carbon-lang/toolchain/check/cpp/call.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/check/cpp/call.h"

#include "clang/Sema/Sema.h"
#include "clang/Sema/Template.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/check/call.h"
#include "toolchain/check/cpp/constant.h"
#include "toolchain/check/cpp/import.h"
#include "toolchain/check/cpp/location.h"
#include "toolchain/check/cpp/operators.h"
#include "toolchain/check/cpp/overload_resolution.h"
#include "toolchain/check/cpp/type_mapping.h"
#include "toolchain/check/literal.h"
#include "toolchain/sem_ir/function.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

// Returns true if the given instruction can only be a template argument, and
// not a function argument. We classify arguments as definitely being template
// arguments if they are types or the name of a template or generic.
// TODO: We should also have a way to specify that an argument is a non-type
// template argument.
static auto IsTemplateArg(Context& context, SemIR::InstId arg_id) -> bool {
  auto arg_type_id = context.insts().Get(arg_id).type_id();
  auto arg_type = context.types().GetAsInst(arg_type_id);
  return arg_type
      .IsOneOf<SemIR::TypeType, SemIR::FacetType, SemIR::CppTemplateNameType,
               SemIR::GenericClassType, SemIR::GenericInterfaceType,
               SemIR::GenericNamedConstraintType>();
}

// Splits a call argument list into a list of template arguments followed by a
// list of function arguments. We split the argument list as early as possible,
// subject to the constraint that if an argument is a template argument, it goes
// in the template argument list.
static auto SplitCallArgumentList(Context& context,
                                  llvm::ArrayRef<SemIR::InstId> arg_ids)
    -> std::pair<llvm::ArrayRef<SemIR::InstId>, llvm::ArrayRef<SemIR::InstId>> {
  for (auto [n, arg_id] : llvm::enumerate(llvm::reverse(arg_ids))) {
    if (IsTemplateArg(context, arg_id)) {
      return {arg_ids.drop_back(n), arg_ids.take_back(n)};
    }
  }
  // No template arguments found.
  return {{}, arg_ids};
}

auto PerformCallToCppFunction(Context& context, SemIR::LocId loc_id,
                              SemIR::CppOverloadSetId overload_set_id,
                              SemIR::InstId self_id,
                              llvm::ArrayRef<SemIR::InstId> arg_ids,
                              bool is_desugared) -> SemIR::InstId {
  auto [template_arg_ids, function_arg_ids] =
      SplitCallArgumentList(context, arg_ids);
  auto callee_id = PerformCppOverloadResolution(
      context, loc_id, context.cpp_overload_sets().Get(overload_set_id),
      template_arg_ids, self_id, function_arg_ids);
  SemIR::Callee callee = GetCallee(context.sem_ir(), callee_id);
  CARBON_KIND_SWITCH(callee) {
    case CARBON_KIND(SemIR::CalleeError _): {
      return SemIR::ErrorInst::InstId;
    }
    case CARBON_KIND(SemIR::CalleeFunction fn): {
      CARBON_CHECK(!fn.self_id.has_value());
      if (self_id.has_value()) {
        // Preserve the `self` argument from the original callee.
        fn.self_id = self_id;
      }
      return PerformCallToFunction(context, loc_id, callee_id, fn,
                                   function_arg_ids, is_desugared);
    }
    case CARBON_KIND(SemIR::CalleeCppOverloadSet _): {
      CARBON_FATAL("overloads can't be recursive");
    }
    case CARBON_KIND(SemIR::CalleeNonFunction _): {
      CARBON_FATAL("overloads should produce functions");
    }
  }
}

// Synthesize a placeholder `void{}` template argument, that will never be a
// valid argument for any template parameter. This is used in order to get Clang
// to diagnose invalid template argument errors for us. The location of the
// Carbon expression is used as the location of the C++ expression, so
// Clang's diagnostics will point into the Carbon code.
//
// TODO: If Clang ever tries to print the type of the expression or to
// pretty-print the expression itself, it would print the wrong thing. Currently
// this doesn't appear to happen, but in principle it could. Ideally we'd add an
// extension point to Clang to represent a "foreign expression" and use it here
// instead of creating a bogus placeholder expression.
static auto MakePlaceholderTemplateArg(Context& context, SemIR::InstId arg_id)
    -> clang::TemplateArgumentLoc {
  auto arg_loc = GetCppLocation(context, SemIR::LocId(arg_id));
  auto void_type = context.ast_context().VoidTy;
  auto* arg = new (context.ast_context()) clang::CXXScalarValueInitExpr(
      void_type,
      context.ast_context().getTrivialTypeSourceInfo(void_type, arg_loc),
      arg_loc);
  return clang::TemplateArgumentLoc(
      clang::TemplateArgument(arg, /*IsCanonical=*/false), arg);
}

