[clang-format] Fix a bug in RemoveParentheses: ReturnStatement (#67911)

Don't remove the outermost parentheses surrounding a return statement
expression when inside a function/lambda that has the decltype(auto)
return type.

Fixed #67892.

clang/lib/Format/UnwrappedLineParser.cpp

@@ -173,10 +173,12 @@ void UnwrappedLineParser::reset() {
   CommentsBeforeNextToken.clear();
   FormatTok = nullptr;
   MustBreakBeforeNextToken = false;
+  IsDecltypeAutoFunction = false;
   PreprocessorDirectives.clear();
   CurrentLines = &Lines;
   DeclarationScopeStack.clear();
   NestedTooDeep.clear();
+  NestedLambdas.clear();
   PPStack.clear();
   Line->FirstStartColumn = FirstStartColumn;
 
@@ -1766,6 +1768,17 @@ void UnwrappedLineParser::parseStructuralElement(
       if (parseStructLike())
         return;
       break;
+    case tok::kw_decltype:
+      nextToken();
+      if (FormatTok->is(tok::l_paren)) {
+        parseParens();
+        assert(FormatTok->Previous);
+        if (FormatTok->Previous->endsSequence(tok::r_paren, tok::kw_auto,
+                                              tok::l_paren)) {
+          Line->SeenDecltypeAuto = true;
+        }
+      }
+      break;
     case tok::period:
       nextToken();
       // In Java, classes have an implicit static member "class".
@@ -1827,6 +1840,7 @@ void UnwrappedLineParser::parseStructuralElement(
       if (InRequiresExpression)
         FormatTok->setFinalizedType(TT_BracedListLBrace);
       if (!tryToParsePropertyAccessor() && !tryToParseBracedList()) {
+        IsDecltypeAutoFunction = Line->SeenDecltypeAuto;
         // A block outside of parentheses must be the last part of a
         // structural element.
         // FIXME: Figure out cases where this is not true, and add projections
@@ -1844,6 +1858,7 @@ void UnwrappedLineParser::parseStructuralElement(
         }
         FormatTok->setFinalizedType(TT_FunctionLBrace);
         parseBlock();
+        IsDecltypeAutoFunction = false;
         addUnwrappedLine();
         return;
       }
@@ -2249,9 +2264,15 @@ bool UnwrappedLineParser::tryToParseLambda() {
       return true;
     }
   }
+
   FormatTok->setFinalizedType(TT_LambdaLBrace);
   LSquare.setFinalizedType(TT_LambdaLSquare);
+
+  NestedLambdas.push_back(Line->SeenDecltypeAuto);
   parseChildBlock();
+  assert(!NestedLambdas.empty());
+  NestedLambdas.pop_back();
+
   return true;
 }
 
@@ -2469,6 +2490,8 @@ bool UnwrappedLineParser::parseParens(TokenType AmpAmpTokenType) {
                PrevPrev->endsSequence(tok::kw_constexpr, tok::kw_if))));
         const bool ReturnParens =
             Style.RemoveParentheses == FormatStyle::RPS_ReturnStatement &&
+            ((NestedLambdas.empty() && !IsDecltypeAutoFunction) ||
+             (!NestedLambdas.empty() && !NestedLambdas.back())) &&
             Prev && Prev->isOneOf(tok::kw_return, tok::kw_co_return) && Next &&
             Next->is(tok::semi);
         if ((DoubleParens && !Blacklisted) || ReturnParens) {
@@ -4379,6 +4402,7 @@ void UnwrappedLineParser::addUnwrappedLine(LineLevel AdjustLevel) {
   Line->MatchingOpeningBlockLineIndex = UnwrappedLine::kInvalidIndex;
   Line->FirstStartColumn = 0;
   Line->IsContinuation = false;
+  Line->SeenDecltypeAuto = false;
 
   if (ClosesWhitesmithsBlock && AdjustLevel == LineLevel::Remove)
     --Line->Level;

clang/lib/Format/UnwrappedLineParser.h

@@ -61,6 +61,9 @@ struct UnwrappedLine {
 
   bool MustBeDeclaration = false;
 
+  /// Whether the parser has seen \c decltype(auto) in this line.
+  bool SeenDecltypeAuto = false;
+
   /// \c True if this line should be indented by ContinuationIndent in
   /// addition to the normal indention level.
   bool IsContinuation = false;
@@ -335,6 +338,14 @@ class UnwrappedLineParser {
   // statement contains more than some predefined number of nested statements).
   SmallVector<bool, 8> NestedTooDeep;
 
+  // Keeps a stack of the states of nested lambdas (true if the return type of
+  // the lambda is `decltype(auto)`).
+  SmallVector<bool, 4> NestedLambdas;
+
+  // Whether the parser is parsing the body of a function whose return type is
+  // `decltype(auto)`.
+  bool IsDecltypeAutoFunction = false;
+
   // Represents preprocessor branch type, so we can find matching
   // #if/#else/#endif directives.
   enum PPBranchKind {

clang/unittests/Format/FormatTest.cpp

@@ -26305,6 +26305,56 @@ TEST_F(FormatTest, RemoveParentheses) {
   verifyFormat("co_return 0;", "co_return ((0));", Style);
   verifyFormat("return 0;", "return (((0)));", Style);
   verifyFormat("return ({ 0; });", "return ((({ 0; })));", Style);
+  verifyFormat("inline decltype(auto) f() {\n"
+               "  if (a) {\n"
+               "    return (a);\n"
+               "  }\n"
+               "  return (b);\n"
+               "}",
+               "inline decltype(auto) f() {\n"
+               "  if (a) {\n"
+               "    return ((a));\n"
+               "  }\n"
+               "  return ((b));\n"
+               "}",
+               Style);
+  verifyFormat("auto g() {\n"
+               "  decltype(auto) x = [] {\n"
+               "    auto y = [] {\n"
+               "      if (a) {\n"
+               "        return a;\n"
+               "      }\n"
+               "      return b;\n"
+               "    };\n"
+               "    if (c) {\n"
+               "      return (c);\n"
+               "    }\n"
+               "    return (d);\n"
+               "  };\n"
+               "  if (e) {\n"
+               "    return e;\n"
+               "  }\n"
+               "  return f;\n"
+               "}",
+               "auto g() {\n"
+               "  decltype(auto) x = [] {\n"
+               "    auto y = [] {\n"
+               "      if (a) {\n"
+               "        return ((a));\n"
+               "      }\n"
+               "      return ((b));\n"
+               "    };\n"
+               "    if (c) {\n"
+               "      return ((c));\n"
+               "    }\n"
+               "    return ((d));\n"
+               "  };\n"
+               "  if (e) {\n"
+               "    return ((e));\n"
+               "  }\n"
+               "  return ((f));\n"
+               "}",
+               Style);
 
   Style.ColumnLimit = 25;
   verifyFormat("return (a + b) - (c + d);",