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

[hubert.reinterpretcast]

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.

[hubert.reinterpretcast]

Update to use BCPL comments for C++ test case

[rsmith]

The code change looks fine. Please add some testcases covering valid constructs too (one of each kind of template, perhaps?).

[hubert.reinterpretcast]

Add tests (with various kinds of template) for valid cases of the requires-clause of a template-declaration.

include/clang/Parse/Parser.h

public:
   enum TypeCastState {
     NotTypeCast = 0,
     MaybeTypeCast,
     IsTypeCast
   };
 
   ExprResult ParseExpression(TypeCastState isTypeCast = NotTypeCast);
   ExprResult ParseConstantExpression(TypeCastState isTypeCast = NotTypeCast);
+  ExprResult ParseConstraintExpression();
   // Expr that doesn't include commas.
   ExprResult ParseAssignmentExpression(TypeCastState isTypeCast = NotTypeCast);
 
   ExprResult ParseMSAsmIdentifier(llvm::SmallVectorImpl<Token> &LineToks,
                                   unsigned &NumLineToksConsumed,
                                   void *Info,
                                   bool IsUnevaluated);
 

lib/Parse/ParseExpr.cpp

ExprResult Parser::ParseConstantExpression(TypeCastState isTypeCast) {
   EnterExpressionEvaluationContext Unevaluated(Actions,
                                                Sema::ConstantEvaluated);
 
   ExprResult LHS(ParseCastExpression(false, false, isTypeCast));
   ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::Conditional));
   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  ||
       K == tok::kw_for  || K == tok::kw_while ||
       K == tok::kw_if   || K == tok::kw_else  ||
       K == tok::kw_goto || K == tok::kw_try)
     return true;
   // If this is a decl-specifier, we can't be at the start of an expression.

lib/Parse/ParseTemplate.cpp

if (!TryConsumeToken(tok::kw_template, TemplateLoc)) {
       return nullptr;
     }
 
     // Parse the '<' template-parameter-list '>'
     SourceLocation LAngleLoc, RAngleLoc;
     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;
     }
 
     ParamLists.push_back(
       Actions.ActOnTemplateParameterList(CurTemplateDepthTracker.getDepth(), 
                                          ExportLoc,
                                          TemplateLoc, LAngleLoc,
                                          TemplateParams.data(),
                                          TemplateParams.size(), RAngleLoc));
 
     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;
     }
   } while (Tok.is(tok::kw_export) || Tok.is(tok::kw_template));
 
   // Parse the actual template declaration.
   return ParseSingleDeclarationAfterTemplate(Context,
                                              ParsedTemplateInfo(&ParamLists,

test/Parser/cxx-concepts-ambig-constraint-expr.cpp

+// RUN: %clang_cc1 -std=c++14 -fconcepts-ts -x c++ %s -verify
+
+// Test parsing of constraint-expressions in cases where the grammar is
+// ambiguous with the expectation that the longest token sequence which matches
+// the syntax is consumed without backtracking.
+
+// type-specifier-seq in conversion-type-id
+template <typename T> requires (bool)&T::operator short
+unsigned int foo(); // expected-error {{C++ requires a type specifier for all declarations}}
+
+// type-specifier-seq in new-type-id
+template <typename T> requires (bool)sizeof new (T::f()) short
+unsigned int bar(); // expected-error {{C++ requires a type specifier for all declarations}}
+
+template<typename T> requires (bool)sizeof new (T::f()) unsigned // expected-error {{'struct' cannot be signed or unsigned}}
+struct X { }; // expected-error {{'X' cannot be defined in a type specifier}}
+
+// C-style cast
+// of function call on function-style cast
+template <typename T> requires (bool(T()))
+T (*fp)(); // expected-error {{use of undeclared identifier 'fp'}}
+
+// function-style cast
+// as the callee in a function call
+struct A {
+  static int t;
+  template <typename T> requires bool(T())
+  (A(T (&t))) { } // expected-error {{called object type 'bool' is not a function or function pointer}}
+};

test/Parser/cxx-concepts-requires-clause.cpp

+// RUN: %clang_cc1 -std=c++14 -fconcepts-ts -x c++ %s -verify
+// expected-no-diagnostics
+
+// Test parsing of the optional requires-clause in a template-declaration.
+
+template <typename T> requires true
+void foo() { }
+
+
+template <typename T> requires !0
+struct A {
+  void foo();
+  struct AA;
+  enum E : int;
+  static int x;
+
+  template <typename> requires true
+  void Mfoo();
+
+  template <typename> requires true
+  struct M;
+
+  template <typename> requires true
+  static int Mx;
+
+  template <typename TT> requires true
+  using MQ = M<TT>;
+};
+
+template <typename T> requires !0
+void A<T>::foo() { }
+
+template <typename T> requires !0
+struct A<T>::AA { };
+
+template <typename T> requires !0
+enum A<T>::E : int { E0 };
+
+template <typename T> requires !0
+int A<T>::x = 0;
+
+template <typename T> requires !0
+template <typename> requires true
+void A<T>::Mfoo() { }
+
+template <typename T> requires !0
+template <typename> requires true
+struct A<T>::M { };
+
+template <typename T> requires !0
+template <typename> requires true
+int A<T>::Mx = 0;
+
+
+template <typename T> requires true
+int x = 0;
+
+template <typename T> requires true
+using Q = A<T>;
+
+struct C {
+  template <typename> requires true
+  void Mfoo();
+
+  template <typename> requires true
+  struct M;
+
+  template <typename> requires true
+  static int Mx;
+
+  template <typename T> requires true
+  using MQ = M<T>;
+};
+
+template <typename> requires true
+void C::Mfoo() { }
+
+template <typename> requires true
+struct C::M { };
+
+template <typename> requires true
+int C::Mx = 0;