carbon-lang/executable_semantics/interpreter/typecheck.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 "executable_semantics/interpreter/typecheck.h"

#include <algorithm>
#include <iterator>
#include <map>
#include <set>
#include <vector>

#include "executable_semantics/ast/function_definition.h"
#include "executable_semantics/interpreter/interpreter.h"
#include "executable_semantics/tracing_flag.h"

namespace Carbon {

void ExpectType(int line_num, const std::string& context, const Value* expected,
                const Value* actual) {
  if (!TypeEqual(expected, actual)) {
    llvm::errs() << line_num << ": type error in " << context << "\n"
                 << "expected: " << *expected << "\n"
                 << "actual: " << *actual << "\n";
    exit(-1);
  }
}

void ExpectPointerType(int line_num, const std::string& context,
                       const Value* actual) {
  if (actual->tag() != ValKind::PointerType) {
    llvm::errs() << line_num << ": type error in " << context << "\n"
                 << "expected a pointer type\n"
                 << "actual: " << *actual << "\n";
    exit(-1);
  }
}

// Reify type to type expression.
auto ReifyType(const Value* t, int line_num) -> const Expression* {
  switch (t->tag()) {
    case ValKind::IntType:
      return Expression::MakeIntTypeLiteral(0);
    case ValKind::BoolType:
      return Expression::MakeBoolTypeLiteral(0);
    case ValKind::TypeType:
      return Expression::MakeTypeTypeLiteral(0);
    case ValKind::ContinuationType:
      return Expression::MakeContinuationTypeLiteral(0);
    case ValKind::FunctionType:
      return Expression::MakeFunctionTypeLiteral(
          0, ReifyType(t->GetFunctionType().param, line_num),
          ReifyType(t->GetFunctionType().ret, line_num));
    case ValKind::TupleValue: {
      std::vector<FieldInitializer> args;
      for (const TupleElement& field : t->GetTupleValue().elements) {
        args.push_back({.name = field.name,
                        .expression = ReifyType(field.value, line_num)});
      }
      return Expression::MakeTupleLiteral(0, args);
    }
    case ValKind::StructType:
      return Expression::MakeIdentifierExpression(0, t->GetStructType().name);
    case ValKind::ChoiceType:
      return Expression::MakeIdentifierExpression(0, t->GetChoiceType().name);
    case ValKind::PointerType:
      return Expression::MakePrimitiveOperatorExpression(
          0, Operator::Ptr, {ReifyType(t->GetPointerType().type, line_num)});
    default:
      llvm::errs() << line_num << ": expected a type, not " << *t << "\n";
      exit(-1);
  }
}

