[clang-format] Fix a bug in indenting lambda trailing arrows (#94560)

Closes #94181

Author: c8ef

clang/lib/Format/ContinuationIndenter.cpp

@@ -1257,6 +1257,11 @@ unsigned ContinuationIndenter::getNewLineColumn(const LineState &State) {
     }
     return CurrentState.Indent;
   }
+  if (Current.is(TT_TrailingReturnArrow) &&
+      Previous.isOneOf(tok::kw_noexcept, tok::kw_mutable, tok::kw_constexpr,
+                       tok::kw_consteval, tok::kw_static, TT_AttributeSquare)) {
+    return ContinuationIndent;
+  }
   if ((Current.isOneOf(tok::r_brace, tok::r_square) ||
        (Current.is(tok::greater) && (Style.isProto() || Style.isTableGen()))) &&
       State.Stack.size() > 1) {

clang/unittests/Format/FormatTest.cpp

@@ -22866,6 +22866,22 @@ TEST_F(FormatTest, FormatsLambdas) {
       "      //\n"
       "    });");
 
+  FormatStyle LLVMStyle = getLLVMStyleWithColumns(60);
+  verifyFormat("very_long_function_name_yes_it_is_really_long(\n"
+               "    [](auto n) noexcept [[back_attr]]\n"
+               "        -> std::unordered_map<very_long_type_name_A,\n"
+               "                              very_long_type_name_B> {\n"
+               "      really_do_something();\n"
+               "    });",
+               LLVMStyle);
+  verifyFormat("very_long_function_name_yes_it_is_really_long(\n"
+               "    [](auto n) constexpr\n"
+               "        -> std::unordered_map<very_long_type_name_A,\n"
+               "                              very_long_type_name_B> {\n"
+               "      really_do_something();\n"
+               "    });",
+               LLVMStyle);
+
   FormatStyle DoNotMerge = getLLVMStyle();
   DoNotMerge.AllowShortLambdasOnASingleLine = FormatStyle::SLS_None;
   verifyFormat("auto c = []() {\n"