[C++20] add Basic consteval specifier

Summary:
this revision adds Lexing, Parsing and Basic Semantic for the consteval specifier as specified by http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1073r3.html

with this patch, the consteval specifier is treated as constexpr but can only be applied to function declaration.

Changes:
 - add the consteval keyword.
 - add parsing of consteval specifier for normal declarations and lambdas expressions.
 - add the whether a declaration is constexpr is now represented by and enum everywhere except for variable because they can't be consteval.
 - adapt diagnostic about constexpr to print constexpr or consteval depending on the case.
 - add tests for basic semantic.

Reviewers: rsmith, martong, shafik

Reviewed By: rsmith

Subscribers: eraman, efriedma, rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61790

llvm-svn: 363362

1 files changed, 2 insertions, 0 deletions

diff --git a/clang/lib/Parse/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
index 13d43d5e68e..a413f9a9414 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -1221,6 +1221,7 @@ public:
 ///           'friend'
 ///           'typedef'
 /// [C++11]   'constexpr'
+/// [C++20]   'consteval'
 /// [GNU]     attributes declaration-specifiers[opt]
 ///
 ///         storage-class-specifier:
@@ -1406,6 +1407,7 @@ Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
   case tok::kw_friend:
   case tok::kw_typedef:
   case tok::kw_constexpr:
+  case tok::kw_consteval:
     // storage-class-specifier
   case tok::kw_register:
   case tok::kw_static: