rebase: bump the golang-dependencies group with 1 update

Bumps the golang-dependencies group with 1 update: [golang.org/x/crypto](https://github.com/golang/crypto).


Updates `golang.org/x/crypto` from 0.16.0 to 0.17.0
- [Commits](https://github.com/golang/crypto/compare/v0.16.0...v0.17.0)

---
updated-dependencies:
- dependency-name: golang.org/x/crypto
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: golang-dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>

vendor/github.com/google/cel-go/checker/checker.go

@@ -18,15 +18,13 @@ package checker
 
 import (
 	"fmt"
-	"reflect"
 
-	"github.com/google/cel-go/checker/decls"
 	"github.com/google/cel-go/common"
+	"github.com/google/cel-go/common/ast"
 	"github.com/google/cel-go/common/containers"
+	"github.com/google/cel-go/common/decls"
 	"github.com/google/cel-go/common/operators"
-	"github.com/google/cel-go/common/types/ref"
-
-	"google.golang.org/protobuf/proto"
+	"github.com/google/cel-go/common/types"
 
 	exprpb "google.golang.org/genproto/googleapis/api/expr/v1alpha1"
 )
@@ -37,8 +35,8 @@ type checker struct {
 	mappings           *mapping
 	freeTypeVarCounter int
 	sourceInfo         *exprpb.SourceInfo
-	types              map[int64]*exprpb.Type
-	references         map[int64]*exprpb.Reference
+	types              map[int64]*types.Type
+	references         map[int64]*ast.ReferenceInfo
 }
 
 // Check performs type checking, giving a typed AST.
@@ -47,40 +45,38 @@ type checker struct {
 // descriptions of protocol buffers, and a registry for errors.
 // Returns a CheckedExpr proto, which might not be usable if
 // there are errors in the error registry.
-func Check(parsedExpr *exprpb.ParsedExpr,
-	source common.Source,
-	env *Env) (*exprpb.CheckedExpr, *common.Errors) {
+func Check(parsedExpr *exprpb.ParsedExpr, source common.Source, env *Env) (*ast.CheckedAST, *common.Errors) {
+	errs := common.NewErrors(source)
 	c := checker{
 		env:                env,
-		errors:             &typeErrors{common.NewErrors(source)},
+		errors:             &typeErrors{errs: errs},
 		mappings:           newMapping(),
 		freeTypeVarCounter: 0,
 		sourceInfo:         parsedExpr.GetSourceInfo(),
-		types:              make(map[int64]*exprpb.Type),
-		references:         make(map[int64]*exprpb.Reference),
+		types:              make(map[int64]*types.Type),
+		references:         make(map[int64]*ast.ReferenceInfo),
 	}
 	c.check(parsedExpr.GetExpr())
 
 	// Walk over the final type map substituting any type parameters either by their bound value or
 	// by DYN.
-	m := make(map[int64]*exprpb.Type)
-	for k, v := range c.types {
-		m[k] = substitute(c.mappings, v, true)
+	m := make(map[int64]*types.Type)
+	for id, t := range c.types {
+		m[id] = substitute(c.mappings, t, true)
 	}
 
-	return &exprpb.CheckedExpr{
+	return &ast.CheckedAST{
 		Expr:         parsedExpr.GetExpr(),
 		SourceInfo:   parsedExpr.GetSourceInfo(),
 		TypeMap:      m,
 		ReferenceMap: c.references,
-	}, c.errors.Errors
+	}, errs
 }
 
 func (c *checker) check(e *exprpb.Expr) {
 	if e == nil {
 		return
 	}
-
 	switch e.GetExprKind().(type) {
 	case *exprpb.Expr_ConstExpr:
 		literal := e.GetConstExpr()
@@ -113,53 +109,51 @@ func (c *checker) check(e *exprpb.Expr) {
 	case *exprpb.Expr_ComprehensionExpr:
 		c.checkComprehension(e)
 	default:
-		c.errors.ReportError(
-			c.location(e), "Unrecognized ast type: %v", reflect.TypeOf(e))
+		c.errors.unexpectedASTType(e.GetId(), c.location(e), e)
 	}
 }
 
 func (c *checker) checkInt64Literal(e *exprpb.Expr) {
-	c.setType(e, decls.Int)
+	c.setType(e, types.IntType)
 }
 
 func (c *checker) checkUint64Literal(e *exprpb.Expr) {
-	c.setType(e, decls.Uint)
+	c.setType(e, types.UintType)
 }
 
 func (c *checker) checkStringLiteral(e *exprpb.Expr) {
-	c.setType(e, decls.String)
+	c.setType(e, types.StringType)
 }
 
