[P3816] Implement std::consteval_hash<std::meta::info> (semi-stable implementation 2)

clang/lib/AST/ExprConstantMeta.cpp

@@ -711,6 +711,15 @@ static bool reflect_invoke(APValue &Result, ASTContext &C, MetaActions &Meta,
                            SourceRange Range, ArrayRef<Expr *> Args,
                            Decl *ContainingDecl);
 
+          // ==========================================================
+          // std::consteval_hash<std::meta::info> specialization helper
+          // ==========================================================
+
+static bool reflection_hash(APValue &Result, ASTContext &C, MetaActions &Meta,
+                            EvalFn Evaluator, DiagFn Diagnoser, bool AllowInjection,
+                            QualType ResultTy, SourceRange Range,
+                            ArrayRef<Expr *> Args, Decl *ContainingDecl);
+
 // -----------------------------------------------------------------------------
 // Metafunction table
 //
@@ -844,6 +853,9 @@ static constexpr Metafunction Metafunctions[] = {
   // Other bespoke functions (not proposed at this time)
   { Metafunction::MFRK_bool, 1, 1, is_access_specified },
   { Metafunction::MFRK_metaInfo, 5, 5, reflect_invoke },
+
+  // std::consteval_hash<std::meta::info> specialization helper
+  { Metafunction::MFRK_sizeT, 1, 1, reflection_hash },
 };
 constexpr const unsigned NumMetafunctions = sizeof(Metafunctions) /
                                             sizeof(Metafunction);
@@ -1664,6 +1676,7 @@ StringRef DescriptionOf(APValue RV, bool Granular = true) {
     return "an annotation";
   }
   }
+  return "unknown reflection";
 }
 
 bool DiagnoseReflectionKind(DiagFn Diagnoser, SourceRange Range,
@@ -6405,4 +6418,29 @@ bool reflect_invoke(APValue &Result, ASTContext &C, MetaActions &Meta,
   return SetAndSucceed(Result, EvalResult.Val.Lift(CallExpr->getType()));
 }
 
+bool reflection_hash(APValue &Result, ASTContext &C, MetaActions &Meta,
+                    EvalFn Evaluator, DiagFn Diagnoser, bool AllowInjection,
+                    QualType ResultTy, SourceRange Range, ArrayRef<Expr *> Args,
+                    Decl *ContainingDecl) {
+  assert(Args[0]->getType()->isReflectionType());
+  assert(ResultTy == C.getSizeType());
+
+  APValue R;
+  if (!Evaluator(R, Args[0], true)) {
+    return true;
+  }
+
+  static std::unordered_map<const void*, std::size_t> s_values;
+  static std::size_t s_count = 0;
+
+  const auto [it, success] = s_values.insert({R.getOpaqueReflectionData(), s_count});
+  if (success) {
+    ++s_count;
+  }
+
+  return SetAndSucceed(
+    Result,
+    APValue(C.MakeIntValue(it->second, C.getSizeType())));
+}
+
 }  // end namespace clang

libcxx/include/meta

@@ -549,6 +549,8 @@ enum : unsigned {
   // Other bespoke functions (not proposed at this time)
   __metafn_is_access_specified,
   __metafn_reflect_invoke,
+
+  __metafn_reflection_hash,
 };
 
 consteval auto __workaround_expand_compiler_builtins(info type) -> info;
@@ -3023,6 +3025,27 @@ consteval auto u8display_string_of(info R) -> u8string_view {
 
 _LIBCPP_END_NAMESPACE_REFLECTION_V2
 
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <typename T> struct consteval_hash;
+
+template <>
+struct consteval_hash<meta::info>
+{
+  consteval consteval_hash() = default;
+  consteval consteval_hash(const consteval_hash&) = default;
+  consteval consteval_hash(consteval_hash&&) = default;
+  consteval auto operator()(meta::info r) const noexcept -> size_t
+  {
+    return __metafunction(meta::detail::__metafn_reflection_hash, r);
+  }
+
+private:
+  const meta::info unused = ^^::; // required to make type consteval-only
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
 #endif  // __has_feature(reflection)
 
 #endif  // _LIBCPP_META