// Converts an argument in a call to a C++ template name into a corresponding
// clang template argument, given the template parameter it will be matched
// against.
static auto ConvertArgToTemplateArg(
    Context& context, clang::TemplateDecl* template_decl,
    clang::NamedDecl* param_decl, SemIR::InstId arg_id,
    clang::SmallVector<clang::TemplateArgument>* template_args,
    unsigned argument_pack_index, bool diagnose)
    -> std::optional<clang::TemplateArgumentLoc> {
  if (isa<clang::TemplateTypeParmDecl>(param_decl)) {
    auto type = ExprAsType(context, SemIR::LocId(arg_id), arg_id, diagnose);
    if (type.type_id == SemIR::ErrorInst::TypeId) {
      return std::nullopt;
    }
    auto clang_type = MapToCppType(context, type.type_id);
    if (clang_type.isNull()) {
      if (diagnose) {
        context.TODO(arg_id, "unsupported type used as template argument");
      }
      return std::nullopt;
    }
    return clang::TemplateArgumentLoc(
        clang_type,
        context.ast_context().getTrivialTypeSourceInfo(
            clang_type, GetCppLocation(context, SemIR::LocId(arg_id))));
  }

  if (isa<clang::TemplateTemplateParmDecl>(param_decl)) {
    auto inst = context.sem_ir().insts().Get(arg_id);
    if (auto template_name_type =
            context.types().TryGetAs<SemIR::CppTemplateNameType>(
                inst.type_id())) {
      clang::TemplateName name(cast<clang::TemplateDecl>(
          context.clang_decls().Get(template_name_type->decl_id).key.decl));
      return clang::TemplateArgumentLoc(
          context.ast_context(), clang::TemplateArgument(name),
          /*TemplateKWLoc=*/clang::SourceLocation(),
          clang::NestedNameSpecifierLoc(),
          GetCppLocation(context, SemIR::LocId(arg_id)));
    }

    // TODO: Eventually we should also support passing Carbon generics as
    // template template arguments.
    return MakePlaceholderTemplateArg(context, arg_id);
  }

  if (auto* non_type = dyn_cast<clang::NonTypeTemplateParmDecl>(param_decl)) {
    auto param_type = non_type->getType();
    if (non_type->isParameterPack() && non_type->isExpandedParameterPack()) {
      param_type = non_type->getExpansionType(argument_pack_index);
    }

    // Handle non-type parameters with a dependent type. For example:
    //
    // C++:    template<typename T, T N> struct S{};
    // Carbon: Cpp.S(i32, 42)
    //
    // When evaluating the second template argument, the generic type of
    // `T` should be substituted with `i32`.
    if (param_type->isInstantiationDependentType()) {
      // If we don't want to diagnose errors, create a SFINAE context so that
      // Clang knows to suppress error messages.
      std::optional<clang::Sema::SFINAETrap> sfinae;
      if (!diagnose) {
        sfinae.emplace(context.clang_sema());
      }

      clang::Sema::InstantiatingTemplate inst(
          context.clang_sema(), clang::SourceLocation(), param_decl, non_type,
          *template_args, clang::SourceRange());
      if (inst.isInvalid()) {
        return std::nullopt;
      }
      clang::MultiLevelTemplateArgumentList mltal(template_decl, *template_args,
                                                  /*Final=*/true);

      mltal.addOuterRetainedLevels(non_type->getDepth());
      if (const auto* pet = param_type->getAs<clang::PackExpansionType>()) {
        clang::Sema::ArgPackSubstIndexRAII subst_index(context.clang_sema(),
                                                       argument_pack_index);
        param_type = context.clang_sema().SubstType(pet->getPattern(), mltal,
                                                    non_type->getLocation(),
                                                    non_type->getDeclName());
      } else {
        param_type = context.clang_sema().SubstType(param_type, mltal,
                                                    non_type->getLocation(),
                                                    non_type->getDeclName());
      }

      if (!param_type.isNull()) {
        param_type = context.clang_sema().CheckNonTypeTemplateParameterType(
            param_type, non_type->getLocation());
      }
      if (param_type.isNull() || (sfinae && sfinae->hasErrorOccurred())) {
        return std::nullopt;
      }
    }

    // Get the Carbon type corresponding to the parameter's Clang type.
    const auto type_expr =
        ImportCppType(context, SemIR::LocId(arg_id), param_type);

    // Try to convert the argument to the parameter type.
    const auto converted_inst_id =
        Convert(context, SemIR::LocId(arg_id), arg_id,
                {
                    .kind = ConversionTarget::Value,
                    .type_id = type_expr.type_id,
                    .diagnose = diagnose,
                });

    if (converted_inst_id == SemIR::ErrorInst::InstId) {
      return std::nullopt;
    }

    // TODO: provide a better location.
    auto template_loc = clang::TemplateArgumentLocInfo(context.ast_context(),
                                                       clang::SourceLocation());

    auto const_inst_id =
        context.constant_values().GetConstantInstId(converted_inst_id);
    if (const_inst_id.has_value()) {
      if (param_type->isPointerType()) {
        if (auto addr_of =
                context.insts().TryGetAs<SemIR::AddrOf>(const_inst_id)) {
          if (auto* var_decl = GetAsClangVarDecl(context, addr_of->lvalue_id)) {
            clang::TemplateArgument template_arg(var_decl, param_type);
            return clang::TemplateArgumentLoc(template_arg, template_loc);
          }

