[Concepts] Parsing of requires-clause in template-declaration

Summary:
This change implements parse-only acceptance of the optional
requires-clause in a template-declaration. Diagnostic testing is added
for cases where the grammar is ambiguous with the expectation that the
longest token sequence which matches the syntax of a
constraint-expression is consumed without backtracking.

Reviewers: faisalv, fraggamuffin, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10462

llvm-svn: 240611

2 files changed, 30 insertions, 1 deletions

diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index da759c76522..36a302e52e6 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -205,6 +205,24 @@ ExprResult Parser::ParseConstantExpression(TypeCastState isTypeCast) {
   return Actions.ActOnConstantExpression(Res);
 }
 
+/// \brief Parse a constraint-expression.
+///
+/// \verbatim
+///       constraint-expression: [Concepts TS temp.constr.decl p1]
+///         logical-or-expression
+/// \endverbatim
+ExprResult Parser::ParseConstraintExpression() {
+  // FIXME: this may erroneously consume a function-body as the braced
+  // initializer list of a compound literal
+  //
+  // FIXME: this may erroneously consume a parenthesized rvalue reference
+  // declarator as a parenthesized address-of-label expression
+  ExprResult LHS(ParseCastExpression(/*isUnaryExpression=*/false));
+  ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::LogicalOr));
+
+  return Res;
+}
+
 bool Parser::isNotExpressionStart() {
   tok::TokenKind K = Tok.getKind();
   if (K == tok::l_brace || K == tok::r_brace  ||
diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index a8116785fdf..2a9becbc857 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/clang/lib/Parse/ParseTemplate.cpp
@@ -116,7 +116,7 @@ Parser::ParseTemplateDeclarationOrSpecialization(unsigned Context,
     SmallVector<Decl*, 4> TemplateParams;
     if (ParseTemplateParameters(CurTemplateDepthTracker.getDepth(),
                                 TemplateParams, LAngleLoc, RAngleLoc)) {
-      // Skip until the semi-colon or a }.
+      // Skip until the semi-colon or a '}'.
       SkipUntil(tok::r_brace, StopAtSemi | StopBeforeMatch);
       TryConsumeToken(tok::semi);
       return nullptr;
@@ -132,6 +132,17 @@ Parser::ParseTemplateDeclarationOrSpecialization(unsigned Context,
     if (!TemplateParams.empty()) {
       isSpecialization = false;
       ++CurTemplateDepthTracker;
+
+      if (TryConsumeToken(tok::kw_requires)) {
+        ExprResult ER =
+            Actions.CorrectDelayedTyposInExpr(ParseConstraintExpression());
+        if (!ER.isUsable()) {
+          // Skip until the semi-colon or a '}'.
+          SkipUntil(tok::r_brace, StopAtSemi | StopBeforeMatch);
+          TryConsumeToken(tok::semi);
+          return nullptr;
+        }
+      }
     } else {
       LastParamListWasEmpty = true;
     }