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// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package types declares the data types and implements
// the algorithms for type-checking of Go packages. Use
// Config.Check to invoke the type checker for a package.
// Alternatively, create a new type checker with NewChecker
// and invoke it incrementally by calling Checker.Files.
// Type-checking consists of several interdependent phases:
// Name resolution maps each identifier (ast.Ident) in the program to the
// language object (Object) it denotes.
// Use Info.{Defs,Uses,Implicits} for the results of name resolution.
// Constant folding computes the exact constant value (constant.Value)
// for every expression (ast.Expr) that is a compile-time constant.
// Use Info.Types[expr].Value for the results of constant folding.
// Type inference computes the type (Type) of every expression (ast.Expr)
// and checks for compliance with the language specification.
// Use Info.Types[expr].Type for the results of type inference.
// For a tutorial, see
package types
import (
_ "unsafe" // for go:linkname
// An Error describes a type-checking error; it implements the error interface.
// A "soft" error is an error that still permits a valid interpretation of a
// package (such as "unused variable"); "hard" errors may lead to unpredictable
// behavior if ignored.
type Error struct {
Fset *token.FileSet // file set for interpretation of Pos
Pos token.Pos // error position
Msg string // error message
Soft bool // if set, error is "soft"
// Error returns an error string formatted as follows:
// filename:line:column: message
func (err Error) Error() string {
return fmt.Sprintf("%s: %s", err.Fset.Position(err.Pos), err.Msg)
// An Importer resolves import paths to Packages.
// CAUTION: This interface does not support the import of locally
// vendored packages. See
// If possible, external implementations should implement ImporterFrom.
type Importer interface {
// Import returns the imported package for the given import path.
// The semantics is like for ImporterFrom.ImportFrom except that
// dir and mode are ignored (since they are not present).
Import(path string) (*Package, error)
// ImportMode is reserved for future use.
type ImportMode int
// An ImporterFrom resolves import paths to packages; it
// supports vendoring per
// Use go/importer to obtain an ImporterFrom implementation.
type ImporterFrom interface {
// Importer is present for backward-compatibility. Calling
// Import(path) is the same as calling ImportFrom(path, "", 0);
// i.e., locally vendored packages may not be found.
// The types package does not call Import if an ImporterFrom
// is present.
// ImportFrom returns the imported package for the given import
// path when imported by a package file located in dir.
// If the import failed, besides returning an error, ImportFrom
// is encouraged to cache and return a package anyway, if one
// was created. This will reduce package inconsistencies and
// follow-on type checker errors due to the missing package.
// The mode value must be 0; it is reserved for future use.
// Two calls to ImportFrom with the same path and dir must
// return the same package.
ImportFrom(path, dir string, mode ImportMode) (*Package, error)
// A Config specifies the configuration for type checking.
// The zero value for Config is a ready-to-use default configuration.
type Config struct {
// If IgnoreFuncBodies is set, function bodies are not
// type-checked.
IgnoreFuncBodies bool
// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
// declares an empty "C" package and errors are omitted for qualified
// identifiers referring to package C (which won't find an object).
// This feature is intended for the standard library cmd/api tool.
// Caution: Effects may be unpredictable due to follow-on errors.
// Do not use casually!
FakeImportC bool
// If go115UsesCgo is set, the type checker expects the
// _cgo_gotypes.go file generated by running cmd/cgo to be
// provided as a package source file. Qualified identifiers
// referring to package C will be resolved to cgo-provided
// declarations within _cgo_gotypes.go.
// It is an error to set both FakeImportC and go115UsesCgo.
go115UsesCgo bool
// If Error != nil, it is called with each error found
// during type checking; err has dynamic type Error.
// Secondary errors (for instance, to enumerate all types
// involved in an invalid recursive type declaration) have
// error strings that start with a '\t' character.
// If Error == nil, type-checking stops with the first
// error found.
Error func(err error)
// An importer is used to import packages referred to from
// import declarations.
// If the installed importer implements ImporterFrom, the type
// checker calls ImportFrom instead of Import.
// The type checker reports an error if an importer is needed
// but none was installed.
Importer Importer
// If Sizes != nil, it provides the sizing functions for package unsafe.
