[dev.typeparams] cmd/dist: disable -G=3 on the std go tests for now

[gostls13.git] / src / go / types / errorcodes.go

// Copyright 2020 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

type errorCode int

// This file defines the error codes that can be produced during type-checking.
// Collectively, these codes provide an identifier that may be used to
// implement special handling for certain types of errors.
//
// Error codes should be fine-grained enough that the exact nature of the error
// can be easily determined, but coarse enough that they are not an
// implementation detail of the type checking algorithm. As a rule-of-thumb,
// errors should be considered equivalent if there is a theoretical refactoring
// of the type checker in which they are emitted in exactly one place. For
// example, the type checker emits different error messages for "too many
// arguments" and "too few arguments", but one can imagine an alternative type
// checker where this check instead just emits a single "wrong number of
// arguments", so these errors should have the same code.
//
// Error code names should be as brief as possible while retaining accuracy and
// distinctiveness. In most cases names should start with an adjective
// describing the nature of the error (e.g. "invalid", "unused", "misplaced"),
// and end with a noun identifying the relevant language object. For example,
// "_DuplicateDecl" or "_InvalidSliceExpr". For brevity, naming follows the
// convention that "bad" implies a problem with syntax, and "invalid" implies a
// problem with types.

const (
        _ errorCode = iota

        // _Test is reserved for errors that only apply while in self-test mode.
        _Test

        /* package names */

        // _BlankPkgName occurs when a package name is the blank identifier "_".
        //
        // Per the spec:
        //  "The PackageName must not be the blank identifier."
        _BlankPkgName

        // _MismatchedPkgName occurs when a file's package name doesn't match the
        // package name already established by other files.
        _MismatchedPkgName

        // _InvalidPkgUse occurs when a package identifier is used outside of a
        // selector expression.
        //
        // Example:
        //  import "fmt"
        //
        //  var _ = fmt
        _InvalidPkgUse

        /* imports */

        // _BadImportPath occurs when an import path is not valid.
        _BadImportPath

        // _BrokenImport occurs when importing a package fails.
        //
        // Example:
        //  import "amissingpackage"
        _BrokenImport

        // _ImportCRenamed occurs when the special import "C" is renamed. "C" is a
        // pseudo-package, and must not be renamed.
        //
        // Example:
        //  import _ "C"
        _ImportCRenamed

        // _UnusedImport occurs when an import is unused.
        //
        // Example:
        //  import "fmt"
        //
        //  func main() {}
        _UnusedImport

        /* initialization */

        // _InvalidInitCycle occurs when an invalid cycle is detected within the
        // initialization graph.
        //
        // Example:
        //  var x int = f()
        //
        //  func f() int { return x }
        _InvalidInitCycle

        /* decls */

        // _DuplicateDecl occurs when an identifier is declared multiple times.
        //
        // Example:
        //  var x = 1
        //  var x = 2
        _DuplicateDecl

        // _InvalidDeclCycle occurs when a declaration cycle is not valid.
        //
        // Example:
        //  import "unsafe"
        //
        //  type T struct {
        //      a [n]int
        //  }
        //
        //  var n = unsafe.Sizeof(T{})
        _InvalidDeclCycle

        // _InvalidTypeCycle occurs when a cycle in type definitions results in a
        // type that is not well-defined.
        //
        // Example:
        //  import "unsafe"
        //
        //  type T [unsafe.Sizeof(T{})]int
        _InvalidTypeCycle

        /* decls > const */

        // _InvalidConstInit occurs when a const declaration has a non-constant
        // initializer.
        //
        // Example:
        //  var x int
        //  const _ = x
        _InvalidConstInit

        // _InvalidConstVal occurs when a const value cannot be converted to its
        // target type.
        //
        // TODO(findleyr): this error code and example are not very clear. Consider
        // removing it.
        //
        // Example:
        //  const _ = 1 << "hello"
        _InvalidConstVal

        // _InvalidConstType occurs when the underlying type in a const declaration
        // is not a valid constant type.
        //
        // Example:
        //  const c *int = 4
        _InvalidConstType

