carbon-lang/toolchain/check/deduce.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/deduce.h"

#include "llvm/ADT/SmallBitVector.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/check/context.h"
#include "toolchain/check/convert.h"
#include "toolchain/check/generic.h"
#include "toolchain/check/subst.h"
#include "toolchain/diagnostics/diagnostic.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/impl.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

namespace {
// A list of pairs of (instruction from generic, corresponding instruction from
// call to of generic) for which we still need to perform deduction, along with
// methods to add and pop pending deductions from the list. Deductions are
// popped in order from most- to least-recently pushed, with the intent that
// they are visited in depth-first order, although the order is not expected to
// matter except when it influences which error is diagnosed.
class DeductionWorklist {
 public:
  explicit DeductionWorklist(Context& context) : context_(context) {}

  struct PendingDeduction {
    SemIR::InstId param;
    SemIR::InstId arg;
    bool needs_substitution;
  };

  // Adds a single (param, arg) deduction.
  auto Add(SemIR::InstId param, SemIR::InstId arg, bool needs_substitution)
      -> void {
    deductions_.push_back(
        {.param = param, .arg = arg, .needs_substitution = needs_substitution});
  }

  // Adds a single (param, arg) type deduction.
  auto Add(SemIR::TypeId param, SemIR::TypeId arg, bool needs_substitution)
      -> void {
    Add(context_.types().GetInstId(param), context_.types().GetInstId(arg),
        needs_substitution);
  }

  // Adds a single (param, arg) deduction of a specific.
  auto Add(SemIR::SpecificId param, SemIR::SpecificId arg,
           bool needs_substitution) -> void {
    if (!param.has_value() || !arg.has_value()) {
      return;
    }
    auto& param_specific = context_.specifics().Get(param);
    auto& arg_specific = context_.specifics().Get(arg);
    if (param_specific.generic_id != arg_specific.generic_id) {
      // TODO: Decide whether to error on this or just treat the specific as
      // non-deduced. For now we treat it as non-deduced.
      return;
    }
    AddAll(param_specific.args_id, arg_specific.args_id, needs_substitution);
  }

  // Adds a list of (param, arg) deductions. These are added in reverse order so
  // they are popped in forward order.
  template <typename ElementId>
  auto AddAll(llvm::ArrayRef<ElementId> params, llvm::ArrayRef<ElementId> args,
              bool needs_substitution) -> void {
    if (params.size() != args.size()) {
      // TODO: Decide whether to error on this or just treat the parameter list
      // as non-deduced. For now we treat it as non-deduced.
      return;
    }
    for (auto [param, arg] : llvm::reverse(llvm::zip_equal(params, args))) {
      Add(param, arg, needs_substitution);
    }
  }

  auto AddAll(SemIR::InstBlockId params, llvm::ArrayRef<SemIR::InstId> args,
              bool needs_substitution) -> void {
    AddAll(context_.inst_blocks().Get(params), args, needs_substitution);
  }

  auto AddAll(SemIR::StructTypeFieldsId params, SemIR::StructTypeFieldsId args,
              bool needs_substitution) -> void {
    const auto& param_fields = context_.struct_type_fields().Get(params);
    const auto& arg_fields = context_.struct_type_fields().Get(args);
    if (param_fields.size() != arg_fields.size()) {
      // TODO: Decide whether to error on this or just treat the parameter list
      // as non-deduced. For now we treat it as non-deduced.
      return;
    }
    // Don't do deduction unless the names match in order.
    // TODO: Support reordering of names.
    for (auto [param, arg] : llvm::zip_equal(param_fields, arg_fields)) {
      if (param.name_id != arg.name_id) {
        return;
      }
    }
    for (auto [param, arg] :
         llvm::reverse(llvm::zip_equal(param_fields, arg_fields))) {
      Add(param.type_id, arg.type_id, needs_substitution);
    }
  }

  auto AddAll(SemIR::InstBlockId params, SemIR::InstBlockId args,
              bool needs_substitution) -> void {
    AddAll(context_.inst_blocks().Get(params), context_.inst_blocks().Get(args),
           needs_substitution);
  }

  auto AddAll(SemIR::TypeBlockId params, SemIR::TypeBlockId args,
              bool needs_substitution) -> void {
    AddAll(context_.type_blocks().Get(params), context_.type_blocks().Get(args),
           needs_substitution);
  }

