[clangd] Return earlier when snippet is empty

Fixes github.com/clangd/clangd/issues/1216

If the Snippet string is empty, Snippet.front() would trigger a crash.
Move the Snippet->empty() check up a few lines to avoid this. Should not
break any existing behavior.

Differential Revision: https://reviews.llvm.org/D134137

(cherry picked from commit 60528c6)

Author: tom-anders

clang-tools-extra/clangd/CodeComplete.cpp

@@ -486,6 +486,9 @@ struct CodeCompletionBuilder {
       // we need to complete 'forward<$1>($0)'.
       return "($0)";
 
+    if (Snippet->empty())
+      return "";
+
     bool MayHaveArgList = Completion.Kind == CompletionItemKind::Function ||
                           Completion.Kind == CompletionItemKind::Method ||
                           Completion.Kind == CompletionItemKind::Constructor ||
@@ -524,8 +527,6 @@ struct CodeCompletionBuilder {
       return *Snippet;
 
     // Replace argument snippets with a simplified pattern.
-    if (Snippet->empty())
-      return "";
     if (MayHaveArgList) {
       // Functions snippets can be of 2 types:
       // - containing only function arguments, e.g.

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

@@ -1014,6 +1014,23 @@ TEST(CodeCompleteTest, NoColonColonAtTheEnd) {
   EXPECT_THAT(Results.Completions, Not(Contains(labeled("clang::"))));
 }
 
+TEST(CompletionTests, EmptySnippetDoesNotCrash) {
+    // See https://github.com/clangd/clangd/issues/1216
+    auto Results = completions(R"cpp(
+        int main() {
+          auto w = [&](auto &&f) { return f(f); };
+          auto f = w([&](auto &&f) {
+            return [&](auto &&n) {
+              if (n == 0) {
+                return 1;
+              }
+              return n * ^(f)(n - 1);
+            };
+          })(10);
+        }
+    )cpp");
+}
+
 TEST(CompletionTest, BacktrackCrashes) {
   // Sema calls code completion callbacks twice in these cases.
   auto Results = completions(R"cpp(