 func (c *checker) checkBytesLiteral(e *exprpb.Expr) {
-	c.setType(e, decls.Bytes)
+	c.setType(e, types.BytesType)
 }
 
 func (c *checker) checkDoubleLiteral(e *exprpb.Expr) {
-	c.setType(e, decls.Double)
+	c.setType(e, types.DoubleType)
 }
 
 func (c *checker) checkBoolLiteral(e *exprpb.Expr) {
-	c.setType(e, decls.Bool)
+	c.setType(e, types.BoolType)
 }
 
 func (c *checker) checkNullLiteral(e *exprpb.Expr) {
-	c.setType(e, decls.Null)
+	c.setType(e, types.NullType)
 }
 
 func (c *checker) checkIdent(e *exprpb.Expr) {
 	identExpr := e.GetIdentExpr()
 	// Check to see if the identifier is declared.
 	if ident := c.env.LookupIdent(identExpr.GetName()); ident != nil {
-		c.setType(e, ident.GetIdent().GetType())
-		c.setReference(e, newIdentReference(ident.GetName(), ident.GetIdent().GetValue()))
+		c.setType(e, ident.Type())
+		c.setReference(e, ast.NewIdentReference(ident.Name(), ident.Value()))
 		// Overwrite the identifier with its fully qualified name.
-		identExpr.Name = ident.GetName()
+		identExpr.Name = ident.Name()
 		return
 	}
 
-	c.setType(e, decls.Error)
-	c.errors.undeclaredReference(
-		c.location(e), c.env.container.Name(), identExpr.GetName())
+	c.setType(e, types.ErrorType)
+	c.errors.undeclaredReference(e.GetId(), c.location(e), c.env.container.Name(), identExpr.GetName())
 }
 
 func (c *checker) checkSelect(e *exprpb.Expr) {
@@ -174,9 +168,9 @@ func (c *checker) checkSelect(e *exprpb.Expr) {
 
 			// Rewrite the node to be a variable reference to the resolved fully-qualified
 			// variable name.
-			c.setType(e, ident.GetIdent().GetType())
-			c.setReference(e, newIdentReference(ident.GetName(), ident.GetIdent().GetValue()))
-			identName := ident.GetName()
+			c.setType(e, ident.Type())
+			c.setReference(e, ast.NewIdentReference(ident.Name(), ident.Value()))
+			identName := ident.Name()
 			e.ExprKind = &exprpb.Expr_IdentExpr{
 				IdentExpr: &exprpb.Expr_Ident{
 					Name: identName,
@@ -188,7 +182,7 @@ func (c *checker) checkSelect(e *exprpb.Expr) {
 
 	resultType := c.checkSelectField(e, sel.GetOperand(), sel.GetField(), false)
 	if sel.TestOnly {
-		resultType = decls.Bool
+		resultType = types.BoolType
 	}
 	c.setType(e, substitute(c.mappings, resultType, false))
 }
@@ -200,16 +194,17 @@ func (c *checker) checkOptSelect(e *exprpb.Expr) {
 	field := call.GetArgs()[1]
 	fieldName, isString := maybeUnwrapString(field)
 	if !isString {
-		c.errors.ReportError(c.location(field), "unsupported optional field selection: %v", field)
+		c.errors.notAnOptionalFieldSelection(field.GetId(), c.location(field), field)
 		return
 	}
 
 	// Perform type-checking using the field selection logic.
 	resultType := c.checkSelectField(e, operand, fieldName, true)
 	c.setType(e, substitute(c.mappings, resultType, false))
+	c.setReference(e, ast.NewFunctionReference("select_optional_field"))
 }
 
-func (c *checker) checkSelectField(e, operand *exprpb.Expr, field string, optional bool) *exprpb.Type {
+func (c *checker) checkSelectField(e, operand *exprpb.Expr, field string, optional bool) *types.Type {
 	// Interpret as field selection, first traversing down the operand.
 	c.check(operand)
 	operandType := substitute(c.mappings, c.getType(operand), false)
@@ -218,38 +213,37 @@ func (c *checker) checkSelectField(e, operand *exprpb.Expr, field string, option
 	targetType, isOpt := maybeUnwrapOptional(operandType)
 