        /* decls > var (+ other variable assignment codes) */

        // _UntypedNil occurs when the predeclared (untyped) value nil is used to
        // initialize a variable declared without an explicit type.
        //
        // Example:
        //  var x = nil
        _UntypedNil

        // _WrongAssignCount occurs when the number of values on the right-hand side
        // of an assignment or or initialization expression does not match the number
        // of variables on the left-hand side.
        //
        // Example:
        //  var x = 1, 2
        _WrongAssignCount

        // _UnassignableOperand occurs when the left-hand side of an assignment is
        // not assignable.
        //
        // Example:
        //  func f() {
        //      const c = 1
        //      c = 2
        //  }
        _UnassignableOperand

        // _NoNewVar occurs when a short variable declaration (':=') does not declare
        // new variables.
        //
        // Example:
        //  func f() {
        //      x := 1
        //      x := 2
        //  }
        _NoNewVar

        // _MultiValAssignOp occurs when an assignment operation (+=, *=, etc) does
        // not have single-valued left-hand or right-hand side.
        //
        // Per the spec:
        //  "In assignment operations, both the left- and right-hand expression lists
        //  must contain exactly one single-valued expression"
        //
        // Example:
        //  func f() int {
        //      x, y := 1, 2
        //      x, y += 1
        //      return x + y
        //  }
        _MultiValAssignOp

        // _InvalidIfaceAssign occurs when a value of type T is used as an
        // interface, but T does not implement a method of the expected interface.
        //
        // Example:
        //  type I interface {
        //      f()
        //  }
        //
        //  type T int
        //
        //  var x I = T(1)
        _InvalidIfaceAssign

        // _InvalidChanAssign occurs when a chan assignment is invalid.
        //
        // Per the spec, a value x is assignable to a channel type T if:
        //  "x is a bidirectional channel value, T is a channel type, x's type V and
        //  T have identical element types, and at least one of V or T is not a
        //  defined type."
        //
        // Example:
        //  type T1 chan int
        //  type T2 chan int
        //
        //  var x T1
        //  // Invalid assignment because both types are named
        //  var _ T2 = x
        _InvalidChanAssign

        // _IncompatibleAssign occurs when the type of the right-hand side expression
        // in an assignment cannot be assigned to the type of the variable being
        // assigned.
        //
        // Example:
        //  var x []int
        //  var _ int = x
        _IncompatibleAssign

        // _UnaddressableFieldAssign occurs when trying to assign to a struct field
        // in a map value.
        //
        // Example:
        //  func f() {
        //      m := make(map[string]struct{i int})
        //      m["foo"].i = 42
        //  }
        _UnaddressableFieldAssign

        /* decls > type (+ other type expression codes) */

        // _NotAType occurs when the identifier used as the underlying type in a type
        // declaration or the right-hand side of a type alias does not denote a type.
        //
        // Example:
        //  var S = 2
        //
        //  type T S
        _NotAType

        // _InvalidArrayLen occurs when an array length is not a constant value.
        //
        // Example:
        //  var n = 3
        //  var _ = [n]int{}
        _InvalidArrayLen

        // _BlankIfaceMethod occurs when a method name is '_'.
        //
        // Per the spec:
        //  "The name of each explicitly specified method must be unique and not
        //  blank."
        //
        // Example:
        //  type T interface {
        //      _(int)
        //  }
        _BlankIfaceMethod

        // _IncomparableMapKey occurs when a map key type does not support the == and
        // != operators.
        //
        // Per the spec:
        //  "The comparison operators == and != must be fully defined for operands of
        //  the key type; thus the key type must not be a function, map, or slice."
        //
        // Example:
        //  var x map[T]int
        //
        //  type T []int
        _IncomparableMapKey

        // _InvalidIfaceEmbed occurs when a non-interface type is embedded in an
        // interface.
        //
        // Example:
        //  type T struct {}
        //
        //  func (T) m()
        //
        //  type I interface {
        //      T
        //  }
        _InvalidIfaceEmbed