  auto AddAll(SemIR::FacetTypeId params, SemIR::FacetTypeId args,
              bool needs_substitution) -> void {
    const auto& param_impls =
        context_.facet_types().Get(params).impls_constraints;
    const auto& arg_impls = context_.facet_types().Get(args).impls_constraints;
    // TODO: Decide whether to error on these or just treat the parameter list
    // as non-deduced. For now we treat it as non-deduced.
    if (param_impls.size() != 1 || arg_impls.size() != 1) {
      return;
    }
    auto param = param_impls.front();
    auto arg = arg_impls.front();
    if (param.interface_id != arg.interface_id) {
      return;
    }
    Add(param.specific_id, arg.specific_id, needs_substitution);
  }

  // Adds a (param, arg) pair for an instruction argument, given its kind.
  auto AddInstArg(SemIR::IdKind kind, int32_t param, int32_t arg,
                  bool needs_substitution) -> void {
    switch (kind) {
      case SemIR::IdKind::None:
      case SemIR::IdKind::For<SemIR::ClassId>:
      case SemIR::IdKind::For<SemIR::IntKind>:
        break;
      case SemIR::IdKind::For<SemIR::InstId>:
        Add(SemIR::InstId(param), SemIR::InstId(arg), needs_substitution);
        break;
      case SemIR::IdKind::For<SemIR::TypeId>:
        Add(SemIR::TypeId(param), SemIR::TypeId(arg), needs_substitution);
        break;
      case SemIR::IdKind::For<SemIR::StructTypeFieldsId>:
        AddAll(SemIR::StructTypeFieldsId(param), SemIR::StructTypeFieldsId(arg),
               needs_substitution);
        break;
      case SemIR::IdKind::For<SemIR::InstBlockId>:
        AddAll(SemIR::InstBlockId(param), SemIR::InstBlockId(arg),
               needs_substitution);
        break;
      case SemIR::IdKind::For<SemIR::TypeBlockId>:
        AddAll(SemIR::TypeBlockId(param), SemIR::TypeBlockId(arg),
               needs_substitution);
        break;
      case SemIR::IdKind::For<SemIR::SpecificId>:
        Add(SemIR::SpecificId(param), SemIR::SpecificId(arg),
            needs_substitution);
        break;
      case SemIR::IdKind::For<SemIR::FacetTypeId>:
        AddAll(SemIR::FacetTypeId(param), SemIR::FacetTypeId(arg),
               needs_substitution);
        break;
      default:
        CARBON_FATAL("unexpected argument kind");
    }
  }

  // Returns whether we have completed all deductions.
  auto Done() -> bool { return deductions_.empty(); }

  // Pops the next deduction. Requires `!Done()`.
  auto PopNext() -> PendingDeduction { return deductions_.pop_back_val(); }

 private:
  Context& context_;
  llvm::SmallVector<PendingDeduction> deductions_;
};

// State that is tracked throughout the deduction process.
class DeductionContext {
 public:
  // Preparse to perform deduction. If an enclosing specific or self type
  // are provided, adds the corresponding arguments as known arguments that will
  // not be deduced.
  DeductionContext(Context& context, SemIR::LocId loc_id,
                   SemIR::GenericId generic_id,
                   SemIR::SpecificId enclosing_specific_id,
                   SemIR::InstId self_type_id, bool diagnose);

  auto context() const -> Context& { return *context_; }

  // Adds a pending deduction of `param` from `arg`. `needs_substitution`
  // indicates whether we need to substitute known generic parameters into
  // `param`.
  template <typename ParamT, typename ArgT>
  auto Add(ParamT param, ArgT arg, bool needs_substitution) -> void {
    worklist_.Add(param, arg, needs_substitution);
  }

  // Same as `Add` but for an array or block of operands.
  template <typename ParamT, typename ArgT>
  auto AddAll(ParamT param, ArgT arg, bool needs_substitution) -> void {
    worklist_.AddAll(param, arg, needs_substitution);
  }

  // Performs all deductions in the deduction worklist. Returns whether
  // deduction succeeded.
  auto Deduce() -> bool;

  // Returns whether every generic parameter has a corresponding deduced generic
  // argument. If not, issues a suitable diagnostic.
  auto CheckDeductionIsComplete() -> bool;