// The TypeCheckExp function performs semantic analysis on an expression.
// It returns a new version of the expression, its type, and an
// updated environment which are bundled into a TCResult object.
// The purpose of the updated environment is
// to bring pattern variables into scope, for example, in a match case.
// The new version of the expression may include more information,
// for example, the type arguments deduced for the type parameters of a
// generic.
//
// e is the expression to be analyzed.
// types maps variable names to the type of their run-time value.
// values maps variable names to their compile-time values. It is not
//    directly used in this function but is passed to InterExp.
// expected is the type that this expression is expected to have.
//    This parameter is non-null when the expression is in a pattern context
//    and it is used to implement `auto`, otherwise it is null.
// context says what kind of position this expression is nested in,
//    whether it's a position that expects a value, a pattern, or a type.
auto TypeCheckExp(const Expression* e, TypeEnv types, Env values,
                  const Value* expected, TCContext context) -> TCResult {
  if (tracing_output) {
    switch (context) {
      case TCContext::ValueContext:
        llvm::outs() << "checking expression ";
        break;
      case TCContext::PatternContext:
        llvm::outs() << "checking pattern, ";
        if (expected) {
          llvm::outs() << "expecting " << *expected;
        }
        llvm::outs() << ", ";
        break;
      case TCContext::TypeContext:
        llvm::outs() << "checking type ";
        break;
    }
    llvm::outs() << *e << "\n";
  }
  switch (e->tag()) {
    case ExpressionKind::BindingExpression: {
      if (context != TCContext::PatternContext) {
        llvm::errs()
            << e->line_num
            << ": compilation error, pattern variables are only allowed in "
               "pattern context"
            << "\n";
        exit(-1);
      }
      auto t = InterpExp(values, e->GetBindingExpression().type);
      if (t->tag() == ValKind::AutoType) {
        if (expected == nullptr) {
          llvm::errs() << e->line_num
                       << ": compilation error, auto not allowed here"
                       << "\n";
          exit(-1);
        } else {
          t = expected;
        }
      } else if (expected) {
        ExpectType(e->line_num, "pattern variable", t, expected);
      }
      const std::optional<std::string>& name = e->GetBindingExpression().name;
      auto new_e = Expression::MakeBindingExpression(e->line_num, name,
                                                     ReifyType(t, e->line_num));
      if (name.has_value()) {
        types.Set(*name, t);
      }
      return TCResult(new_e, t, types);
    }
    case ExpressionKind::IndexExpression: {
      auto res = TypeCheckExp(e->GetIndexExpression().aggregate, types, values,
                              nullptr, TCContext::ValueContext);
      auto t = res.type;
      switch (t->tag()) {
        case ValKind::TupleValue: {
          auto i =
              InterpExp(values, e->GetIndexExpression().offset)->GetIntValue();
          std::string f = std::to_string(i);
          const Value* field_t = t->GetTupleValue().FindField(f);
          if (field_t == nullptr) {
            llvm::errs() << e->line_num << ": compilation error, field " << f
                         << " is not in the tuple " << *t << "\n";
            exit(-1);
          }
          auto new_e = Expression::MakeIndexExpression(
              e->line_num, res.exp, Expression::MakeIntLiteral(e->line_num, i));
          return TCResult(new_e, field_t, res.types);
        }
        default:
          llvm::errs() << e->line_num
                       << ": compilation error, expected a tuple\n";
          exit(-1);
      }
    }
    case ExpressionKind::TupleLiteral: {
      std::vector<FieldInitializer> new_args;
      std::vector<TupleElement> arg_types;
      auto new_types = types;
      if (expected && expected->tag() != ValKind::TupleValue) {
        llvm::errs() << e->line_num
                     << ": compilation error, didn't expect a tuple\n";
        exit(-1);
      }
      if (expected && e->GetTupleLiteral().fields.size() !=