// Otherwise SizesFor("gc", "amd64") is used instead.
Sizes Sizes
// If DisableUnusedImportCheck is set, packages are not checked
// for unused imports.
DisableUnusedImportCheck bool
//go:linkname srcimporter_setUsesCgo
func srcimporter_setUsesCgo(conf *Config) {
conf.go115UsesCgo = true
// Info holds result type information for a type-checked package.
// Only the information for which a map is provided is collected.
// If the package has type errors, the collected information may
// be incomplete.
type Info struct {
// Types maps expressions to their types, and for constant
// expressions, also their values. Invalid expressions are
// omitted.
// For (possibly parenthesized) identifiers denoting built-in
// functions, the recorded signatures are call-site specific:
// if the call result is not a constant, the recorded type is
// an argument-specific signature. Otherwise, the recorded type
// is invalid.
// The Types map does not record the type of every identifier,
// only those that appear where an arbitrary expression is
// permitted. For instance, the identifier f in a selector
// expression x.f is found only in the Selections map, the
// identifier z in a variable declaration 'var z int' is found
// only in the Defs map, and identifiers denoting packages in
// qualified identifiers are collected in the Uses map.
Types map[ast.Expr]TypeAndValue
// Defs maps identifiers to the objects they define (including
// package names, dots "." of dot-imports, and blank "_" identifiers).
// For identifiers that do not denote objects (e.g., the package name
// in package clauses, or symbolic variables t in t := x.(type) of
// type switch headers), the corresponding objects are nil.
// For an embedded field, Defs returns the field *Var it defines.
// Invariant: Defs[id] == nil || Defs[id].Pos() == id.Pos()
Defs map[*ast.Ident]Object
// Uses maps identifiers to the objects they denote.
// For an embedded field, Uses returns the *TypeName it denotes.
// Invariant: Uses[id].Pos() != id.Pos()
Uses map[*ast.Ident]Object
// Implicits maps nodes to their implicitly declared objects, if any.
// The following node and object types may appear:
// node declared object
// *ast.ImportSpec *PkgName for imports without renames
// *ast.CaseClause type-specific *Var for each type switch case clause (incl. default)
// *ast.Field anonymous parameter *Var (incl. unnamed results)
Implicits map[ast.Node]Object
// Selections maps selector expressions (excluding qualified identifiers)
// to their corresponding selections.
Selections map[*ast.SelectorExpr]*Selection
// Scopes maps ast.Nodes to the scopes they define. Package scopes are not
// associated with a specific node but with all files belonging to a package.
// Thus, the package scope can be found in the type-checked Package object.
// Scopes nest, with the Universe scope being the outermost scope, enclosing
// the package scope, which contains (one or more) files scopes, which enclose
// function scopes which in turn enclose statement and function literal scopes.
// Note that even though package-level functions are declared in the package
// scope, the function scopes are embedded in the file scope of the file
// containing the function declaration.
// The following node types may appear in Scopes:
// *ast.File
// *ast.FuncType
// *ast.BlockStmt
// *ast.IfStmt
// *ast.SwitchStmt
// *ast.TypeSwitchStmt
// *ast.CaseClause
// *ast.CommClause
// *ast.ForStmt
// *ast.RangeStmt
Scopes map[ast.Node]*Scope
// InitOrder is the list of package-level initializers in the order in which
// they must be executed. Initializers referring to variables related by an
// initialization dependency appear in topological order, the others appear
// in source order. Variables without an initialization expression do not
// appear in this list.
InitOrder []*Initializer
// TypeOf returns the type of expression e, or nil if not found.
// Precondition: the Types, Uses and Defs maps are populated.
func (info *Info) TypeOf(e ast.Expr) Type {
if t, ok := info.Types[e]; ok {
return t.Type
if id, _ := e.(*ast.Ident); id != nil {
if obj := info.ObjectOf(id); obj != nil {
return obj.Type()
return nil
// ObjectOf returns the object denoted by the specified id,
// or nil if not found.
// If id is an embedded struct field, ObjectOf returns the field (*Var)
// it defines, not the type (*TypeName) it uses.
