[c++1z] N4295: fold-expressions.

This is a new form of expression of the form:

  (expr op ... op expr)

where one of the exprs is a parameter pack. It expands into

  (expr1 op (expr2onwards op ... op expr))

(and likewise if the pack is on the right). The non-pack operand can be
omitted; in that case, an empty pack gives a fallback value or an error,
depending on the operator.

llvm-svn: 221573

1 files changed, 81 insertions, 2 deletions

diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index 60840c6f510..208ead86466 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -216,6 +216,13 @@ bool Parser::isNotExpressionStart() {
   return isKnownToBeDeclarationSpecifier();
 }
 
+static bool isFoldOperator(prec::Level Level) {
+  return Level > prec::Unknown && Level != prec::Conditional;
+}
+static bool isFoldOperator(tok::TokenKind Kind) {
+  return isFoldOperator(getBinOpPrecedence(Kind, false, true));
+}
+
 /// \brief Parse a binary expression that starts with \p LHS and has a
 /// precedence of at least \p MinPrec.
 ExprResult
@@ -247,6 +254,16 @@ Parser::ParseRHSOfBinaryExpression(ExprResult LHS, prec::Level MinPrec) {
       return LHS;
     }
 
+    // If the next token is an ellipsis, then this is a fold-expression. Leave
+    // it alone so we can handle it in the paren expression.
+    if (isFoldOperator(NextTokPrec) && Tok.is(tok::ellipsis)) {
+      // FIXME: We can't check this via lookahead before we consume the token
+      // because that tickles a lexer bug.
+      PP.EnterToken(Tok);
+      Tok = OpToken;
+      return LHS;
+    }
+
     // Special case handling for the ternary operator.
     ExprResult TernaryMiddle(true);
     if (NextTokPrec == prec::Conditional) {
@@ -365,7 +382,6 @@ Parser::ParseRHSOfBinaryExpression(ExprResult LHS, prec::Level MinPrec) {
       NextTokPrec = getBinOpPrecedence(Tok.getKind(), GreaterThanIsOperator,
                                        getLangOpts().CPlusPlus11);
     }
-    assert(NextTokPrec <= ThisPrec && "Recursion didn't work!");
 
     if (!RHS.isInvalid() && RHSIsInitList) {
       if (ThisPrec == prec::Assignment) {
@@ -1989,11 +2005,15 @@ ExprResult Parser::ParseBuiltinPrimaryExpression() {
 ///       cast-expression: [C99 6.5.4]
 ///         '(' type-name ')' cast-expression
 /// [ARC]   bridged-cast-expression
-/// 
 /// [ARC] bridged-cast-expression:
 ///         (__bridge type-name) cast-expression
 ///         (__bridge_transfer type-name) cast-expression
 ///         (__bridge_retained type-name) cast-expression
+///       fold-expression: [C++1z]
+///         '(' cast-expression fold-operator '...' ')'
+///         '(' '...' fold-operator cast-expression ')'
+///         '(' cast-expression fold-operator '...'
+///                 fold-operator cast-expression ')'
 /// \endverbatim
 ExprResult
 Parser::ParseParenExpression(ParenParseOption &ExprType, bool stopIfCastExpr,
@@ -2194,12 +2214,18 @@ Parser::ParseParenExpression(ParenParseOption &ExprType, bool stopIfCastExpr,
       Result = Actions.ActOnParenListExpr(OpenLoc, Tok.getLocation(),
                                           ArgExprs);
     }
+  } else if (Tok.is(tok::ellipsis) &&
+             isFoldOperator(NextToken().getKind())) {
+    return ParseFoldExpression(ExprResult(), T);
   } else {
     InMessageExpressionRAIIObject InMessage(*this, false);
     
     Result = ParseExpression(MaybeTypeCast);
     ExprType = SimpleExpr;
 
+    if (isFoldOperator(Tok.getKind()) && NextToken().is(tok::ellipsis))
+      return ParseFoldExpression(Result, T);
+
     // Don't build a paren expression unless we actually match a ')'.
     if (!Result.isInvalid() && Tok.is(tok::r_paren))
       Result =
@@ -2357,6 +2383,59 @@ ExprResult Parser::ParseGenericSelectionExpression() {
                                            Types, Exprs);
 }
 
+/// \brief Parse A C++1z fold-expression after the opening paren and optional
+/// left-hand-side expression.
+///
+/// \verbatim
+///   fold-expression:
+///       ( cast-expression fold-operator ... )
+///       ( ... fold-operator cast-expression )
+///       ( cast-expression fold-operator ... fold-operator cast-expression )
+ExprResult Parser::ParseFoldExpression(ExprResult LHS,
+                                       BalancedDelimiterTracker &T) {
+  if (LHS.isInvalid()) {
+    T.skipToEnd();
+    return true;
+  }
+
+  tok::TokenKind Kind = tok::unknown;
+  SourceLocation FirstOpLoc;
+  if (LHS.isUsable()) {
+    Kind = Tok.getKind();
+    assert(isFoldOperator(Kind) && "missing fold-operator");
+    FirstOpLoc = ConsumeToken();
+  }
+
+  assert(Tok.is(tok::ellipsis) && "not a fold-expression");
+  SourceLocation EllipsisLoc = ConsumeToken();
+
+  ExprResult RHS;
+  if (Tok.isNot(tok::r_paren)) {
+    if (!isFoldOperator(Tok.getKind()))
+      return Diag(Tok.getLocation(), diag::err_expected_fold_operator);
+
+    if (Kind != tok::unknown && Tok.getKind() != Kind)
+      Diag(Tok.getLocation(), diag::err_fold_operator_mismatch)
+        << SourceRange(FirstOpLoc);
+    Kind = Tok.getKind();
+    ConsumeToken();
+
+    RHS = ParseExpression();
+    if (RHS.isInvalid()) {
+      T.skipToEnd();
+      return true;
+    }
+  }
+
+  Diag(EllipsisLoc, getLangOpts().CPlusPlus1z
+                        ? diag::warn_cxx14_compat_fold_expression
+                        : diag::ext_fold_expression);
+
+  T.consumeClose();
+  return Actions.ActOnCXXFoldExpr(T.getOpenLocation(), LHS.get(), Kind,
+                                  EllipsisLoc, RHS.get(), T.getCloseLocation());
+}
+
 /// ParseExpressionList - Used for C/C++ (argument-)expression-list.
 ///
 /// \verbatim