                          expected->GetTupleValue().elements.size()) {
        llvm::errs() << e->line_num
                     << ": compilation error, tuples of different length\n";
        exit(-1);
      }
      int i = 0;
      for (auto arg = e->GetTupleLiteral().fields.begin();
           arg != e->GetTupleLiteral().fields.end(); ++arg, ++i) {
        const Value* arg_expected = nullptr;
        if (expected && expected->tag() == ValKind::TupleValue) {
          if (expected->GetTupleValue().elements[i].name != arg->name) {
            llvm::errs()
                << e->line_num
                << ": compilation error, field names do not match, expected "
                << expected->GetTupleValue().elements[i].name << " but got "
                << arg->name << "\n";
            exit(-1);
          }
          arg_expected = expected->GetTupleValue().elements[i].value;
        }
        auto arg_res = TypeCheckExp(arg->expression, new_types, values,
                                    arg_expected, context);
        new_types = arg_res.types;
        new_args.push_back({.name = arg->name, .expression = arg_res.exp});
        arg_types.push_back({.name = arg->name, .value = arg_res.type});
      }
      auto tuple_e = Expression::MakeTupleLiteral(e->line_num, new_args);
      auto tuple_t = Value::MakeTupleValue(std::move(arg_types));
      return TCResult(tuple_e, tuple_t, new_types);
    }
    case ExpressionKind::FieldAccessExpression: {
      auto res = TypeCheckExp(e->GetFieldAccessExpression().aggregate, types,
                              values, nullptr, TCContext::ValueContext);
      auto t = res.type;
      switch (t->tag()) {
        case ValKind::StructType:
          // Search for a field
          for (auto& field : t->GetStructType().fields) {
            if (e->GetFieldAccessExpression().field == field.first) {
              const Expression* new_e = Expression::MakeFieldAccessExpression(
                  e->line_num, res.exp, e->GetFieldAccessExpression().field);
              return TCResult(new_e, field.second, res.types);
            }
          }
          // Search for a method
          for (auto& method : t->GetStructType().methods) {
            if (e->GetFieldAccessExpression().field == method.first) {
              const Expression* new_e = Expression::MakeFieldAccessExpression(
                  e->line_num, res.exp, e->GetFieldAccessExpression().field);
              return TCResult(new_e, method.second, res.types);
            }
          }
          llvm::errs() << e->line_num << ": compilation error, struct "
                       << t->GetStructType().name
                       << " does not have a field named "
                       << e->GetFieldAccessExpression().field << "\n";
          exit(-1);
        case ValKind::TupleValue:
          for (const TupleElement& field : t->GetTupleValue().elements) {
            if (e->GetFieldAccessExpression().field == field.name) {
              auto new_e = Expression::MakeFieldAccessExpression(
                  e->line_num, res.exp, e->GetFieldAccessExpression().field);
              return TCResult(new_e, field.value, res.types);
            }
          }
          llvm::errs() << e->line_num << ": compilation error, struct "
                       << t->GetStructType().name
                       << " does not have a field named "
                       << e->GetFieldAccessExpression().field << "\n";
          exit(-1);
        case ValKind::ChoiceType:
          for (auto vt = t->GetChoiceType().alternatives.begin();
               vt != t->GetChoiceType().alternatives.end(); ++vt) {
            if (e->GetFieldAccessExpression().field == vt->first) {
              const Expression* new_e = Expression::MakeFieldAccessExpression(
                  e->line_num, res.exp, e->GetFieldAccessExpression().field);
              auto fun_ty = Value::MakeFunctionType(vt->second, t);
              return TCResult(new_e, fun_ty, res.types);
            }
          }
          llvm::errs() << e->line_num << ": compilation error, struct "
                       << t->GetStructType().name
                       << " does not have a field named "
                       << e->GetFieldAccessExpression().field << "\n";
          exit(-1);

