[clang-format] Correctly format lambdas with variadic template parameters.

[curdeius]

Summary

Fixes https://github.com/llvm/llvm-project/issues/53405.

[MyDeveloperDay]

LGTM

[HazardyKnusperkeks]

You guys have an enormous speed currently.

[MyDeveloperDay]

In D118220#3273416, @HazardyKnusperkeks wrote:

You guys have an enormous speed currently.

I'm just trying to keep up with @curdeius, but my real work keeps getting in the way, but I'm loving the momentum and trying to get back most days to at least help move things forward!

[curdeius]

Haha, and I want to have a few things fixed before release 14! :)

clang/lib/Format/UnwrappedLineParser.cpp

if (!Style.isCpp()) {
     return false;
   }
   assert(FormatTok->is(tok::l_square));
   FormatToken &LSquare = *FormatTok;
   if (!tryToParseLambdaIntroducer())
     return false;
 
   bool SeenArrow = false;
+  bool InTemplateParameterList = false;
 
   while (FormatTok->isNot(tok::l_brace)) {
     if (FormatTok->isSimpleTypeSpecifier()) {
       nextToken();
       continue;
     }
     switch (FormatTok->Tok.getKind()) {
     case tok::l_brace:
       break;
     case tok::l_paren:
       parseParens();
       break;
     case tok::l_square:
       parseSquare();
       break;
+    case tok::kw_class:
+    case tok::kw_template:
+    case tok::kw_typename:
+      assert(FormatTok->Previous);
+      if (FormatTok->Previous->is(tok::less))
+        InTemplateParameterList = true;
+      nextToken();
+      break;
     case tok::amp:
     case tok::star:
     case tok::kw_const:
     case tok::comma:
     case tok::less:
     case tok::greater:
     case tok::identifier:
     case tok::numeric_constant:
     case tok::coloncolon:
-    case tok::kw_class:
     case tok::kw_mutable:
     case tok::kw_noexcept:
-    case tok::kw_template:
-    case tok::kw_typename:
       nextToken();
       break;
     // Specialization of a template with an integer parameter can contain
     // arithmetic, logical, comparison and ternary operators.
     //
     // FIXME: This also accepts sequences of operators that are not in the scope
     // of a template argument list.
     //
while (FormatTok->isNot(tok::l_brace)) {
     case tok::exclaimequal:
     case tok::greaterequal:
     case tok::lessequal:
     case tok::question:
     case tok::colon:
     case tok::ellipsis:
     case tok::kw_true:
     case tok::kw_false:
-      if (SeenArrow) {
+      if (SeenArrow || InTemplateParameterList) {
         nextToken();
         break;
       }
       return true;
     case tok::arrow:
       // This might or might not actually be a lambda arrow (this could be an
       // ObjC method invocation followed by a dereferencing arrow). We might
       // reset this back to TT_Unknown in TokenAnnotator.

clang/unittests/Format/FormatTest.cpp

TEST_F(FormatTest, FormatsLambdas) {
   verifyFormat("SomeFunction({[]() -> int *[] { return {}; }});");
   verifyFormat("SomeFunction({[]() -> int (*)[] { return {}; }});");
   verifyFormat("SomeFunction({[]() -> ns::type<int (*)[]> { return {}; }});");
   verifyFormat("return int{[x = x]() { return x; }()};");
 
   // Lambdas with explicit template argument lists.
   verifyFormat(
       "auto L = []<template <typename> class T, class U>(T<U> &&a) {};\n");
+  verifyFormat("auto L = []<class T>(T) {\n"
+               "  {\n"
+               "    f();\n"
+               "    g();\n"
+               "  }\n"
+               "};\n");
+  verifyFormat("auto L = []<class... T>(T...) {\n"
+               "  {\n"
+               "    f();\n"
+               "    g();\n"
+               "  }\n"
+               "};\n");
+  verifyFormat("auto L = []<typename... T>(T...) {\n"
+               "  {\n"
+               "    f();\n"
+               "    g();\n"
+               "  }\n"
+               "};\n");
+  verifyFormat("auto L = []<template <typename...> class T>(T...) {\n"
+               "  {\n"
+               "    f();\n"
+               "    g();\n"
+               "  }\n"
+               "};\n");
+  verifyFormat("auto L = []</*comment*/ class... T>(T...) {\n"
+               "  {\n"
+               "    f();\n"
+               "    g();\n"
+               "  }\n"
+               "};\n");
 
   // Multiple lambdas in the same parentheses change indentation rules. These
   // lambdas are forced to start on new lines.
   verifyFormat("SomeFunction(\n"
                "    []() {\n"
                "      //\n"
                "    },\n"
                "    []() {\n"