[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, 18 insertions, 7 deletions

diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 08c3b0ff3273..21d7f6bd426c 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/clang/lib/Sema/DeclSpec.cpp
@@ -564,6 +564,15 @@ const char *DeclSpec::getSpecifierName(DeclSpec::TST T,
   llvm_unreachable("Unknown typespec!");
 }
 
+const char *DeclSpec::getSpecifierName(ConstexprSpecKind C) {
+  switch (C) {
+  case CSK_unspecified: return "unspecified";
+  case CSK_constexpr:   return "constexpr";
+  case CSK_consteval:   return "consteval";
+  }
+  llvm_unreachable("Unknown ConstexprSpecKind");
+}
+
 const char *DeclSpec::getSpecifierName(TQ T) {
   switch (T) {
   case DeclSpec::TQ_unspecified: return "unspecified";
@@ -1025,16 +1034,17 @@ bool DeclSpec::setModulePrivateSpec(SourceLocation Loc, const char *&PrevSpec,
   return false;
 }
 
-bool DeclSpec::SetConstexprSpec(SourceLocation Loc, const char *&PrevSpec,
+bool DeclSpec::SetConstexprSpec(ConstexprSpecKind ConstexprKind,
+                                SourceLocation Loc, const char *&PrevSpec,
                                 unsigned &DiagID) {
-  // 'constexpr constexpr' is ok, but warn as this is likely not what the user
-  // intended.
-  if (Constexpr_specified) {
+  if (ConstexprSpecifier != CSK_unspecified) {
+    if (ConstexprSpecifier == CSK_consteval || ConstexprKind == CSK_consteval)
+      return BadSpecifier(ConstexprKind, ConstexprSpecifier, PrevSpec, DiagID);
     DiagID = diag::warn_duplicate_declspec;
     PrevSpec = "constexpr";
     return true;
   }
-  Constexpr_specified = true;
+  ConstexprSpecifier = ConstexprKind;
   ConstexprLoc = Loc;
   return false;
 }
@@ -1280,9 +1290,10 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
   else if (TypeSpecType == TST_char16 || TypeSpecType == TST_char32)
     S.Diag(TSTLoc, diag::warn_cxx98_compat_unicode_type)
       << (TypeSpecType == TST_char16 ? "char16_t" : "char32_t");
-  if (Constexpr_specified)
+  if (getConstexprSpecifier() == CSK_constexpr)
     S.Diag(ConstexprLoc, diag::warn_cxx98_compat_constexpr);
-
+  if (getConstexprSpecifier() == CSK_consteval)
+    S.Diag(ConstexprLoc, diag::warn_cxx20_compat_consteval);
   // C++ [class.friend]p6:
   //   No storage-class-specifier shall appear in the decl-specifier-seq
   //   of a friend declaration.