          // TODO: support pointers to variables declared in Carbon.
        }
      } else if (auto ap_value =
                     MapConstantToAPValue(context, const_inst_id, param_type)) {
        clang::TemplateArgument template_arg(context.ast_context(), param_type,
                                             *ap_value);
        return clang::TemplateArgumentLoc(template_arg, template_loc);
      }
    }

    // TODO: Support other types.
    if (diagnose) {
      context.TODO(arg_id,
                   "unsupported argument type for non-type template parameter");
    }
    return std::nullopt;
  }

  CARBON_FATAL("Unknown declaration kind for template parameter");
}

auto ConvertArgsToTemplateArgs(Context& context,
                               clang::TemplateDecl* template_decl,
                               llvm::ArrayRef<SemIR::InstId> arg_ids,
                               clang::TemplateArgumentListInfo& arg_list,
                               bool diagnose) -> bool {
  clang::SmallVector<clang::TemplateArgument> template_args;
  for (auto* param_decl : template_decl->getTemplateParameters()->asArray()) {
    if (arg_ids.empty()) {
      return true;
    }

    // A parameter pack consumes all remaining arguments; otherwise, it consumes
    // a single argument.
    // TODO: Handle expanded template parameter packs, which have a known, fixed
    // arity.
    llvm::ArrayRef<SemIR::InstId> args_for_param =
        param_decl->isTemplateParameterPack() ? std::exchange(arg_ids, {})
                                              : arg_ids.consume_front();
    for (auto [argument_pack_index, arg_id] : llvm::enumerate(args_for_param)) {
      if (auto arg = ConvertArgToTemplateArg(context, template_decl, param_decl,
                                             arg_id, &template_args,
                                             argument_pack_index, diagnose)) {
        arg_list.addArgument(*arg);
        template_args.push_back(arg->getArgument());
        argument_pack_index++;
      } else {
        return false;
      }
    }
  }

  // If there are any remaining arguments, that's an error; convert them to
  // placeholder template arguments so that Clang will diagnose it for us.
  for (auto arg_id : arg_ids) {
    // Synthesize a placeholder `void{}` template argument.
    arg_list.addArgument(MakePlaceholderTemplateArg(context, arg_id));
  }

  return true;
}

// Given a template and an template argument list, builds a Carbon value
// describing the corresponding C++ template-id.
static auto BuildTemplateId(Context& context, SemIR::LocId loc_id,
                            clang::SourceLocation loc,
                            clang::TemplateDecl* template_decl,
                            clang::TemplateArgumentListInfo& arg_list)
    -> SemIR::InstId {
  if (auto* var_template_decl =
          dyn_cast<clang::VarTemplateDecl>(template_decl)) {
    auto decl_result = context.clang_sema().CheckVarTemplateId(
        var_template_decl, /*TemplateLoc=*/clang::SourceLocation(), loc,
        arg_list, /*SetWrittenArgs=*/false);
    return decl_result.isInvalid()
               ? SemIR::ErrorInst::InstId
               : ImportCppDecl(context, loc_id,
                               SemIR::ClangDeclKey::ForNonFunctionDecl(
                                   decl_result.get()));
  }

  if (auto* concept_decl = dyn_cast<clang::ConceptDecl>(template_decl)) {
    auto expr_result = context.clang_sema().CheckConceptTemplateId(
        clang::CXXScopeSpec(), /*TemplateKWLoc=*/clang::SourceLocation(),
        clang::DeclarationNameInfo(concept_decl->getDeclName(), loc),
        concept_decl, concept_decl, &arg_list);
    if (expr_result.isInvalid()) {
      return SemIR::ErrorInst::InstId;
    }
    auto* expr = expr_result.getAs<clang::ConceptSpecializationExpr>();
    return MakeBoolLiteral(context, loc_id,
                           SemIR::BoolValue::From(expr->isSatisfied()));
  }

  clang::TemplateName template_name(template_decl);
  auto clang_type = context.clang_sema().CheckTemplateIdType(
      clang::ElaboratedTypeKeyword::None, template_name, loc, arg_list,
      /*Scope=*/nullptr, /*ForNestedNameSpecifier=*/false);
  if (clang_type.isNull()) {
    return SemIR::ErrorInst::InstId;
  }
  return ImportCppType(context, loc_id, clang_type).inst_id;
}

auto PerformCallToCppTemplateName(Context& context, SemIR::LocId loc_id,
                                  SemIR::ClangDeclId template_decl_id,
                                  llvm::ArrayRef<SemIR::InstId> arg_ids)
    -> SemIR::InstId {
  auto* template_decl = dyn_cast<clang::TemplateDecl>(
      context.clang_decls().Get(template_decl_id).key.decl);
  auto loc = GetCppLocation(context, loc_id);

  // Form a template argument list for this template.
  clang::TemplateArgumentListInfo arg_list(loc, loc);
  if (!ConvertArgsToTemplateArgs(context, template_decl, arg_ids, arg_list)) {
    return SemIR::ErrorInst::InstId;
  }

  return BuildTemplateId(context, loc_id, loc, template_decl, arg_list);
}

}  // namespace Carbon::Check