        // _InvalidPtrEmbed occurs when an embedded field is of the pointer form *T,
        // and T itself is itself a pointer, an unsafe.Pointer, or an interface.
        //
        // Per the spec:
        //  "An embedded field must be specified as a type name T or as a pointer to
        //  a non-interface type name *T, and T itself may not be a pointer type."
        //
        // Example:
        //  type T *int
        //
        //  type S struct {
        //      *T
        //  }
        _InvalidPtrEmbed

        /* decls > func and method */

        // _BadRecv occurs when a method declaration does not have exactly one
        // receiver parameter.
        //
        // Example:
        //  func () _() {}
        _BadRecv

        // _InvalidRecv occurs when a receiver type expression is not of the form T
        // or *T, or T is a pointer type.
        //
        // Example:
        //  type T struct {}
        //
        //  func (**T) m() {}
        _InvalidRecv

        // _DuplicateFieldAndMethod occurs when an identifier appears as both a field
        // and method name.
        //
        // Example:
        //  type T struct {
        //      m int
        //  }
        //
        //  func (T) m() {}
        _DuplicateFieldAndMethod

        // _DuplicateMethod occurs when two methods on the same receiver type have
        // the same name.
        //
        // Example:
        //  type T struct {}
        //  func (T) m() {}
        //  func (T) m(i int) int { return i }
        _DuplicateMethod

        /* decls > special */

        // _InvalidBlank occurs when a blank identifier is used as a value or type.
        //
        // Per the spec:
        //  "The blank identifier may appear as an operand only on the left-hand side
        //  of an assignment."
        //
        // Example:
        //  var x = _
        _InvalidBlank

        // _InvalidIota occurs when the predeclared identifier iota is used outside
        // of a constant declaration.
        //
        // Example:
        //  var x = iota
        _InvalidIota

        // _MissingInitBody occurs when an init function is missing its body.
        //
        // Example:
        //  func init()
        _MissingInitBody

        // _InvalidInitSig occurs when an init function declares parameters or
        // results.
        //
        // Deprecated: no longer emitted by the type checker. _InvalidInitDecl is
        // used instead.
        _InvalidInitSig

        // _InvalidInitDecl occurs when init is declared as anything other than a
        // function.
        //
        // Example:
        //  var init = 1
        //
        // Example:
        //  func init() int { return 1 }
        _InvalidInitDecl

        // _InvalidMainDecl occurs when main is declared as anything other than a
        // function, in a main package.
        _InvalidMainDecl

        /* exprs */

        // _TooManyValues occurs when a function returns too many values for the
        // expression context in which it is used.
        //
        // Example:
        //  func ReturnTwo() (int, int) {
        //      return 1, 2
        //  }
        //
        //  var x = ReturnTwo()
        _TooManyValues

        // _NotAnExpr occurs when a type expression is used where a value expression
        // is expected.
        //
        // Example:
        //  type T struct {}
        //
        //  func f() {
        //      T
        //  }
        _NotAnExpr

        /* exprs > const */

        // _TruncatedFloat occurs when a float constant is truncated to an integer
        // value.
        //
        // Example:
        //  var _ int = 98.6
        _TruncatedFloat

        // _NumericOverflow occurs when a numeric constant overflows its target type.
        //
        // Example:
        //  var x int8 = 1000
        _NumericOverflow

        /* exprs > operation */

        // _UndefinedOp occurs when an operator is not defined for the type(s) used
        // in an operation.
        //
        // Example:
        //  var c = "a" - "b"
        _UndefinedOp

        // _MismatchedTypes occurs when operand types are incompatible in a binary
        // operation.
        //
        // Example:
        //  var a = "hello"
        //  var b = 1
        //  var c = a - b
        _MismatchedTypes

        // _DivByZero occurs when a division operation is provable at compile
        // time to be a division by zero.
        //
        // Example:
        //  const divisor = 0
        //  var x int = 1/divisor
        _DivByZero

        // _NonNumericIncDec occurs when an increment or decrement operator is
        // applied to a non-numeric value.
        //
        // Example:
        //  func f() {
        //      var c = "c"
        //      c++
        //  }
        _NonNumericIncDec

