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go/types: document that predicates are undefined on generic types

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Fixes #50887

Change-Id: I451d66b067badcfb7cf2e2756ea2b062366ac9d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/390039
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Robert Findley 2022-03-04 14:39:43 -05:00
parent 43b09c096a
commit 20dd9a42fb
2 changed files with 36 additions and 8 deletions
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22
src/cmd/compile/internal/types2/api.go
View File

@ -421,8 +421,11 @@ func (conf *Config) Check(path string, files []*syntax.File, info *Info) (*Packa
} }
// AssertableTo reports whether a value of type V can be asserted to have type T. // AssertableTo reports whether a value of type V can be asserted to have type T.
// The behavior of AssertableTo is undefined if V is a generalized interface; i.e., //
// an interface that may only be used as a type constraint in Go code. // The behavior of AssertableTo is undefined in two cases:
// - if V is a generalized interface; i.e., an interface that may only be used
// as a type constraint in Go code
// - if T is an uninstantiated generic type
func AssertableTo(V *Interface, T Type) bool { func AssertableTo(V *Interface, T Type) bool {
// Checker.newAssertableTo suppresses errors for invalid types, so we need special // Checker.newAssertableTo suppresses errors for invalid types, so we need special
// handling here. // handling here.
@ -432,20 +435,31 @@ func AssertableTo(V *Interface, T Type) bool {
return (*Checker)(nil).newAssertableTo(V, T) == nil return (*Checker)(nil).newAssertableTo(V, T) == nil
} }
// AssignableTo reports whether a value of type V is assignable to a variable of type T. // AssignableTo reports whether a value of type V is assignable to a variable
// of type T.
//
// The behavior of AssignableTo is undefined if V or T is an uninstantiated
// generic type.
func AssignableTo(V, T Type) bool { func AssignableTo(V, T Type) bool {
x := operand{mode: value, typ: V} x := operand{mode: value, typ: V}
ok, _ := x.assignableTo(nil, T, nil) // check not needed for non-constant x ok, _ := x.assignableTo(nil, T, nil) // check not needed for non-constant x
return ok return ok
} }
// ConvertibleTo reports whether a value of type V is convertible to a value of type T. // ConvertibleTo reports whether a value of type V is convertible to a value of
// type T.
//
// The behavior of ConvertibleTo is undefined if V or T is an uninstantiated
// generic type.
func ConvertibleTo(V, T Type) bool { func ConvertibleTo(V, T Type) bool {
x := operand{mode: value, typ: V} x := operand{mode: value, typ: V}
return x.convertibleTo(nil, T, nil) // check not needed for non-constant x return x.convertibleTo(nil, T, nil) // check not needed for non-constant x
} }
// Implements reports whether type V implements interface T. // Implements reports whether type V implements interface T.
//
// The behavior of Implements is undefined if V is an uninstantiated generic
// type.
func Implements(V Type, T *Interface) bool { func Implements(V Type, T *Interface) bool {
if T.Empty() { if T.Empty() {
// All types (even Typ[Invalid]) implement the empty interface. // All types (even Typ[Invalid]) implement the empty interface.

22
src/go/types/api.go
View File

@ -417,8 +417,11 @@ func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, i
} }
// AssertableTo reports whether a value of type V can be asserted to have type T. // AssertableTo reports whether a value of type V can be asserted to have type T.
// The behavior of AssertableTo is undefined if V is a generalized interface; i.e., //
// an interface that may only be used as a type constraint in Go code. // The behavior of AssertableTo is undefined in two cases:
// - if V is a generalized interface; i.e., an interface that may only be used
// as a type constraint in Go code
// - if T is an uninstantiated generic type
func AssertableTo(V *Interface, T Type) bool { func AssertableTo(V *Interface, T Type) bool {
// Checker.newAssertableTo suppresses errors for invalid types, so we need special // Checker.newAssertableTo suppresses errors for invalid types, so we need special
// handling here. // handling here.
@ -428,20 +431,31 @@ func AssertableTo(V *Interface, T Type) bool {
return (*Checker)(nil).newAssertableTo(V, T) == nil return (*Checker)(nil).newAssertableTo(V, T) == nil
} }
// AssignableTo reports whether a value of type V is assignable to a variable of type T. // AssignableTo reports whether a value of type V is assignable to a variable
// of type T.
//
// The behavior of AssignableTo is undefined if V or T is an uninstantiated
// generic type.
func AssignableTo(V, T Type) bool { func AssignableTo(V, T Type) bool {
x := operand{mode: value, typ: V} x := operand{mode: value, typ: V}
ok, _ := x.assignableTo(nil, T, nil) // check not needed for non-constant x ok, _ := x.assignableTo(nil, T, nil) // check not needed for non-constant x
return ok return ok
} }
// ConvertibleTo reports whether a value of type V is convertible to a value of type T. // ConvertibleTo reports whether a value of type V is convertible to a value of
// type T.
//
// The behavior of ConvertibleTo is undefined if V or T is an uninstantiated
// generic type.
func ConvertibleTo(V, T Type) bool { func ConvertibleTo(V, T Type) bool {
x := operand{mode: value, typ: V} x := operand{mode: value, typ: V}
return x.convertibleTo(nil, T, nil) // check not needed for non-constant x return x.convertibleTo(nil, T, nil) // check not needed for non-constant x
} }
// Implements reports whether type V implements interface T. // Implements reports whether type V implements interface T.
//
// The behavior of Implements is undefined if V is an uninstantiated generic
// type.
func Implements(V Type, T *Interface) bool { func Implements(V Type, T *Interface) bool {
if T.Empty() { if T.Empty() {
// All types (even Typ[Invalid]) implement the empty interface. // All types (even Typ[Invalid]) implement the empty interface.