// Precondition: the Uses and Defs maps are populated.
func (info *Info) ObjectOf(id *ast.Ident) Object {
if obj := info.Defs[id]; obj != nil {
return obj
return info.Uses[id]
// TypeAndValue reports the type and value (for constants)
// of the corresponding expression.
type TypeAndValue struct {
mode operandMode
Type Type
Value constant.Value
// IsVoid reports whether the corresponding expression
// is a function call without results.
func (tv TypeAndValue) IsVoid() bool {
return tv.mode == novalue
// IsType reports whether the corresponding expression specifies a type.
func (tv TypeAndValue) IsType() bool {
return tv.mode == typexpr
// IsBuiltin reports whether the corresponding expression denotes
// a (possibly parenthesized) built-in function.
func (tv TypeAndValue) IsBuiltin() bool {
return tv.mode == builtin
// IsValue reports whether the corresponding expression is a value.
// Builtins are not considered values. Constant values have a non-
// nil Value.
func (tv TypeAndValue) IsValue() bool {
switch tv.mode {
case constant_, variable, mapindex, value, commaok, commaerr:
return true
return false
// IsNil reports whether the corresponding expression denotes the
// predeclared value nil.
func (tv TypeAndValue) IsNil() bool {
return tv.mode == value && tv.Type == Typ[UntypedNil]
// Addressable reports whether the corresponding expression
// is addressable (
func (tv TypeAndValue) Addressable() bool {
return tv.mode == variable
// Assignable reports whether the corresponding expression
// is assignable to (provided a value of the right type).
func (tv TypeAndValue) Assignable() bool {
return tv.mode == variable || tv.mode == mapindex
// HasOk reports whether the corresponding expression may be
// used on the rhs of a comma-ok assignment.
func (tv TypeAndValue) HasOk() bool {
return tv.mode == commaok || tv.mode == mapindex
// An Initializer describes a package-level variable, or a list of variables in case
// of a multi-valued initialization expression, and the corresponding initialization
// expression.
type Initializer struct {
Lhs []*Var // var Lhs = Rhs
Rhs ast.Expr
func (init *Initializer) String() string {
var buf bytes.Buffer
for i, lhs := range init.Lhs {
if i > 0 {
buf.WriteString(", ")
buf.WriteString(" = ")
WriteExpr(&buf, init.Rhs)
return buf.String()
// Check type-checks a package and returns the resulting package object and
// the first error if any. Additionally, if info != nil, Check populates each
// of the non-nil maps in the Info struct.
// The package is marked as complete if no errors occurred, otherwise it is
// incomplete. See Config.Error for controlling behavior in the presence of
// errors.
// The package is specified by a list of *ast.Files and corresponding
// file set, and the package path the package is identified with.
// The clean path must not be empty or dot (".").
func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, info *Info) (*Package, error) {
pkg := NewPackage(path, "")
return pkg, NewChecker(conf, fset, pkg, info).Files(files)
// AssertableTo reports whether a value of type V can be asserted to have type T.
func AssertableTo(V *Interface, T Type) bool {
m, _ := (*Checker)(nil).assertableTo(V, T)
return m == nil
// AssignableTo reports whether a value of type V is assignable to a variable of type T.
func AssignableTo(V, T Type) bool {
x := operand{mode: value, typ: V}
return x.assignableTo(nil, T, nil) // check not needed for non-constant x
// ConvertibleTo reports whether a value of type V is convertible to a value of type T.
func ConvertibleTo(V, T Type) bool {
x := operand{mode: value, typ: V}
return x.convertibleTo(nil, T) // check not needed for non-constant x
// Implements reports whether type V implements interface T.
func Implements(V Type, T *Interface) bool {
f, _ := MissingMethod(V, T, true)
return f == nil
// Identical reports whether x and y are identical types.
// Receivers of Signature types are ignored.
func Identical(x, y Type) bool {
return (*Checker)(nil).identical(x, y)
// IdenticalIgnoreTags reports whether x and y are identical types if tags are ignored.
// Receivers of Signature types are ignored.
func IdenticalIgnoreTags(x, y Type) bool {
return (*Checker)(nil).identicalIgnoreTags(x, y)