go.tools/go/types: export Comparable and Assertable, simplify Implements

The "static bool" parameter to Implements was confusing; typically we think about interface implementation and type assertion as separate but related concepts, but here they were merged.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/58540044

go/types/api.go

@@ -226,6 +226,12 @@ func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, i
 	return pkg, err
 }
 
+// Assertable reports whether a value of type V can be asserted to have type T.
+func Assertable(V *Interface, T Type) bool {
+	f, _ := MissingMethod(T, V, false)
+	return f == 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}
@@ -238,19 +244,8 @@ func ConvertibleTo(V, T Type) bool {
 	return x.convertibleTo(nil, T) // config not needed for non-constant x
 }
 
-// Implements reports whether a value of type V implements T, as follows:
-//
-// 1) For non-interface types V, or if static is set, V implements T if all
-// methods of T are present in V. Informally, this reports whether V is a
-// subtype of T.
-//
-// 2) For interface types V, and if static is not set, V implements T if all
-// methods of T which are also present in V have matching types. Informally,
-// this indicates whether a type assertion x.(T) where x is of type V would
-// be legal (the concrete dynamic type of x may implement T even if V does
-// not statically implement it).
-//
-func Implements(V Type, T *Interface, static bool) bool {
-	f, _ := MissingMethod(V, T, static)
+// Implements reports whether type V implements interface T.
+func Implements(V Type, T *Interface) bool {
+	f, _ := MissingMethod(V, T, true)
 	return f == nil
 }

go/types/expr.go

@@ -567,7 +567,7 @@ func (check *checker) comparison(x, y *operand, op token.Token) {
 		switch op {
 		case token.EQL, token.NEQ:
 			// spec: "The equality operators == and != apply to operands that are comparable."
-			defined = isComparable(x.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
+			defined = Comparable(x.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
 		case token.LSS, token.LEQ, token.GTR, token.GEQ:
 			// spec: The ordering operators <, <=, >, and >= apply to operands that are ordered."
 			defined = isOrdered(x.typ)

go/types/lookup.go

@@ -242,7 +242,7 @@ func consolidateMultiples(list []embeddedType) []embeddedType {
 // methods of T are present in V. Otherwise (V is an interface and static
 // is not set), MissingMethod only checks that methods of T which are also
 // present in V have matching types (e.g., for a type assertion x.(T) where
-// x is of interface type typ).
+// x is of interface type V).
 //
 func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
 	// fast path for common case

go/types/operand.go

@@ -217,7 +217,7 @@ func (x *operand) assignableTo(conf *Config, T Type) bool {
 	// T is an interface type and x implements T
 	// (Do this check first as it might succeed early.)
 	if Ti, ok := Tu.(*Interface); ok {
-		if m, _ := MissingMethod(x.typ, Ti, true); m == nil {
+		if Implements(x.typ, Ti) {
 			return true
 		}
 	}

go/types/predicates.go

@@ -76,8 +76,9 @@ func isInterface(typ Type) bool {
 	return ok
 }
 
-func isComparable(typ Type) bool {
-	switch t := typ.Underlying().(type) {
+// Comparable reports whether values of type T are comparable.
+func Comparable(T Type) bool {
+	switch t := T.Underlying().(type) {
 	case *Basic:
 		// assume invalid types to be comparable
 		// to avoid follow-up errors
@@ -86,13 +87,13 @@ func isComparable(typ Type) bool {
 		return true
 	case *Struct:
 		for _, f := range t.fields {
-			if !isComparable(f.typ) {
+			if !Comparable(f.typ) {
 				return false
 			}
 		}
 		return true
 	case *Array:
-		return isComparable(t.elem)
+		return Comparable(t.elem)
 	}
 	return false
 }

go/types/typexpr.go

@@ -296,7 +296,7 @@ func (check *checker) typInternal(e ast.Expr, def *Named, path []*TypeName) Type
 		// Delay this check because it requires fully setup types;
 		// it is safe to continue in any case (was issue 6667).
 		check.delay(func() {
-			if !isComparable(typ.key) {
+			if !Comparable(typ.key) {
 				check.errorf(e.Key.Pos(), "invalid map key type %s", typ.key)
 			}
 		})