[clangd] Make EnableFunctionArgSnippets option string-typed (#121178)

Fixes clangd/clangd#2232

Author: HighCommander4

clang-tools-extra/clangd/tool/ClangdMain.cpp

@@ -242,13 +242,13 @@ opt<std::string> FallbackStyle{
     init(clang::format::DefaultFallbackStyle),
 };
 
-opt<int> EnableFunctionArgSnippets{
+opt<std::string> EnableFunctionArgSnippets{
     "function-arg-placeholders",
     cat(Features),
     desc("When disabled (0), completions contain only parentheses for "
          "function calls. When enabled (1), completions also contain "
          "placeholders for method parameters"),
-    init(-1),
+    init("-1"),
 };
 
 opt<CodeCompleteOptions::IncludeInsertion> HeaderInsertion{
@@ -636,6 +636,22 @@ loadExternalIndex(const Config::ExternalIndexSpec &External,
   llvm_unreachable("Invalid ExternalIndexKind.");
 }
 
+std::optional<bool> shouldEnableFunctionArgSnippets() {
+  std::string Val = EnableFunctionArgSnippets;
+  // Accept the same values that a bool option parser would, but also accept
+  // -1 to indicate "unspecified", in which case the ArgumentListsPolicy
+  // config option will be respected.
+  if (Val == "1" || Val == "true" || Val == "True" || Val == "TRUE")
+    return true;
+  if (Val == "0" || Val == "false" || Val == "False" || Val == "FALSE")
+    return false;
+  if (Val != "-1")
+    elog("Value specified by --function-arg-placeholders is invalid. Provide a "
+         "boolean value or leave unspecified to use ArgumentListsPolicy from "
+         "config instead.");
+  return std::nullopt;
+}
+
 class FlagsConfigProvider : public config::Provider {
 private:
   config::CompiledFragment Frag;
@@ -696,10 +712,9 @@ class FlagsConfigProvider : public config::Provider {
       BGPolicy = Config::BackgroundPolicy::Skip;
     }
 
-    if (EnableFunctionArgSnippets >= 0) {
-      ArgumentLists = EnableFunctionArgSnippets
-                          ? Config::ArgumentListsPolicy::FullPlaceholders
-                          : Config::ArgumentListsPolicy::Delimiters;
+    if (std::optional<bool> Enable = shouldEnableFunctionArgSnippets()) {
+      ArgumentLists = *Enable ? Config::ArgumentListsPolicy::FullPlaceholders
+                              : Config::ArgumentListsPolicy::Delimiters;
     }
 
     Frag = [=](const config::Params &, Config &C) {