  // Forms a specific corresponding to the deduced generic with the deduced
  // argument list. Must not be called before deduction is complete.
  auto MakeSpecific() -> SemIR::SpecificId;

 private:
  auto NoteInitializingParam(SemIR::InstId param_id, auto& builder) -> void {
    if (auto param = context().insts().TryGetAs<SemIR::SymbolicBindingPattern>(
            param_id)) {
      CARBON_DIAGNOSTIC(InitializingGenericParam, Note,
                        "initializing generic parameter `{0}` declared here",
                        SemIR::NameId);
      builder.Note(param_id, InitializingGenericParam,
                   context().entity_names().Get(param->entity_name_id).name_id);
    } else {
      NoteGenericHere(context(), generic_id_, builder);
    }
  }

  Context* context_;
  SemIR::LocId loc_id_;
  SemIR::GenericId generic_id_;
  bool diagnose_;

  DeductionWorklist worklist_;

  llvm::SmallVector<SemIR::InstId> result_arg_ids_;
  llvm::SmallVector<Substitution> substitutions_;
  SemIR::CompileTimeBindIndex first_deduced_index_;
  // Non-deduced indexes, indexed by parameter index - first_deduced_index_.
  llvm::SmallBitVector non_deduced_indexes_;
};

}  // namespace

static auto NoteGenericHere(Context& context, SemIR::GenericId generic_id,
                            Context::DiagnosticBuilder& diag) -> void {
  CARBON_DIAGNOSTIC(DeductionGenericHere, Note,
                    "while deducing parameters of generic declared here");
  diag.Note(context.generics().Get(generic_id).decl_id, DeductionGenericHere);
}

DeductionContext::DeductionContext(Context& context, SemIR::LocId loc_id,
                                   SemIR::GenericId generic_id,
                                   SemIR::SpecificId enclosing_specific_id,
                                   SemIR::InstId self_type_id, bool diagnose)
    : context_(&context),
      loc_id_(loc_id),
      generic_id_(generic_id),
      diagnose_(diagnose),
      worklist_(context),
      first_deduced_index_(0) {
  CARBON_CHECK(generic_id.has_value(),
               "Performing deduction for non-generic entity");

  // Initialize the deduced arguments to `None`.
  result_arg_ids_.resize(
      context.inst_blocks()
          .Get(context.generics().Get(generic_id_).bindings_id)
          .size(),
      SemIR::InstId::None);

  if (enclosing_specific_id.has_value()) {
    // Copy any outer generic arguments from the specified instance and prepare
    // to substitute them into the function declaration.
    auto args = context.inst_blocks().Get(
        context.specifics().Get(enclosing_specific_id).args_id);
    llvm::copy(args, result_arg_ids_.begin());

    // TODO: Subst is linear in the length of the substitutions list. Change
    // it so we can pass in an array mapping indexes to substitutions instead.
    substitutions_.reserve(args.size() + result_arg_ids_.size());
    for (auto [i, subst_inst_id] : llvm::enumerate(args)) {
      substitutions_.push_back(
          {.bind_id = SemIR::CompileTimeBindIndex(i),
           .replacement_id = context.constant_values().Get(subst_inst_id)});
    }
    first_deduced_index_ = SemIR::CompileTimeBindIndex(args.size());
  }

  if (self_type_id.has_value()) {
    // Copy the provided `Self` type as the value of the next binding.
    auto self_index = first_deduced_index_;
    result_arg_ids_[self_index.index] = self_type_id;
    substitutions_.push_back(
        {.bind_id = SemIR::CompileTimeBindIndex(self_index),
         .replacement_id = context.constant_values().Get(self_type_id)});
    first_deduced_index_ = SemIR::CompileTimeBindIndex(self_index.index + 1);
  }

  non_deduced_indexes_.resize(result_arg_ids_.size() -
                              first_deduced_index_.index);
}

auto DeductionContext::Deduce() -> bool {
  while (!worklist_.Done()) {
    auto [param_id, arg_id, needs_substitution] = worklist_.PopNext();

    // TODO: Bail out if there's nothing to deduce: if we're not in a pattern
    // and the parameter doesn't have a symbolic constant value.