 	// Assume error type by default as most types do not support field selection.
-	resultType := decls.Error
-	switch kindOf(targetType) {
-	case kindMap:
+	resultType := types.ErrorType
+	switch targetType.Kind() {
+	case types.MapKind:
 		// Maps yield their value type as the selection result type.
-		mapType := targetType.GetMapType()
-		resultType = mapType.GetValueType()
-	case kindObject:
+		resultType = targetType.Parameters()[1]
+	case types.StructKind:
 		// Objects yield their field type declaration as the selection result type, but only if
 		// the field is defined.
 		messageType := targetType
-		if fieldType, found := c.lookupFieldType(c.location(e), messageType.GetMessageType(), field); found {
-			resultType = fieldType.Type
+		if fieldType, found := c.lookupFieldType(e.GetId(), messageType.TypeName(), field); found {
+			resultType = fieldType
 		}
-	case kindTypeParam:
+	case types.TypeParamKind:
 		// Set the operand type to DYN to prevent assignment to a potentially incorrect type
 		// at a later point in type-checking. The isAssignable call will update the type
 		// substitutions for the type param under the covers.
-		c.isAssignable(decls.Dyn, targetType)
+		c.isAssignable(types.DynType, targetType)
 		// Also, set the result type to DYN.
-		resultType = decls.Dyn
+		resultType = types.DynType
 	default:
 		// Dynamic / error values are treated as DYN type. Errors are handled this way as well
 		// in order to allow forward progress on the check.
 		if !isDynOrError(targetType) {
-			c.errors.typeDoesNotSupportFieldSelection(c.location(e), targetType)
+			c.errors.typeDoesNotSupportFieldSelection(e.GetId(), c.location(e), targetType)
 		}
-		resultType = decls.Dyn
+		resultType = types.DynType
 	}
 
 	// If the target type was optional coming in, then the result must be optional going out.
 	if isOpt || optional {
-		return decls.NewOptionalType(resultType)
+		return types.NewOptionalType(resultType)
 	}
 	return resultType
 }
@@ -277,15 +271,14 @@ func (c *checker) checkCall(e *exprpb.Expr) {
 		// Check for the existence of the function.
 		fn := c.env.LookupFunction(fnName)
 		if fn == nil {
-			c.errors.undeclaredReference(
-				c.location(e), c.env.container.Name(), fnName)
-			c.setType(e, decls.Error)
+			c.errors.undeclaredReference(e.GetId(), c.location(e), c.env.container.Name(), fnName)
+			c.setType(e, types.ErrorType)
 			return
 		}
 		// Overwrite the function name with its fully qualified resolved name.
-		call.Function = fn.GetName()
+		call.Function = fn.Name()
 		// Check to see whether the overload resolves.
-		c.resolveOverloadOrError(c.location(e), e, fn, nil, args)
+		c.resolveOverloadOrError(e, fn, nil, args)
 		return
 	}
 
@@ -303,8 +296,8 @@ func (c *checker) checkCall(e *exprpb.Expr) {
 			// be an inaccurate representation of the desired evaluation behavior.
 			// Overwrite with fully-qualified resolved function name sans receiver target.
 			call.Target = nil
-			call.Function = fn.GetName()
-			c.resolveOverloadOrError(c.location(e), e, fn, nil, args)
+			call.Function = fn.Name()
+			c.resolveOverloadOrError(e, fn, nil, args)
 			return
 		}
 	}
@@ -314,22 +307,21 @@ func (c *checker) checkCall(e *exprpb.Expr) {
 	fn := c.env.LookupFunction(fnName)
 	// Function found, attempt overload resolution.
 	if fn != nil {
-		c.resolveOverloadOrError(c.location(e), e, fn, target, args)
+		c.resolveOverloadOrError(e, fn, target, args)
 		return
 	}
 	// Function name not declared, record error.
-	c.errors.undeclaredReference(c.location(e), c.env.container.Name(), fnName)
+	c.setType(e, types.ErrorType)
+	c.errors.undeclaredReference(e.GetId(), c.location(e), c.env.container.Name(), fnName)
 }
 
 func (c *checker) resolveOverloadOrError(
-	loc common.Location,
-	e *exprpb.Expr,
-	fn *exprpb.Decl, target *exprpb.Expr, args []*exprpb.Expr) {
+	e *exprpb.Expr, fn *decls.FunctionDecl, target *exprpb.Expr, args []*exprpb.Expr) {
 	// Attempt to resolve the overload.
-	resolution := c.resolveOverload(loc, fn, target, args)
+	resolution := c.resolveOverload(e, fn, target, args)
 	// No such overload, error noted in the resolveOverload call, type recorded here.
 	if resolution == nil {
-		c.setType(e, decls.Error)
+		c.setType(e, types.ErrorType)
 		return
 	}
 	// Overload found.
@@ -338,10 +330,9 @@ func (c *checker) resolveOverloadOrError(
 }
 
 func (c *checker) resolveOverload(
-	loc common.Location,
-	fn *exprpb.Decl, target *exprpb.Expr, args []*exprpb.Expr) *overloadResolution {
+	call *exprpb.Expr, fn *decls.FunctionDecl, target *exprpb.Expr, args []*exprpb.Expr) *overloadResolution {
 