        default:
          llvm::errs()
              << e->line_num
              << ": compilation error in field access, expected a struct\n"
              << *e << "\n";
          exit(-1);
      }
    }
    case ExpressionKind::IdentifierExpression: {
      std::optional<const Value*> type =
          types.Get(e->GetIdentifierExpression().name);
      if (type) {
        return TCResult(e, *type, types);
      } else {
        llvm::errs() << e->line_num << ": could not find `"
                     << e->GetIdentifierExpression().name << "`\n";
        exit(-1);
      }
    }
    case ExpressionKind::IntLiteral:
      return TCResult(e, Value::MakeIntType(), types);
    case ExpressionKind::BoolLiteral:
      return TCResult(e, Value::MakeBoolType(), types);
    case ExpressionKind::PrimitiveOperatorExpression: {
      std::vector<const Expression*> es;
      std::vector<const Value*> ts;
      auto new_types = types;
      for (const Expression* argument :
           e->GetPrimitiveOperatorExpression().arguments) {
        auto res = TypeCheckExp(argument, types, values, nullptr,
                                TCContext::ValueContext);
        new_types = res.types;
        es.push_back(res.exp);
        ts.push_back(res.type);
      }
      auto new_e = Expression::MakePrimitiveOperatorExpression(
          e->line_num, e->GetPrimitiveOperatorExpression().op, es);
      switch (e->GetPrimitiveOperatorExpression().op) {
        case Operator::Neg:
          ExpectType(e->line_num, "negation", Value::MakeIntType(), ts[0]);
          return TCResult(new_e, Value::MakeIntType(), new_types);
        case Operator::Add:
          ExpectType(e->line_num, "addition(1)", Value::MakeIntType(), ts[0]);
          ExpectType(e->line_num, "addition(2)", Value::MakeIntType(), ts[1]);
          return TCResult(new_e, Value::MakeIntType(), new_types);
        case Operator::Sub:
          ExpectType(e->line_num, "subtraction(1)", Value::MakeIntType(),
                     ts[0]);
          ExpectType(e->line_num, "subtraction(2)", Value::MakeIntType(),
                     ts[1]);
          return TCResult(new_e, Value::MakeIntType(), new_types);
        case Operator::Mul:
          ExpectType(e->line_num, "multiplication(1)", Value::MakeIntType(),
                     ts[0]);
          ExpectType(e->line_num, "multiplication(2)", Value::MakeIntType(),
                     ts[1]);
          return TCResult(new_e, Value::MakeIntType(), new_types);
        case Operator::And:
          ExpectType(e->line_num, "&&(1)", Value::MakeBoolType(), ts[0]);
          ExpectType(e->line_num, "&&(2)", Value::MakeBoolType(), ts[1]);
          return TCResult(new_e, Value::MakeBoolType(), new_types);
        case Operator::Or:
          ExpectType(e->line_num, "||(1)", Value::MakeBoolType(), ts[0]);
          ExpectType(e->line_num, "||(2)", Value::MakeBoolType(), ts[1]);
          return TCResult(new_e, Value::MakeBoolType(), new_types);
        case Operator::Not:
          ExpectType(e->line_num, "!", Value::MakeBoolType(), ts[0]);
          return TCResult(new_e, Value::MakeBoolType(), new_types);
        case Operator::Eq:
          ExpectType(e->line_num, "==", ts[0], ts[1]);
          return TCResult(new_e, Value::MakeBoolType(), new_types);
        case Operator::Deref:
          ExpectPointerType(e->line_num, "*", ts[0]);
          return TCResult(new_e, ts[0]->GetPointerType().type, new_types);
        case Operator::Ptr:
          ExpectType(e->line_num, "*", Value::MakeTypeType(), ts[0]);
          return TCResult(new_e, Value::MakeTypeType(), new_types);
      }
      break;
    }
    case ExpressionKind::CallExpression: {
      auto fun_res = TypeCheckExp(e->GetCallExpression().function, types,
                                  values, nullptr, TCContext::ValueContext);
      switch (fun_res.type->tag()) {
        case ValKind::FunctionType: {
          auto fun_t = fun_res.type;
          auto arg_res =
              TypeCheckExp(e->GetCallExpression().argument, fun_res.types,
                           values, fun_t->GetFunctionType().param, context);
          ExpectType(e->line_num, "call", fun_t->GetFunctionType().param,
                     arg_res.type);
          auto new_e = Expression::MakeCallExpression(e->line_num, fun_res.exp,
                                                      arg_res.exp);
          return TCResult(new_e, fun_t->GetFunctionType().ret, arg_res.types);
        }
        default: {
          llvm::errs() << e->line_num
                       << ": compilation error in call, expected a function\n"
                       << *e << "\n";
          exit(-1);
        }
      }
      break;
    }
    case ExpressionKind::FunctionTypeLiteral: {
      switch (context) {
        case TCContext::ValueContext:
        case TCContext::TypeContext: {
          auto pt = InterpExp(values, e->GetFunctionTypeLiteral().parameter);
          auto rt = InterpExp(values, e->GetFunctionTypeLiteral().return_type);
          auto new_e = Expression::MakeFunctionTypeLiteral(
              e->line_num, ReifyType(pt, e->line_num),
              ReifyType(rt, e->line_num));
          return TCResult(new_e, Value::MakeTypeType(), types);
        }
        case TCContext::PatternContext: {
          auto param_res = TypeCheckExp(e->GetFunctionTypeLiteral().parameter,
                                        types, values, nullptr, context);
          auto ret_res =
              TypeCheckExp(e->GetFunctionTypeLiteral().return_type,
                           param_res.types, values, nullptr, context);
          auto new_e = Expression::MakeFunctionTypeLiteral(
              e->line_num, ReifyType(param_res.type, e->line_num),
              ReifyType(ret_res.type, e->line_num));
          return TCResult(new_e, Value::MakeTypeType(), ret_res.types);
        }
      }
    }
    case ExpressionKind::IntTypeLiteral:
      return TCResult(e, Value::MakeTypeType(), types);
    case ExpressionKind::BoolTypeLiteral:
      return TCResult(e, Value::MakeTypeType(), types);
    case ExpressionKind::TypeTypeLiteral:
      return TCResult(e, Value::MakeTypeType(), types);
    case ExpressionKind::AutoTypeLiteral:
      return TCResult(e, Value::MakeTypeType(), types);
    case ExpressionKind::ContinuationTypeLiteral:
      return TCResult(e, Value::MakeTypeType(), types);
  }
}