        /* exprs > ptr */

        // _UnaddressableOperand occurs when the & operator is applied to an
        // unaddressable expression.
        //
        // Example:
        //  var x = &1
        _UnaddressableOperand

        // _InvalidIndirection occurs when a non-pointer value is indirected via the
        // '*' operator.
        //
        // Example:
        //  var x int
        //  var y = *x
        _InvalidIndirection

        /* exprs > [] */

        // _NonIndexableOperand occurs when an index operation is applied to a value
        // that cannot be indexed.
        //
        // Example:
        //  var x = 1
        //  var y = x[1]
        _NonIndexableOperand

        // _InvalidIndex occurs when an index argument is not of integer type,
        // negative, or out-of-bounds.
        //
        // Example:
        //  var s = [...]int{1,2,3}
        //  var x = s[5]
        //
        // Example:
        //  var s = []int{1,2,3}
        //  var _ = s[-1]
        //
        // Example:
        //  var s = []int{1,2,3}
        //  var i string
        //  var _ = s[i]
        _InvalidIndex

        // _SwappedSliceIndices occurs when constant indices in a slice expression
        // are decreasing in value.
        //
        // Example:
        //  var _ = []int{1,2,3}[2:1]
        _SwappedSliceIndices

        /* operators > slice */

        // _NonSliceableOperand occurs when a slice operation is applied to a value
        // whose type is not sliceable, or is unaddressable.
        //
        // Example:
        //  var x = [...]int{1, 2, 3}[:1]
        //
        // Example:
        //  var x = 1
        //  var y = 1[:1]
        _NonSliceableOperand

        // _InvalidSliceExpr occurs when a three-index slice expression (a[x:y:z]) is
        // applied to a string.
        //
        // Example:
        //  var s = "hello"
        //  var x = s[1:2:3]
        _InvalidSliceExpr

        /* exprs > shift */

        // _InvalidShiftCount occurs when the right-hand side of a shift operation is
        // either non-integer, negative, or too large.
        //
        // Example:
        //  var (
        //      x string
        //      y int = 1 << x
        //  )
        _InvalidShiftCount

        // _InvalidShiftOperand occurs when the shifted operand is not an integer.
        //
        // Example:
        //  var s = "hello"
        //  var x = s << 2
        _InvalidShiftOperand

        /* exprs > chan */

        // _InvalidReceive occurs when there is a channel receive from a value that
        // is either not a channel, or is a send-only channel.
        //
        // Example:
        //  func f() {
        //      var x = 1
        //      <-x
        //  }
        _InvalidReceive

        // _InvalidSend occurs when there is a channel send to a value that is not a
        // channel, or is a receive-only channel.
        //
        // Example:
        //  func f() {
        //      var x = 1
        //      x <- "hello!"
        //  }
        _InvalidSend

        /* exprs > literal */

        // _DuplicateLitKey occurs when an index is duplicated in a slice, array, or
        // map literal.
        //
        // Example:
        //  var _ = []int{0:1, 0:2}
        //
        // Example:
        //  var _ = map[string]int{"a": 1, "a": 2}
        _DuplicateLitKey

        // _MissingLitKey occurs when a map literal is missing a key expression.
        //
        // Example:
        //  var _ = map[string]int{1}
        _MissingLitKey

        // _InvalidLitIndex occurs when the key in a key-value element of a slice or
        // array literal is not an integer constant.
        //
        // Example:
        //  var i = 0
        //  var x = []string{i: "world"}
        _InvalidLitIndex

        // _OversizeArrayLit occurs when an array literal exceeds its length.
        //
        // Example:
        //  var _ = [2]int{1,2,3}
        _OversizeArrayLit

        // _MixedStructLit occurs when a struct literal contains a mix of positional
        // and named elements.
        //
        // Example:
        //  var _ = struct{i, j int}{i: 1, 2}
        _MixedStructLit

        // _InvalidStructLit occurs when a positional struct literal has an incorrect
        // number of values.
        //
        // Example:
        //  var _ = struct{i, j int}{1,2,3}
        _InvalidStructLit

        // _MissingLitField occurs when a struct literal refers to a field that does
        // not exist on the struct type.
        //
        // Example:
        //  var _ = struct{i int}{j: 2}
        _MissingLitField