-	var argTypes []*exprpb.Type
+	var argTypes []*types.Type
 	if target != nil {
 		argTypes = append(argTypes, c.getType(target))
 	}
@@ -349,55 +340,75 @@ func (c *checker) resolveOverload(
 		argTypes = append(argTypes, c.getType(arg))
 	}
 
-	var resultType *exprpb.Type
-	var checkedRef *exprpb.Reference
-	for _, overload := range fn.GetFunction().GetOverloads() {
+	var resultType *types.Type
+	var checkedRef *ast.ReferenceInfo
+	for _, overload := range fn.OverloadDecls() {
 		// Determine whether the overload is currently considered.
-		if c.env.isOverloadDisabled(overload.GetOverloadId()) {
+		if c.env.isOverloadDisabled(overload.ID()) {
 			continue
 		}
 
 		// Ensure the call style for the overload matches.
-		if (target == nil && overload.GetIsInstanceFunction()) ||
-			(target != nil && !overload.GetIsInstanceFunction()) {
+		if (target == nil && overload.IsMemberFunction()) ||
+			(target != nil && !overload.IsMemberFunction()) {
 			// not a compatible call style.
 			continue
 		}
 
-		overloadType := decls.NewFunctionType(overload.ResultType, overload.Params...)
-		if len(overload.GetTypeParams()) > 0 {
+		// Alternative type-checking behavior when the logical operators are compacted into
+		// variadic AST representations.
+		if fn.Name() == operators.LogicalAnd || fn.Name() == operators.LogicalOr {
+			checkedRef = ast.NewFunctionReference(overload.ID())
+			for i, argType := range argTypes {
+				if !c.isAssignable(argType, types.BoolType) {
+					c.errors.typeMismatch(
+						args[i].GetId(),
+						c.locationByID(args[i].GetId()),
+						types.BoolType,
+						argType)
+					resultType = types.ErrorType
+				}
+			}
+			if isError(resultType) {
+				return nil
+			}
+			return newResolution(checkedRef, types.BoolType)
+		}
+
+		overloadType := newFunctionType(overload.ResultType(), overload.ArgTypes()...)
+		typeParams := overload.TypeParams()
+		if len(typeParams) != 0 {
 			// Instantiate overload's type with fresh type variables.
 			substitutions := newMapping()
-			for _, typePar := range overload.GetTypeParams() {
-				substitutions.add(decls.NewTypeParamType(typePar), c.newTypeVar())
+			for _, typePar := range typeParams {
+				substitutions.add(types.NewTypeParamType(typePar), c.newTypeVar())
 			}
 			overloadType = substitute(substitutions, overloadType, false)
 		}
 
-		candidateArgTypes := overloadType.GetFunction().GetArgTypes()
+		candidateArgTypes := overloadType.Parameters()[1:]
 		if c.isAssignableList(argTypes, candidateArgTypes) {
 			if checkedRef == nil {
-				checkedRef = newFunctionReference(overload.GetOverloadId())
+				checkedRef = ast.NewFunctionReference(overload.ID())
 			} else {
-				checkedRef.OverloadId = append(checkedRef.GetOverloadId(), overload.GetOverloadId())
+				checkedRef.AddOverload(overload.ID())
 			}
 
 			// First matching overload, determines result type.
-			fnResultType := substitute(c.mappings, overloadType.GetFunction().GetResultType(), false)
+			fnResultType := substitute(c.mappings, overloadType.Parameters()[0], false)
 			if resultType == nil {
 				resultType = fnResultType
-			} else if !isDyn(resultType) && !proto.Equal(fnResultType, resultType) {
-				resultType = decls.Dyn
+			} else if !isDyn(resultType) && !fnResultType.IsExactType(resultType) {
+				resultType = types.DynType
 			}
 		}
 	}
 
 	if resultType == nil {
-		for i, arg := range argTypes {
-			argTypes[i] = substitute(c.mappings, arg, true)
+		for i, argType := range argTypes {
+			argTypes[i] = substitute(c.mappings, argType, true)
 		}
-		c.errors.noMatchingOverload(loc, fn.GetName(), argTypes, target != nil)
-		resultType = decls.Error
+		c.errors.noMatchingOverload(call.GetId(), c.location(call), fn.Name(), argTypes, target != nil)
 		return nil
 	}
 