auto TypecheckCase(const Value* expected, const Expression* pat,
                   const Statement* body, TypeEnv types, Env values,
                   const Value*& ret_type)
    -> std::pair<const Expression*, const Statement*> {
  auto pat_res =
      TypeCheckExp(pat, types, values, expected, TCContext::PatternContext);
  auto res = TypeCheckStmt(body, pat_res.types, values, ret_type);
  return std::make_pair(pat, res.stmt);
}

// The TypeCheckStmt function performs semantic analysis on a statement.
// It returns a new version of the statement and a new type environment.
//
// The ret_type parameter is used for analyzing return statements.
// It is the declared return type of the enclosing function definition.
// If the return type is "auto", then the return type is inferred from
// the first return statement.
auto TypeCheckStmt(const Statement* s, TypeEnv types, Env values,
                   const Value*& ret_type) -> TCStatement {
  if (!s) {
    return TCStatement(s, types);
  }
  switch (s->tag()) {
    case StatementKind::Match: {
      auto res = TypeCheckExp(s->GetMatch().exp, types, values, nullptr,
                              TCContext::ValueContext);
      auto res_type = res.type;
      auto new_clauses =
          new std::list<std::pair<const Expression*, const Statement*>>();
      for (auto& clause : *s->GetMatch().clauses) {
        new_clauses->push_back(TypecheckCase(
            res_type, clause.first, clause.second, types, values, ret_type));
      }
      const Statement* new_s =
          Statement::MakeMatch(s->line_num, res.exp, new_clauses);
      return TCStatement(new_s, types);
    }
    case StatementKind::While: {
      auto cnd_res = TypeCheckExp(s->GetWhile().cond, types, values, nullptr,
                                  TCContext::ValueContext);
      ExpectType(s->line_num, "condition of `while`", Value::MakeBoolType(),
                 cnd_res.type);
      auto body_res =
          TypeCheckStmt(s->GetWhile().body, types, values, ret_type);
      auto new_s =
          Statement::MakeWhile(s->line_num, cnd_res.exp, body_res.stmt);
      return TCStatement(new_s, types);
    }
    case StatementKind::Break:
    case StatementKind::Continue:
      return TCStatement(s, types);
    case StatementKind::Block: {
      auto stmt_res =
          TypeCheckStmt(s->GetBlock().stmt, types, values, ret_type);
      return TCStatement(Statement::MakeBlock(s->line_num, stmt_res.stmt),
                         types);
    }
    case StatementKind::VariableDefinition: {
      auto res = TypeCheckExp(s->GetVariableDefinition().init, types, values,
                              nullptr, TCContext::ValueContext);
      const Value* rhs_ty = res.type;
      auto lhs_res = TypeCheckExp(s->GetVariableDefinition().pat, types, values,
                                  rhs_ty, TCContext::PatternContext);
      const Statement* new_s = Statement::MakeVariableDefinition(
          s->line_num, s->GetVariableDefinition().pat, res.exp);
      return TCStatement(new_s, lhs_res.types);
    }
    case StatementKind::Sequence: {
      auto stmt_res =
          TypeCheckStmt(s->GetSequence().stmt, types, values, ret_type);
      auto types2 = stmt_res.types;
      auto next_res =
          TypeCheckStmt(s->GetSequence().next, types2, values, ret_type);
      auto types3 = next_res.types;
      return TCStatement(
          Statement::MakeSequence(s->line_num, stmt_res.stmt, next_res.stmt),
          types3);
    }
    case StatementKind::Assign: {
      auto rhs_res = TypeCheckExp(s->GetAssign().rhs, types, values, nullptr,
                                  TCContext::ValueContext);
      auto rhs_t = rhs_res.type;
      auto lhs_res = TypeCheckExp(s->GetAssign().lhs, types, values, rhs_t,
                                  TCContext::ValueContext);
      auto lhs_t = lhs_res.type;
      ExpectType(s->line_num, "assign", lhs_t, rhs_t);
      auto new_s = Statement::MakeAssign(s->line_num, lhs_res.exp, rhs_res.exp);
      return TCStatement(new_s, lhs_res.types);
    }
    case StatementKind::ExpressionStatement: {
      auto res = TypeCheckExp(s->GetExpressionStatement().exp, types, values,
                              nullptr, TCContext::ValueContext);
      auto new_s = Statement::MakeExpressionStatement(s->line_num, res.exp);
      return TCStatement(new_s, types);
    }
    case StatementKind::If: {
      auto cnd_res = TypeCheckExp(s->GetIf().cond, types, values, nullptr,
                                  TCContext::ValueContext);
      ExpectType(s->line_num, "condition of `if`", Value::MakeBoolType(),
                 cnd_res.type);
      auto thn_res =
          TypeCheckStmt(s->GetIf().then_stmt, types, values, ret_type);
      auto els_res =
          TypeCheckStmt(s->GetIf().else_stmt, types, values, ret_type);
      auto new_s = Statement::MakeIf(s->line_num, cnd_res.exp, thn_res.stmt,
                                     els_res.stmt);
      return TCStatement(new_s, types);
    }
    case StatementKind::Return: {
      auto res = TypeCheckExp(s->GetReturn().exp, types, values, nullptr,
                              TCContext::ValueContext);
      if (ret_type->tag() == ValKind::AutoType) {
        // The following infers the return type from the first 'return'
        // statement. This will get more difficult with subtyping, when we
        // should infer the least-upper bound of all the 'return' statements.
        ret_type = res.type;
      } else {
        ExpectType(s->line_num, "return", ret_type, res.type);
      }
      return TCStatement(Statement::MakeReturn(s->line_num, res.exp), types);
    }
    case StatementKind::Continuation: {
      TCStatement body_result =
          TypeCheckStmt(s->GetContinuation().body, types, values, ret_type);
      const Statement* new_continuation = Statement::MakeContinuation(
          s->line_num, s->GetContinuation().continuation_variable,
          body_result.stmt);
      types.Set(s->GetContinuation().continuation_variable,
                Value::MakeContinuationType());
      return TCStatement(new_continuation, types);
    }
    case StatementKind::Run: {
      TCResult argument_result =
          TypeCheckExp(s->GetRun().argument, types, values, nullptr,
                       TCContext::ValueContext);
      ExpectType(s->line_num, "argument of `run`",
                 Value::MakeContinuationType(), argument_result.type);
      const Statement* new_run =
          Statement::MakeRun(s->line_num, argument_result.exp);
      return TCStatement(new_run, types);
    }
    case StatementKind::Await: {
      // nothing to do here
      return TCStatement(s, types);
    }
  }  // switch
}