        // _DuplicateLitField occurs when a struct literal contains duplicated
        // fields.
        //
        // Example:
        //  var _ = struct{i int}{i: 1, i: 2}
        _DuplicateLitField

        // _UnexportedLitField occurs when a positional struct literal implicitly
        // assigns an unexported field of an imported type.
        _UnexportedLitField

        // _InvalidLitField occurs when a field name is not a valid identifier.
        //
        // Example:
        //  var _ = struct{i int}{1: 1}
        _InvalidLitField

        // _UntypedLit occurs when a composite literal omits a required type
        // identifier.
        //
        // Example:
        //  type outer struct{
        //      inner struct { i int }
        //  }
        //
        //  var _ = outer{inner: {1}}
        _UntypedLit

        // _InvalidLit occurs when a composite literal expression does not match its
        // type.
        //
        // Example:
        //  type P *struct{
        //      x int
        //  }
        //  var _ = P {}
        _InvalidLit

        /* exprs > selector */

        // _AmbiguousSelector occurs when a selector is ambiguous.
        //
        // Example:
        //  type E1 struct { i int }
        //  type E2 struct { i int }
        //  type T struct { E1; E2 }
        //
        //  var x T
        //  var _ = x.i
        _AmbiguousSelector

        // _UndeclaredImportedName occurs when a package-qualified identifier is
        // undeclared by the imported package.
        //
        // Example:
        //  import "go/types"
        //
        //  var _ = types.NotAnActualIdentifier
        _UndeclaredImportedName

        // _UnexportedName occurs when a selector refers to an unexported identifier
        // of an imported package.
        //
        // Example:
        //  import "reflect"
        //
        //  type _ reflect.flag
        _UnexportedName

        // _UndeclaredName occurs when an identifier is not declared in the current
        // scope.
        //
        // Example:
        //  var x T
        _UndeclaredName

        // _MissingFieldOrMethod occurs when a selector references a field or method
        // that does not exist.
        //
        // Example:
        //  type T struct {}
        //
        //  var x = T{}.f
        _MissingFieldOrMethod

        /* exprs > ... */

        // _BadDotDotDotSyntax occurs when a "..." occurs in a context where it is
        // not valid.
        //
        // Example:
        //  var _ = map[int][...]int{0: {}}
        _BadDotDotDotSyntax

        // _NonVariadicDotDotDot occurs when a "..." is used on the final argument to
        // a non-variadic function.
        //
        // Example:
        //  func printArgs(s []string) {
        //      for _, a := range s {
        //              println(a)
        //      }
        //  }
        //
        //  func f() {
        //      s := []string{"a", "b", "c"}
        //      printArgs(s...)
        //  }
        _NonVariadicDotDotDot

        // _MisplacedDotDotDot occurs when a "..." is used somewhere other than the
        // final argument in a function declaration.
        //
        // Example:
        //      func f(...int, int)
        _MisplacedDotDotDot

        // _InvalidDotDotDot occurs when a "..." is used in a non-variadic built-in
        // function.
        //
        // Example:
        //  var s = []int{1, 2, 3}
        //  var l = len(s...)
        _InvalidDotDotDot

        /* exprs > built-in */

        // _UncalledBuiltin occurs when a built-in function is used as a
        // function-valued expression, instead of being called.
        //
        // Per the spec:
        //  "The built-in functions do not have standard Go types, so they can only
        //  appear in call expressions; they cannot be used as function values."
        //
        // Example:
        //  var _ = copy
        _UncalledBuiltin

        // _InvalidAppend occurs when append is called with a first argument that is
        // not a slice.
        //
        // Example:
        //  var _ = append(1, 2)
        _InvalidAppend

        // _InvalidCap occurs when an argument to the cap built-in function is not of
        // supported type.
        //
        // See https://golang.org/ref/spec#Length_and_capacity for information on
        // which underlying types are supported as arguments to cap and len.
        //
        // Example:
        //  var s = 2
        //  var x = cap(s)
        _InvalidCap