@@ -406,7 +417,7 @@ func (c *checker) resolveOverload(
 
 func (c *checker) checkCreateList(e *exprpb.Expr) {
 	create := e.GetListExpr()
-	var elemsType *exprpb.Type
+	var elemsType *types.Type
 	optionalIndices := create.GetOptionalIndices()
 	optionals := make(map[int32]bool, len(optionalIndices))
 	for _, optInd := range optionalIndices {
@@ -419,16 +430,16 @@ func (c *checker) checkCreateList(e *exprpb.Expr) {
 			var isOptional bool
 			elemType, isOptional = maybeUnwrapOptional(elemType)
 			if !isOptional && !isDyn(elemType) {
-				c.errors.typeMismatch(c.location(e), decls.NewOptionalType(elemType), elemType)
+				c.errors.typeMismatch(e.GetId(), c.location(e), types.NewOptionalType(elemType), elemType)
 			}
 		}
-		elemsType = c.joinTypes(c.location(e), elemsType, elemType)
+		elemsType = c.joinTypes(e, elemsType, elemType)
 	}
 	if elemsType == nil {
 		// If the list is empty, assign free type var to elem type.
 		elemsType = c.newTypeVar()
 	}
-	c.setType(e, decls.NewListType(elemsType))
+	c.setType(e, types.NewListType(elemsType))
 }
 
 func (c *checker) checkCreateStruct(e *exprpb.Expr) {
@@ -442,12 +453,12 @@ func (c *checker) checkCreateStruct(e *exprpb.Expr) {
 
 func (c *checker) checkCreateMap(e *exprpb.Expr) {
 	mapVal := e.GetStructExpr()
-	var mapKeyType *exprpb.Type
-	var mapValueType *exprpb.Type
+	var mapKeyType *types.Type
+	var mapValueType *types.Type
 	for _, ent := range mapVal.GetEntries() {
 		key := ent.GetMapKey()
 		c.check(key)
-		mapKeyType = c.joinTypes(c.location(key), mapKeyType, c.getType(key))
+		mapKeyType = c.joinTypes(key, mapKeyType, c.getType(key))
 
 		val := ent.GetValue()
 		c.check(val)
@@ -456,50 +467,54 @@ func (c *checker) checkCreateMap(e *exprpb.Expr) {
 			var isOptional bool
 			valType, isOptional = maybeUnwrapOptional(valType)
 			if !isOptional && !isDyn(valType) {
-				c.errors.typeMismatch(c.location(val), decls.NewOptionalType(valType), valType)
+				c.errors.typeMismatch(val.GetId(), c.location(val), types.NewOptionalType(valType), valType)
 			}
 		}
-		mapValueType = c.joinTypes(c.location(val), mapValueType, valType)
+		mapValueType = c.joinTypes(val, mapValueType, valType)
 	}
 	if mapKeyType == nil {
 		// If the map is empty, assign free type variables to typeKey and value type.
 		mapKeyType = c.newTypeVar()
 		mapValueType = c.newTypeVar()
 	}
-	c.setType(e, decls.NewMapType(mapKeyType, mapValueType))
+	c.setType(e, types.NewMapType(mapKeyType, mapValueType))
 }
 
 func (c *checker) checkCreateMessage(e *exprpb.Expr) {
 	msgVal := e.GetStructExpr()
 	// Determine the type of the message.
-	messageType := decls.Error
-	decl := c.env.LookupIdent(msgVal.GetMessageName())
-	if decl == nil {
+	resultType := types.ErrorType
+	ident := c.env.LookupIdent(msgVal.GetMessageName())
+	if ident == nil {
 		c.errors.undeclaredReference(
-			c.location(e), c.env.container.Name(), msgVal.GetMessageName())
+			e.GetId(), c.location(e), c.env.container.Name(), msgVal.GetMessageName())
+		c.setType(e, types.ErrorType)
 		return
 	}
 	// Ensure the type name is fully qualified in the AST.
-	msgVal.MessageName = decl.GetName()
-	c.setReference(e, newIdentReference(decl.GetName(), nil))
-	ident := decl.GetIdent()
-	identKind := kindOf(ident.GetType())
-	if identKind != kindError {
-		if identKind != kindType {
-			c.errors.notAType(c.location(e), ident.GetType())
+	typeName := ident.Name()
+	msgVal.MessageName = typeName
+	c.setReference(e, ast.NewIdentReference(ident.Name(), nil))
+	identKind := ident.Type().Kind()
+	if identKind != types.ErrorKind {
+		if identKind != types.TypeKind {
+			c.errors.notAType(e.GetId(), c.location(e), ident.Type().DeclaredTypeName())
 		} else {
-			messageType = ident.GetType().GetType()
-			if kindOf(messageType) != kindObject {
-				c.errors.notAMessageType(c.location(e), messageType)
-				messageType = decls.Error
+			resultType = ident.Type().Parameters()[0]
+			// Backwards compatibility test between well-known types and message types
+			// In this context, the type is being instantiated by its protobuf name which
+			// is not ideal or recommended, but some users expect this to work.
+			if isWellKnownType(resultType) {
+				typeName = getWellKnownTypeName(resultType)
+			} else if resultType.Kind() == types.StructKind {
+				typeName = resultType.DeclaredTypeName()
+			} else {
+				c.errors.notAMessageType(e.GetId(), c.location(e), resultType.DeclaredTypeName())
+				resultType = types.ErrorType
 			}
 		}
 	}
-	if isObjectWellKnownType(messageType) {
-		c.setType(e, getObjectWellKnownType(messageType))
-	} else {
-		c.setType(e, messageType)
-	}
+	c.setType(e, resultType)
 