auto CheckOrEnsureReturn(const Statement* stmt, bool void_return, int line_num)
    -> const Statement* {
  if (!stmt) {
    if (void_return) {
      return Statement::MakeReturn(line_num,
                                   Expression::MakeTupleLiteral(line_num, {}));
    } else {
      llvm::errs()
          << "control-flow reaches end of non-void function without a return\n";
      exit(-1);
    }
  }
  switch (stmt->tag()) {
    case StatementKind::Match: {
      auto new_clauses =
          new std::list<std::pair<const Expression*, const Statement*>>();
      for (auto i = stmt->GetMatch().clauses->begin();
           i != stmt->GetMatch().clauses->end(); ++i) {
        auto s = CheckOrEnsureReturn(i->second, void_return, stmt->line_num);
        new_clauses->push_back(std::make_pair(i->first, s));
      }
      return Statement::MakeMatch(stmt->line_num, stmt->GetMatch().exp,
                                  new_clauses);
    }
    case StatementKind::Block:
      return Statement::MakeBlock(
          stmt->line_num, CheckOrEnsureReturn(stmt->GetBlock().stmt,
                                              void_return, stmt->line_num));
    case StatementKind::If:
      return Statement::MakeIf(
          stmt->line_num, stmt->GetIf().cond,
          CheckOrEnsureReturn(stmt->GetIf().then_stmt, void_return,
                              stmt->line_num),
          CheckOrEnsureReturn(stmt->GetIf().else_stmt, void_return,
                              stmt->line_num));
    case StatementKind::Return:
      return stmt;
    case StatementKind::Sequence:
      if (stmt->GetSequence().next) {
        return Statement::MakeSequence(
            stmt->line_num, stmt->GetSequence().stmt,
            CheckOrEnsureReturn(stmt->GetSequence().next, void_return,
                                stmt->line_num));
      } else {
        return CheckOrEnsureReturn(stmt->GetSequence().stmt, void_return,
                                   stmt->line_num);
      }
    case StatementKind::Continuation:
    case StatementKind::Run:
    case StatementKind::Await:
      return stmt;
    case StatementKind::Assign:
    case StatementKind::ExpressionStatement:
    case StatementKind::While:
    case StatementKind::Break:
    case StatementKind::Continue:
    case StatementKind::VariableDefinition:
      if (void_return) {
        return Statement::MakeSequence(
            stmt->line_num, stmt,
            Statement::MakeReturn(stmt->line_num, Expression::MakeTupleLiteral(
                                                      stmt->line_num, {})));
      } else {
        llvm::errs()
            << stmt->line_num
            << ": control-flow reaches end of non-void function without a "
               "return\n";
        exit(-1);
      }
  }
}