        // _InvalidClose occurs when close(...) is called with an argument that is
        // not of channel type, or that is a receive-only channel.
        //
        // Example:
        //  func f() {
        //      var x int
        //      close(x)
        //  }
        _InvalidClose

        // _InvalidCopy occurs when the arguments are not of slice type or do not
        // have compatible type.
        //
        // See https://golang.org/ref/spec#Appending_and_copying_slices for more
        // information on the type requirements for the copy built-in.
        //
        // Example:
        //  func f() {
        //      var x []int
        //      y := []int64{1,2,3}
        //      copy(x, y)
        //  }
        _InvalidCopy

        // _InvalidComplex occurs when the complex built-in function is called with
        // arguments with incompatible types.
        //
        // Example:
        //  var _ = complex(float32(1), float64(2))
        _InvalidComplex

        // _InvalidDelete occurs when the delete built-in function is called with a
        // first argument that is not a map.
        //
        // Example:
        //  func f() {
        //      m := "hello"
        //      delete(m, "e")
        //  }
        _InvalidDelete

        // _InvalidImag occurs when the imag built-in function is called with an
        // argument that does not have complex type.
        //
        // Example:
        //  var _ = imag(int(1))
        _InvalidImag

        // _InvalidLen occurs when an argument to the len built-in function is not of
        // supported type.
        //
        // See https://golang.org/ref/spec#Length_and_capacity for information on
        // which underlying types are supported as arguments to cap and len.
        //
        // Example:
        //  var s = 2
        //  var x = len(s)
        _InvalidLen

        // _SwappedMakeArgs occurs when make is called with three arguments, and its
        // length argument is larger than its capacity argument.
        //
        // Example:
        //  var x = make([]int, 3, 2)
        _SwappedMakeArgs

        // _InvalidMake occurs when make is called with an unsupported type argument.
        //
        // See https://golang.org/ref/spec#Making_slices_maps_and_channels for
        // information on the types that may be created using make.
        //
        // Example:
        //  var x = make(int)
        _InvalidMake

        // _InvalidReal occurs when the real built-in function is called with an
        // argument that does not have complex type.
        //
        // Example:
        //  var _ = real(int(1))
        _InvalidReal

        /* exprs > assertion */

        // _InvalidAssert occurs when a type assertion is applied to a
        // value that is not of interface type.
        //
        // Example:
        //  var x = 1
        //  var _ = x.(float64)
        _InvalidAssert

        // _ImpossibleAssert occurs for a type assertion x.(T) when the value x of
        // interface cannot have dynamic type T, due to a missing or mismatching
        // method on T.
        //
        // Example:
        //  type T int
        //
        //  func (t *T) m() int { return int(*t) }
        //
        //  type I interface { m() int }
        //
        //  var x I
        //  var _ = x.(T)
        _ImpossibleAssert

        /* exprs > conversion */

        // _InvalidConversion occurs when the argument type cannot be converted to the
        // target.
        //
        // See https://golang.org/ref/spec#Conversions for the rules of
        // convertibility.
        //
        // Example:
        //  var x float64
        //  var _ = string(x)
        _InvalidConversion

        // _InvalidUntypedConversion occurs when an there is no valid implicit
        // conversion from an untyped value satisfying the type constraints of the
        // context in which it is used.
        //
        // Example:
        //  var _ = 1 + ""
        _InvalidUntypedConversion

        /* offsetof */

        // _BadOffsetofSyntax occurs when unsafe.Offsetof is called with an argument
        // that is not a selector expression.
        //
        // Example:
        //  import "unsafe"
        //
        //  var x int
        //  var _ = unsafe.Offsetof(x)
        _BadOffsetofSyntax

        // _InvalidOffsetof occurs when unsafe.Offsetof is called with a method
        // selector, rather than a field selector, or when the field is embedded via
        // a pointer.
        //
        // Per the spec:
        //
        //  "If f is an embedded field, it must be reachable without pointer
        //  indirections through fields of the struct. "
        //
        // Example:
        //  import "unsafe"
        //
        //  type T struct { f int }
        //  type S struct { *T }
        //  var s S
        //  var _ = unsafe.Offsetof(s.f)
        //
        // Example:
        //  import "unsafe"
        //
        //  type S struct{}
        //
        //  func (S) m() {}
        //
        //  var s S
        //  var _ = unsafe.Offsetof(s.m)
        _InvalidOffsetof