 	// Check the field initializers.
 	for _, ent := range msgVal.GetEntries() {
@@ -507,10 +522,10 @@ func (c *checker) checkCreateMessage(e *exprpb.Expr) {
 		value := ent.GetValue()
 		c.check(value)
 
-		fieldType := decls.Error
-		ft, found := c.lookupFieldType(c.locationByID(ent.GetId()), messageType.GetMessageType(), field)
+		fieldType := types.ErrorType
+		ft, found := c.lookupFieldType(ent.GetId(), typeName, field)
 		if found {
-			fieldType = ft.Type
+			fieldType = ft
 		}
 
 		valType := c.getType(value)
@@ -518,11 +533,11 @@ func (c *checker) checkCreateMessage(e *exprpb.Expr) {
 			var isOptional bool
 			valType, isOptional = maybeUnwrapOptional(valType)
 			if !isOptional && !isDyn(valType) {
-				c.errors.typeMismatch(c.location(value), decls.NewOptionalType(valType), valType)
+				c.errors.typeMismatch(value.GetId(), c.location(value), types.NewOptionalType(valType), valType)
 			}
 		}
 		if !c.isAssignable(fieldType, valType) {
-			c.errors.fieldTypeMismatch(c.locationByID(ent.Id), field, fieldType, valType)
+			c.errors.fieldTypeMismatch(ent.GetId(), c.locationByID(ent.GetId()), field, fieldType, valType)
 		}
 	}
 }
@@ -533,36 +548,36 @@ func (c *checker) checkComprehension(e *exprpb.Expr) {
 	c.check(comp.GetAccuInit())
 	accuType := c.getType(comp.GetAccuInit())
 	rangeType := substitute(c.mappings, c.getType(comp.GetIterRange()), false)
-	var varType *exprpb.Type
+	var varType *types.Type
 
-	switch kindOf(rangeType) {
-	case kindList:
-		varType = rangeType.GetListType().GetElemType()
-	case kindMap:
+	switch rangeType.Kind() {
+	case types.ListKind:
+		varType = rangeType.Parameters()[0]
+	case types.MapKind:
 		// Ranges over the keys.
-		varType = rangeType.GetMapType().GetKeyType()
-	case kindDyn, kindError, kindTypeParam:
+		varType = rangeType.Parameters()[0]
+	case types.DynKind, types.ErrorKind, types.TypeParamKind:
 		// Set the range type to DYN to prevent assignment to a potentially incorrect type
 		// at a later point in type-checking. The isAssignable call will update the type
 		// substitutions for the type param under the covers.
-		c.isAssignable(decls.Dyn, rangeType)
+		c.isAssignable(types.DynType, rangeType)
 		// Set the range iteration variable to type DYN as well.
-		varType = decls.Dyn
+		varType = types.DynType
 	default:
-		c.errors.notAComprehensionRange(c.location(comp.GetIterRange()), rangeType)
-		varType = decls.Error
+		c.errors.notAComprehensionRange(comp.GetIterRange().GetId(), c.location(comp.GetIterRange()), rangeType)
+		varType = types.ErrorType
 	}
 
