inconvenience

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

EXPECT_THAT(
       signatures(Sig1, {Foo0, Foo1, Foo2}).signatures,
       ElementsAre(AllOf(Sig("foo() -> int", {}), SigDoc("Doc from the index")),
                   AllOf(Sig("foo(int) -> int", {"int"}),
                         SigDoc("Overriden doc from sema")),
                   AllOf(Sig("foo(int, int) -> int", {"int", "int"}),
                         SigDoc("Doc from sema"))));
 }
 
+TEST(SignatureHelpTest, DynamicIndexDocumentation) {
+  MockFSProvider FS;
+  MockCompilationDatabase CDB;
+  IgnoreDiagnostics DiagConsumer;
+  ClangdServer::Options Opts = ClangdServer::optsForTest();
+  Opts.BuildDynamicSymbolIndex = true;
+  ClangdServer Server(CDB, FS, DiagConsumer, Opts);
+
+  FS.Files[testPath("foo.h")] = R"cpp(
+    struct Foo {
+       // Member doc
+       int foo();
+    };
+  )cpp";
+  Annotations FileContent(R"cpp(
+    #include "foo.h"
+    void test() {
+      Foo f;
+      f.foo(^);
+    }
+  )cpp");
+  auto File = testPath("test.cpp");
+  Server.addDocument(File, FileContent.code());
+  // Wait for the dynamic index being built.
+  ASSERT_TRUE(Server.blockUntilIdleForTest());
+  EXPECT_THAT(
+      llvm::cantFail(runSignatureHelp(Server, File, FileContent.point()))
+          .signatures,
+      ElementsAre(AllOf(Sig("foo() -> int", {}), SigDoc("Member doc"))));
+}
+
 TEST(CompletionTest, CompletionFunctionArgsDisabled) {
   CodeCompleteOptions Opts;
   Opts.EnableSnippets = true;
   Opts.EnableFunctionArgSnippets = false;
 
   {
     auto Results = completions(
         R"cpp(