    auto param_type_id = context().insts().Get(param_id).type_id();
    // If the parameter has a symbolic type, deduce against that.
    if (param_type_id.AsConstantId().is_symbolic()) {
      Add(context().types().GetInstId(param_type_id),
          context().types().GetInstId(context().insts().Get(arg_id).type_id()),
          needs_substitution);
    } else {
      // The argument (e.g. a TupleLiteral of types) may be convertible to a
      // compile-time value (e.g. TupleType) that we can decompose further.
      // So we do this conversion here, even though we will later try convert
      // again when we have deduced all of the bindings.
      DiagnosticAnnotationScope annotate_diagnostics(
          &context().emitter(),
          [&](auto& builder) { NoteInitializingParam(param_id, builder); });
      // TODO: The call logic should reuse the conversion here (if any) instead
      // of doing the same conversion again. At the moment we throw away the
      // converted arg_id.
      arg_id = ConvertToValueOfType(context(), loc_id_, arg_id, param_type_id);
      if (arg_id == SemIR::ErrorInst::SingletonInstId) {
        return false;
      }
    }

    // Attempt to match `param_inst` against `arg_id`. If the match succeeds,
    // this should `continue` the outer loop. On `break`, we will try to desugar
    // the parameter to continue looking for a match.
    auto param_inst = context().insts().Get(param_id);
    CARBON_KIND_SWITCH(param_inst) {
      // Deducing a symbolic binding pattern from an argument deduces the
      // binding as having that constant value. For example, deducing
      // `(T:! type)` against `(i32)` deduces `T` to be `i32`. This only arises
      // when initializing a generic parameter from an explicitly specified
      // argument, and in this case, the argument is required to be a
      // compile-time constant.
      case CARBON_KIND(SemIR::SymbolicBindingPattern bind): {
        auto& entity_name = context().entity_names().Get(bind.entity_name_id);
        auto index = entity_name.bind_index;
        if (!index.has_value()) {
          break;
        }
        CARBON_CHECK(
            index >= first_deduced_index_ &&
                static_cast<size_t>(index.index) < result_arg_ids_.size(),
            "Unexpected index {0} for symbolic binding pattern; "
            "expected to be in range [{1}, {2})",
            index.index, first_deduced_index_.index, result_arg_ids_.size());
        CARBON_CHECK(!result_arg_ids_[index.index].has_value(),
                     "Deduced a value for parameter prior to its declaration");

        auto arg_const_inst_id =
            context().constant_values().GetConstantInstId(arg_id);
        if (!arg_const_inst_id.has_value()) {
          if (diagnose_) {
            CARBON_DIAGNOSTIC(CompTimeArgumentNotConstant, Error,
                              "argument for generic parameter is not a "
                              "compile-time constant");
            auto diag =
                context().emitter().Build(loc_id_, CompTimeArgumentNotConstant);
            NoteInitializingParam(param_id, diag);
            diag.Emit();
          }
          return false;
        }

        result_arg_ids_[index.index] = arg_const_inst_id;
        // This parameter index should not be deduced if it appears later.
        non_deduced_indexes_[index.index - first_deduced_index_.index] = true;
        continue;
      }

      // Deducing a symbolic binding appearing within an expression against a
      // constant value deduces the binding as having that value. For example,
      // deducing `[T:! type](x: T)` against `("foo")` deduces `T` as `String`.
      case CARBON_KIND(SemIR::BindSymbolicName bind): {
        auto& entity_name = context().entity_names().Get(bind.entity_name_id);
        auto index = entity_name.bind_index;
        if (!index.has_value() || index < first_deduced_index_ ||
            non_deduced_indexes_[index.index - first_deduced_index_.index]) {
          break;
        }

        CARBON_CHECK(static_cast<size_t>(index.index) < result_arg_ids_.size(),
                     "Deduced value for unexpected index {0}; expected to "
                     "deduce {1} arguments.",
                     index, result_arg_ids_.size());
        auto arg_const_inst_id =
            context().constant_values().GetConstantInstId(arg_id);
        if (arg_const_inst_id.has_value()) {
          if (result_arg_ids_[index.index].has_value() &&
              result_arg_ids_[index.index] != arg_const_inst_id) {
            if (diagnose_) {
              // TODO: Include the two different deduced values.
              CARBON_DIAGNOSTIC(DeductionInconsistent, Error,
                                "inconsistent deductions for value of generic "
                                "parameter `{0}`",
                                SemIR::NameId);
              auto diag = context().emitter().Build(
                  loc_id_, DeductionInconsistent, entity_name.name_id);
              NoteGenericHere(context(), generic_id_, diag);
              diag.Emit();
            }
            return false;
          }
          result_arg_ids_[index.index] = arg_const_inst_id;
        }
        continue;
      }