auto TypeCheckFunDef(const FunctionDefinition* f, TypeEnv types, Env values)
    -> struct FunctionDefinition* {
  auto param_res = TypeCheckExp(f->param_pattern, types, values, nullptr,
                                TCContext::PatternContext);
  auto return_type = InterpExp(values, f->return_type);
  if (f->name == "main") {
    ExpectType(f->line_num, "return type of `main`", Value::MakeIntType(),
               return_type);
    // TODO: Check that main doesn't have any parameters.
  }
  auto res = TypeCheckStmt(f->body, param_res.types, values, return_type);
  bool void_return = TypeEqual(return_type, Value::MakeUnitTypeVal());
  auto body = CheckOrEnsureReturn(res.stmt, void_return, f->line_num);
  return new FunctionDefinition(f->line_num, f->name, f->param_pattern,
                                ReifyType(return_type, f->line_num), body);
}

auto TypeOfFunDef(TypeEnv types, Env values, const FunctionDefinition* fun_def)
    -> const Value* {
  auto param_res = TypeCheckExp(fun_def->param_pattern, types, values, nullptr,
                                TCContext::PatternContext);
  auto ret = InterpExp(values, fun_def->return_type);
  if (ret->tag() == ValKind::AutoType) {
    auto f = TypeCheckFunDef(fun_def, types, values);
    ret = InterpExp(values, f->return_type);
  }
  return Value::MakeFunctionType(param_res.type, ret);
}

auto TypeOfStructDef(const StructDefinition* sd, TypeEnv /*types*/, Env ct_top)
    -> const Value* {
  VarValues fields;
  VarValues methods;
  for (const Member* m : sd->members) {
    switch (m->tag()) {
      case MemberKind::FieldMember:
        auto t = InterpExp(ct_top, m->GetFieldMember().type);
        fields.push_back(std::make_pair(m->GetFieldMember().name, t));
        break;
    }
  }
  return Value::MakeStructType(sd->name, std::move(fields), std::move(methods));
}

static auto GetName(const Declaration& d) -> const std::string& {
  switch (d.tag()) {
    case DeclarationKind::FunctionDeclaration:
      return d.GetFunctionDeclaration().definition.name;
    case DeclarationKind::StructDeclaration:
      return d.GetStructDeclaration().definition.name;
    case DeclarationKind::ChoiceDeclaration:
      return d.GetChoiceDeclaration().name;
    case DeclarationKind::VariableDeclaration:
      return d.GetVariableDeclaration().name;
  }
}