 	// Create a scope for the comprehension since it has a local accumulation variable.
 	// This scope will contain the accumulation variable used to compute the result.
 	c.env = c.env.enterScope()
-	c.env.Add(decls.NewVar(comp.GetAccuVar(), accuType))
+	c.env.AddIdents(decls.NewVariable(comp.GetAccuVar(), accuType))
 	// Create a block scope for the loop.
 	c.env = c.env.enterScope()
-	c.env.Add(decls.NewVar(comp.GetIterVar(), varType))
+	c.env.AddIdents(decls.NewVariable(comp.GetIterVar(), varType))
 	// Check the variable references in the condition and step.
 	c.check(comp.GetLoopCondition())
-	c.assertType(comp.GetLoopCondition(), decls.Bool)
+	c.assertType(comp.GetLoopCondition(), types.BoolType)
 	c.check(comp.GetLoopStep())
 	c.assertType(comp.GetLoopStep(), accuType)
 	// Exit the loop's block scope before checking the result.
@@ -574,9 +589,7 @@ func (c *checker) checkComprehension(e *exprpb.Expr) {
 }
 
 // Checks compatibility of joined types, and returns the most general common type.
-func (c *checker) joinTypes(loc common.Location,
-	previous *exprpb.Type,
-	current *exprpb.Type) *exprpb.Type {
+func (c *checker) joinTypes(e *exprpb.Expr, previous, current *types.Type) *types.Type {
 	if previous == nil {
 		return current
 	}
@@ -584,23 +597,23 @@ func (c *checker) joinTypes(loc common.Location,
 		return mostGeneral(previous, current)
 	}
 	if c.dynAggregateLiteralElementTypesEnabled() {
-		return decls.Dyn
+		return types.DynType
 	}
-	c.errors.typeMismatch(loc, previous, current)
-	return decls.Error
+	c.errors.typeMismatch(e.GetId(), c.location(e), previous, current)
+	return types.ErrorType
 }
 
 func (c *checker) dynAggregateLiteralElementTypesEnabled() bool {
 	return c.env.aggLitElemType == dynElementType
 }
 
-func (c *checker) newTypeVar() *exprpb.Type {
+func (c *checker) newTypeVar() *types.Type {
 	id := c.freeTypeVarCounter
 	c.freeTypeVarCounter++
-	return decls.NewTypeParamType(fmt.Sprintf("_var%d", id))
+	return types.NewTypeParamType(fmt.Sprintf("_var%d", id))
 }
 
-func (c *checker) isAssignable(t1 *exprpb.Type, t2 *exprpb.Type) bool {
+func (c *checker) isAssignable(t1, t2 *types.Type) bool {
 	subs := isAssignable(c.mappings, t1, t2)
 	if subs != nil {
 		c.mappings = subs
@@ -610,7 +623,7 @@ func (c *checker) isAssignable(t1 *exprpb.Type, t2 *exprpb.Type) bool {
 	return false
 }
 
-func (c *checker) isAssignableList(l1 []*exprpb.Type, l2 []*exprpb.Type) bool {
+func (c *checker) isAssignableList(l1, l2 []*types.Type) bool {
 	subs := isAssignableList(c.mappings, l1, l2)
 	if subs != nil {
 		c.mappings = subs
@@ -620,57 +633,52 @@ func (c *checker) isAssignableList(l1 []*exprpb.Type, l2 []*exprpb.Type) bool {
 	return false
 }
 
-func (c *checker) lookupFieldType(l common.Location, messageType string, fieldName string) (*ref.FieldType, bool) {
-	if _, found := c.env.provider.FindType(messageType); !found {
-		// This should not happen, anyway, report an error.
-		c.errors.unexpectedFailedResolution(l, messageType)
-		return nil, false
+func maybeUnwrapString(e *exprpb.Expr) (string, bool) {
+	switch e.GetExprKind().(type) {
+	case *exprpb.Expr_ConstExpr:
+		literal := e.GetConstExpr()
+		switch literal.GetConstantKind().(type) {
+		case *exprpb.Constant_StringValue:
+			return literal.GetStringValue(), true
+		}
 	}
-
-	if ft, found := c.env.provider.FindFieldType(messageType, fieldName); found {
-		return ft, found
-	}
-
-	c.errors.undefinedField(l, fieldName)
-	return nil, false
+	return "", false
 }
 
-func (c *checker) setType(e *exprpb.Expr, t *exprpb.Type) {
-	if old, found := c.types[e.GetId()]; found && !proto.Equal(old, t) {
-		c.errors.ReportError(c.location(e),
-			"(Incompatible) Type already exists for expression: %v(%d) old:%v, new:%v", e, e.GetId(), old, t)
+func (c *checker) setType(e *exprpb.Expr, t *types.Type) {
+	if old, found := c.types[e.GetId()]; found && !old.IsExactType(t) {
+		c.errors.incompatibleType(e.GetId(), c.location(e), e, old, t)
 		return
 	}
 	c.types[e.GetId()] = t
 }
 