        /* control flow > scope */

        // _UnusedExpr occurs when a side-effect free expression is used as a
        // statement. Such a statement has no effect.
        //
        // Example:
        //  func f(i int) {
        //      i*i
        //  }
        _UnusedExpr

        // _UnusedVar occurs when a variable is declared but unused.
        //
        // Example:
        //  func f() {
        //      x := 1
        //  }
        _UnusedVar

        // _MissingReturn occurs when a function with results is missing a return
        // statement.
        //
        // Example:
        //  func f() int {}
        _MissingReturn

        // _WrongResultCount occurs when a return statement returns an incorrect
        // number of values.
        //
        // Example:
        //  func ReturnOne() int {
        //      return 1, 2
        //  }
        _WrongResultCount

        // _OutOfScopeResult occurs when the name of a value implicitly returned by
        // an empty return statement is shadowed in a nested scope.
        //
        // Example:
        //  func factor(n int) (i int) {
        //      for i := 2; i < n; i++ {
        //              if n%i == 0 {
        //                      return
        //              }
        //      }
        //      return 0
        //  }
        _OutOfScopeResult

        /* control flow > if */

        // _InvalidCond occurs when an if condition is not a boolean expression.
        //
        // Example:
        //  func checkReturn(i int) {
        //      if i {
        //              panic("non-zero return")
        //      }
        //  }
        _InvalidCond

        /* control flow > for */

        // _InvalidPostDecl occurs when there is a declaration in a for-loop post
        // statement.
        //
        // Example:
        //  func f() {
        //      for i := 0; i < 10; j := 0 {}
        //  }
        _InvalidPostDecl

        // _InvalidIterVar occurs when two iteration variables are used while ranging
        // over a channel.
        //
        // Example:
        //  func f(c chan int) {
        //      for k, v := range c {
        //              println(k, v)
        //      }
        //  }
        _InvalidIterVar

        // _InvalidRangeExpr occurs when the type of a range expression is not array,
        // slice, string, map, or channel.
        //
        // Example:
        //  func f(i int) {
        //      for j := range i {
        //              println(j)
        //      }
        //  }
        _InvalidRangeExpr

        /* control flow > switch */

        // _MisplacedBreak occurs when a break statement is not within a for, switch,
        // or select statement of the innermost function definition.
        //
        // Example:
        //  func f() {
        //      break
        //  }
        _MisplacedBreak

        // _MisplacedContinue occurs when a continue statement is not within a for
        // loop of the innermost function definition.
        //
        // Example:
        //  func sumeven(n int) int {
        //      proceed := func() {
        //              continue
        //      }
        //      sum := 0
        //      for i := 1; i <= n; i++ {
        //              if i % 2 != 0 {
        //                      proceed()
        //              }
        //              sum += i
        //      }
        //      return sum
        //  }
        _MisplacedContinue

        // _MisplacedFallthrough occurs when a fallthrough statement is not within an
        // expression switch.
        //
        // Example:
        //  func typename(i interface{}) string {
        //      switch i.(type) {
        //      case int64:
        //              fallthrough
        //      case int:
        //              return "int"
        //      }
        //      return "unsupported"
        //  }
        _MisplacedFallthrough

        // _DuplicateCase occurs when a type or expression switch has duplicate
        // cases.
        //
        // Example:
        //  func printInt(i int) {
        //      switch i {
        //      case 1:
        //              println("one")
        //      case 1:
        //              println("One")
        //      }
        //  }
        _DuplicateCase

        // _DuplicateDefault occurs when a type or expression switch has multiple
        // default clauses.
        //
        // Example:
        //  func printInt(i int) {
        //      switch i {
        //      case 1:
        //              println("one")
        //      default:
        //              println("One")
        //      default:
        //              println("1")
        //      }
        //  }
        _DuplicateDefault