      case CARBON_KIND(SemIR::ValueParamPattern pattern): {
        Add(pattern.subpattern_id, arg_id, needs_substitution);
        continue;
      }

      case SemIR::StructValue::Kind:
        // TODO: Match field name order between param and arg.
        break;

      case SemIR::FacetAccessType::Kind:
        // Given `fn F[G:! Interface](g: G)`, the type of `g` is `G as type`.
        // `G` is a symbolic binding, whose type is a facet type, but `G as
        // type` converts into a `FacetAccessType`.
        //
        // When we see a `FacetAccessType` parameter here, we want to deduce the
        // facet type of `G`, not `G as type`, for the argument (so that the
        // argument would be a facet value, whose type is the same facet type of
        // `G`. So here we "undo" the `as type` operation that's built into the
        // `g` parameter's type.
        Add(param_inst.As<SemIR::FacetAccessType>().facet_value_inst_id, arg_id,
            needs_substitution);
        continue;

        // TODO: Handle more cases.

      default:
        if (param_inst.kind().deduce_through()) {
          // Various kinds of parameter should match an argument of the same
          // form, if the operands all match.
          auto arg_inst = context().insts().Get(arg_id);
          if (arg_inst.kind() != param_inst.kind()) {
            break;
          }
          auto [kind0, kind1] = param_inst.ArgKinds();
          worklist_.AddInstArg(kind0, param_inst.arg0(), arg_inst.arg0(),
                               needs_substitution);
          worklist_.AddInstArg(kind1, param_inst.arg1(), arg_inst.arg1(),
                               needs_substitution);
          continue;
        }
        break;
    }

    // We didn't manage to deduce against the syntactic form of the parameter.
    // Convert it to a canonical constant value and try deducing against that.
    auto param_const_id = context().constant_values().Get(param_id);
    if (!param_const_id.has_value() || !param_const_id.is_symbolic()) {
      // It's not a symbolic constant. There's nothing here to deduce.
      continue;
    }
    auto param_const_inst_id =
        context().constant_values().GetInstId(param_const_id);
    if (param_const_inst_id != param_id) {
      Add(param_const_inst_id, arg_id, needs_substitution);
      continue;
    }

    // If we've not yet substituted into the parameter, do so now and try again.
    if (needs_substitution) {
      param_const_id = SubstConstant(context(), param_const_id, substitutions_);
      if (!param_const_id.has_value() || !param_const_id.is_symbolic()) {
        continue;
      }
      Add(context().constant_values().GetInstId(param_const_id), arg_id,
          /*needs_substitution=*/false);
    }
  }

  return true;
}

auto DeductionContext::CheckDeductionIsComplete() -> bool {
  // Check we deduced an argument value for every parameter, and convert each
  // argument to match the final parameter type after substituting any deduced
  // types it depends on.
  for (auto&& [i, deduced_arg_id] :
       llvm::enumerate(llvm::MutableArrayRef(result_arg_ids_)
                           .drop_front(first_deduced_index_.index))) {
    auto binding_index = first_deduced_index_.index + i;
    auto binding_id = context().inst_blocks().Get(
        context().generics().Get(generic_id_).bindings_id)[binding_index];
    if (!deduced_arg_id.has_value()) {
      if (diagnose_) {
        auto entity_name_id = context()
                                  .insts()
                                  .GetAs<SemIR::AnyBindName>(binding_id)
                                  .entity_name_id;
        CARBON_DIAGNOSTIC(DeductionIncomplete, Error,
                          "cannot deduce value for generic parameter `{0}`",
                          SemIR::NameId);
        auto diag = context().emitter().Build(
            loc_id_, DeductionIncomplete,
            context().entity_names().Get(entity_name_id).name_id);
        NoteGenericHere(context(), generic_id_, diag);
        diag.Emit();
      }
      return false;
    }