-func (c *checker) getType(e *exprpb.Expr) *exprpb.Type {
+func (c *checker) getType(e *exprpb.Expr) *types.Type {
 	return c.types[e.GetId()]
 }
 
-func (c *checker) setReference(e *exprpb.Expr, r *exprpb.Reference) {
-	if old, found := c.references[e.GetId()]; found && !proto.Equal(old, r) {
-		c.errors.ReportError(c.location(e),
-			"Reference already exists for expression: %v(%d) old:%v, new:%v", e, e.GetId(), old, r)
+func (c *checker) setReference(e *exprpb.Expr, r *ast.ReferenceInfo) {
+	if old, found := c.references[e.GetId()]; found && !old.Equals(r) {
+		c.errors.referenceRedefinition(e.GetId(), c.location(e), e, old, r)
 		return
 	}
 	c.references[e.GetId()] = r
 }
 
-func (c *checker) assertType(e *exprpb.Expr, t *exprpb.Type) {
+func (c *checker) assertType(e *exprpb.Expr, t *types.Type) {
 	if !c.isAssignable(t, c.getType(e)) {
-		c.errors.typeMismatch(c.location(e), t, c.getType(e))
+		c.errors.typeMismatch(e.GetId(), c.location(e), t, c.getType(e))
 	}
 }
 
 type overloadResolution struct {
-	Reference *exprpb.Reference
-	Type      *exprpb.Type
+	Type      *types.Type
+	Reference *ast.ReferenceInfo
 }
 
-func newResolution(checkedRef *exprpb.Reference, t *exprpb.Type) *overloadResolution {
+func newResolution(r *ast.ReferenceInfo, t *types.Type) *overloadResolution {
 	return &overloadResolution{
-		Reference: checkedRef,
+		Reference: r,
 		Type:      t,
 	}
 }
@@ -697,10 +705,56 @@ func (c *checker) locationByID(id int64) common.Location {
 	return common.NoLocation
 }
 
-func newIdentReference(name string, value *exprpb.Constant) *exprpb.Reference {
-	return &exprpb.Reference{Name: name, Value: value}
+func (c *checker) lookupFieldType(exprID int64, structType, fieldName string) (*types.Type, bool) {
+	if _, found := c.env.provider.FindStructType(structType); !found {
+		// This should not happen, anyway, report an error.
+		c.errors.unexpectedFailedResolution(exprID, c.locationByID(exprID), structType)
+		return nil, false
+	}
+
+	if ft, found := c.env.provider.FindStructFieldType(structType, fieldName); found {
+		return ft.Type, found
+	}
+
+	c.errors.undefinedField(exprID, c.locationByID(exprID), fieldName)
+	return nil, false
 }
 
-func newFunctionReference(overloads ...string) *exprpb.Reference {
-	return &exprpb.Reference{OverloadId: overloads}
+func isWellKnownType(t *types.Type) bool {
+	switch t.Kind() {
+	case types.AnyKind, types.TimestampKind, types.DurationKind, types.DynKind, types.NullTypeKind:
+		return true
+	case types.BoolKind, types.BytesKind, types.DoubleKind, types.IntKind, types.StringKind, types.UintKind:
+		return t.IsAssignableType(types.NullType)
+	case types.ListKind:
+		return t.Parameters()[0] == types.DynType
+	case types.MapKind:
+		return t.Parameters()[0] == types.StringType && t.Parameters()[1] == types.DynType
+	}
+	return false
 }
+
+func getWellKnownTypeName(t *types.Type) string {
+	if name, found := wellKnownTypes[t.Kind()]; found {
+		return name
+	}
+	return ""
+}
+
+var (
+	wellKnownTypes = map[types.Kind]string{
+		types.AnyKind:       "google.protobuf.Any",
+		types.BoolKind:      "google.protobuf.BoolValue",
+		types.BytesKind:     "google.protobuf.BytesValue",
+		types.DoubleKind:    "google.protobuf.DoubleValue",
+		types.DurationKind:  "google.protobuf.Duration",
+		types.DynKind:       "google.protobuf.Value",
+		types.IntKind:       "google.protobuf.Int64Value",
+		types.ListKind:      "google.protobuf.ListValue",
+		types.NullTypeKind:  "google.protobuf.NullValue",
+		types.MapKind:       "google.protobuf.Struct",
+		types.StringKind:    "google.protobuf.StringValue",
+		types.TimestampKind: "google.protobuf.Timestamp",
+		types.UintKind:      "google.protobuf.UInt64Value",
+	}
+)