        // _BadTypeKeyword occurs when a .(type) expression is used anywhere other
        // than a type switch.
        //
        // Example:
        //  type I interface {
        //      m()
        //  }
        //  var t I
        //  var _ = t.(type)
        _BadTypeKeyword

        // _InvalidTypeSwitch occurs when .(type) is used on an expression that is
        // not of interface type.
        //
        // Example:
        //  func f(i int) {
        //      switch x := i.(type) {}
        //  }
        _InvalidTypeSwitch

        // _InvalidExprSwitch occurs when a switch expression is not comparable.
        //
        // Example:
        //  func _() {
        //      var a struct{ _ func() }
        //      switch a /* ERROR cannot switch on a */ {
        //      }
        //  }
        _InvalidExprSwitch

        /* control flow > select */

        // _InvalidSelectCase occurs when a select case is not a channel send or
        // receive.
        //
        // Example:
        //  func checkChan(c <-chan int) bool {
        //      select {
        //      case c:
        //              return true
        //      default:
        //              return false
        //      }
        //  }
        _InvalidSelectCase

        /* control flow > labels and jumps */

        // _UndeclaredLabel occurs when an undeclared label is jumped to.
        //
        // Example:
        //  func f() {
        //      goto L
        //  }
        _UndeclaredLabel

        // _DuplicateLabel occurs when a label is declared more than once.
        //
        // Example:
        //  func f() int {
        //  L:
        //  L:
        //      return 1
        //  }
        _DuplicateLabel

        // _MisplacedLabel occurs when a break or continue label is not on a for,
        // switch, or select statement.
        //
        // Example:
        //  func f() {
        //  L:
        //      a := []int{1,2,3}
        //      for _, e := range a {
        //              if e > 10 {
        //                      break L
        //              }
        //              println(a)
        //      }
        //  }
        _MisplacedLabel

        // _UnusedLabel occurs when a label is declared but not used.
        //
        // Example:
        //  func f() {
        //  L:
        //  }
        _UnusedLabel

        // _JumpOverDecl occurs when a label jumps over a variable declaration.
        //
        // Example:
        //  func f() int {
        //      goto L
        //      x := 2
        //  L:
        //      x++
        //      return x
        //  }
        _JumpOverDecl

        // _JumpIntoBlock occurs when a forward jump goes to a label inside a nested
        // block.
        //
        // Example:
        //  func f(x int) {
        //      goto L
        //      if x > 0 {
        //      L:
        //              print("inside block")
        //      }
        // }
        _JumpIntoBlock

        /* control flow > calls */

        // _InvalidMethodExpr occurs when a pointer method is called but the argument
        // is not addressable.
        //
        // Example:
        //  type T struct {}
        //
        //  func (*T) m() int { return 1 }
        //
        //  var _ = T.m(T{})
        _InvalidMethodExpr

        // _WrongArgCount occurs when too few or too many arguments are passed by a
        // function call.
        //
        // Example:
        //  func f(i int) {}
        //  var x = f()
        _WrongArgCount

        // _InvalidCall occurs when an expression is called that is not of function
        // type.
        //
        // Example:
        //  var x = "x"
        //  var y = x()
        _InvalidCall

        /* control flow > suspended */

        // _UnusedResults occurs when a restricted expression-only built-in function
        // is suspended via go or defer. Such a suspension discards the results of
        // these side-effect free built-in functions, and therefore is ineffectual.
        //
        // Example:
        //  func f(a []int) int {
        //      defer len(a)
        //      return i
        //  }
        _UnusedResults

        // _InvalidDefer occurs when a deferred expression is not a function call,
        // for example if the expression is a type conversion.
        //
        // Example:
        //  func f(i int) int {
        //      defer int32(i)
        //      return i
        //  }
        _InvalidDefer

        // _InvalidGo occurs when a go expression is not a function call, for example
        // if the expression is a type conversion.
        //
        // Example:
        //  func f(i int) int {
        //      go int32(i)
        //      return i
        //  }
        _InvalidGo

        // _Todo is a placeholder for error codes that have not been decided.
        // TODO(rFindley) remove this error code after deciding on errors for generics code.
        _Todo
)