    // If the binding is symbolic it can refer to other earlier bindings in the
    // same generic, or from an enclosing specific. Substitute to replace those
    // and get a non-symbolic type in order for us to know the final type that
    // the argument needs to be converted to.
    //
    // Note that when typechecking a checked generic, the arguments can
    // still be symbolic, so the substitution would also be symbolic. We are
    // unable to get the final type for symbolic bindings until deducing with
    // non-symbolic arguments.
    //
    // TODO: If arguments of different values, but that _convert to_ the same
    // value, are deduced for the same symbolic binding, then we will fail
    // typechecking in Deduce() with conflicting types via the
    // `DeductionInconsistent` diagnostic. If we defer that check until after
    // all conversions are done (after the code below) then we won't diagnose
    // that incorrectly.
    auto arg_type_id = context().insts().Get(deduced_arg_id).type_id();
    auto binding_type_id = context().insts().Get(binding_id).type_id();
    if (!arg_type_id.AsConstantId().is_symbolic() &&
        binding_type_id.AsConstantId().is_symbolic()) {
      auto param_type_const_id = SubstConstant(
          context(), binding_type_id.AsConstantId(), substitutions_);
      CARBON_CHECK(param_type_const_id.has_value());
      binding_type_id = context().GetTypeIdForTypeConstant(param_type_const_id);

      // TODO: Suppress diagnostics here if `diagnose_` is false.
      DiagnosticAnnotationScope annotate_diagnostics(
          &context().emitter(),
          [&](auto& builder) { NoteInitializingParam(binding_id, builder); });
      auto converted_arg_id = ConvertToValueOfType(
          context(), loc_id_, deduced_arg_id, binding_type_id);
      // Replace the deduced arg with its value converted to the parameter
      // type. The conversion of the argument type must produce a constant value
      // to be used in deduction.
      if (context().constant_values().Get(converted_arg_id).is_constant()) {
        deduced_arg_id = converted_arg_id;
      } else {
        CARBON_DIAGNOSTIC(RuntimeConversionDuringCompTimeDeduction, Error,
                          "compile-time value requires runtime conversion, "
                          "constructing value of type {0}",
                          SemIR::TypeId);
        auto diag = context().emitter().Build(
            loc_id_, RuntimeConversionDuringCompTimeDeduction, binding_type_id);
        NoteGenericHere(context(), generic_id_, diag);
        diag.Emit();
        deduced_arg_id = SemIR::ErrorInst::SingletonInstId;
      }
    }

    substitutions_.push_back(
        {.bind_id = SemIR::CompileTimeBindIndex(binding_index),
         .replacement_id = context().constant_values().Get(deduced_arg_id)});
  }
  return true;
}

auto DeductionContext::MakeSpecific() -> SemIR::SpecificId {
  // TODO: Convert the deduced values to the types of the bindings.

  return Check::MakeSpecific(context(), loc_id_, generic_id_, result_arg_ids_);
}

auto DeduceGenericCallArguments(
    Context& context, SemIR::LocId loc_id, SemIR::GenericId generic_id,
    SemIR::SpecificId enclosing_specific_id, SemIR::InstId self_type_id,
    [[maybe_unused]] SemIR::InstBlockId implicit_params_id,
    SemIR::InstBlockId params_id, [[maybe_unused]] SemIR::InstId self_id,
    llvm::ArrayRef<SemIR::InstId> arg_ids) -> SemIR::SpecificId {
  DeductionContext deduction(context, loc_id, generic_id, enclosing_specific_id,
                             self_type_id, /*diagnose=*/true);

  // Prepare to perform deduction of the explicit parameters against their
  // arguments.
  // TODO: Also perform deduction for type of self.
  deduction.AddAll(params_id, arg_ids, /*needs_substitution=*/true);

  if (!deduction.Deduce() || !deduction.CheckDeductionIsComplete()) {
    return SemIR::SpecificId::None;
  }

  return deduction.MakeSpecific();
}

// Deduces the impl arguments to use in a use of a parameterized impl. Returns
// `None` if deduction fails.
auto DeduceImplArguments(Context& context, SemIR::LocId loc_id,
                         const SemIR::Impl& impl, SemIR::ConstantId self_id,
                         SemIR::ConstantId constraint_id) -> SemIR::SpecificId {
  DeductionContext deduction(context, loc_id, impl.generic_id,
                             /*enclosing_specific_id=*/SemIR::SpecificId::None,
                             /*self_type_id=*/SemIR::InstId::None,
                             /*diagnose=*/false);

  // Prepare to perform deduction of the type and interface.
  deduction.Add(impl.self_id, context.constant_values().GetInstId(self_id),
                /*needs_substitution=*/false);
  deduction.Add(impl.constraint_id,
                context.constant_values().GetInstId(constraint_id),
                /*needs_substitution=*/false);

  if (!deduction.Deduce() || !deduction.CheckDeductionIsComplete()) {
    return SemIR::SpecificId::None;
  }

  return deduction.MakeSpecific();
}

}  // namespace Carbon::Check