auto MakeTypeChecked(const Declaration& d, const TypeEnv& types,
                     const Env& values) -> Declaration {
  switch (d.tag()) {
    case DeclarationKind::FunctionDeclaration:
      return Declaration::MakeFunctionDeclaration(*TypeCheckFunDef(
          &d.GetFunctionDeclaration().definition, types, values));

    case DeclarationKind::StructDeclaration: {
      const StructDefinition& struct_def = d.GetStructDeclaration().definition;
      std::list<Member*> fields;
      for (Member* m : struct_def.members) {
        switch (m->tag()) {
          case MemberKind::FieldMember:
            // TODO: Interpret the type expression and store the result.
            fields.push_back(m);
            break;
        }
      }
      return Declaration::MakeStructDeclaration(
          struct_def.line_num, struct_def.name, std::move(fields));
    }

    case DeclarationKind::ChoiceDeclaration:
      // TODO
      return d;

    case DeclarationKind::VariableDeclaration: {
      const auto& var = d.GetVariableDeclaration();
      // Signals a type error if the initializing expression does not have
      // the declared type of the variable, otherwise returns this
      // declaration with annotated types.
      TCResult type_checked_initializer = TypeCheckExp(
          var.initializer, types, values, nullptr, TCContext::ValueContext);
      const Value* declared_type = InterpExp(values, var.type);
      ExpectType(var.source_location, "initializer of variable", declared_type,
                 type_checked_initializer.type);
      return d;
    }
  }
}

static void TopLevel(const Declaration& d, TypeCheckContext* tops) {
  switch (d.tag()) {
    case DeclarationKind::FunctionDeclaration: {
      const FunctionDefinition& func_def =
          d.GetFunctionDeclaration().definition;
      auto t = TypeOfFunDef(tops->types, tops->values, &func_def);
      tops->types.Set(func_def.name, t);
      InitEnv(d, &tops->values);
      break;
    }

    case DeclarationKind::StructDeclaration: {
      const StructDefinition& struct_def = d.GetStructDeclaration().definition;
      auto st = TypeOfStructDef(&struct_def, tops->types, tops->values);
      Address a = state->heap.AllocateValue(st);
      tops->values.Set(struct_def.name, a);  // Is this obsolete?
      std::vector<TupleElement> field_types;
      for (const auto& [field_name, field_value] : st->GetStructType().fields) {
        field_types.push_back({.name = field_name, .value = field_value});
      }
      auto fun_ty = Value::MakeFunctionType(
          Value::MakeTupleValue(std::move(field_types)), st);
      tops->types.Set(struct_def.name, fun_ty);
      break;
    }

    case DeclarationKind::ChoiceDeclaration: {
      const auto& choice = d.GetChoiceDeclaration();
      VarValues alts;
      for (const auto& [name, signature] : choice.alternatives) {
        auto t = InterpExp(tops->values, signature);
        alts.push_back(std::make_pair(name, t));
      }
      auto ct = Value::MakeChoiceType(choice.name, std::move(alts));
      Address a = state->heap.AllocateValue(ct);
      tops->values.Set(choice.name, a);  // Is this obsolete?
      tops->types.Set(choice.name, ct);
      break;
    }

    case DeclarationKind::VariableDeclaration: {
      const auto& var = d.GetVariableDeclaration();
      // Associate the variable name with it's declared type in the
      // compile-time symbol table.
      const Value* declared_type = InterpExp(tops->values, var.type);
      tops->types.Set(var.name, declared_type);
      break;
    }
  }
}

auto TopLevel(std::list<Declaration>* fs) -> TypeCheckContext {
  TypeCheckContext tops;
  bool found_main = false;

  for (auto const& d : *fs) {
    if (GetName(d) == "main") {
      found_main = true;
    }
    TopLevel(d, &tops);
  }

  if (found_main == false) {
    llvm::errs() << "error, program must contain a function named `main`\n";
    exit(-1);
  }
  return tops;
}

}  // namespace Carbon