cmd/compile/internal/types2: add Interface.IsMethodSet, remove Interface.IsContraint

This is a port of CL 352616 from go/types to types2. It also removes
Interface.IsConstraint and adjusts all uses to use IsMethodSet. The
dual changes are made to the (unexported) type set implementation.

Change-Id: I292b741d1f7cdbaefb483eed75faf7b85a8d2792
Reviewed-on: https://go-review.googlesource.com/c/go/+/352872
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/noder/writer.go

@@ -1687,7 +1687,7 @@ func (w *writer) pkgDecl(decl syntax.Decl) {
 		name := w.p.info.Defs[decl.Name].(*types2.TypeName)
 		// Skip type declarations for interfaces that are only usable as
 		// type parameter bounds.
-		if iface, ok := name.Type().Underlying().(*types2.Interface); ok && iface.IsConstraint() {
+		if iface, ok := name.Type().Underlying().(*types2.Interface); ok && !iface.IsMethodSet() {
 			break
 		}
 

src/cmd/compile/internal/types2/call.go

@@ -103,7 +103,7 @@ func (check *Checker) callExpr(x *operand, call *syntax.CallExpr) exprKind {
 			check.expr(x, call.ArgList[0])
 			if x.mode != invalid {
 				if t := asInterface(T); t != nil {
-					if t.IsConstraint() {
+					if !t.IsMethodSet() {
 						check.errorf(call, "cannot use interface %s in conversion (contains type list or is comparable)", T)
 						break
 					}

src/cmd/compile/internal/types2/decl.go

@@ -534,7 +534,7 @@ func (check *Checker) isImportedConstraint(typ Type) bool {
 		return false
 	}
 	u, _ := named.under().(*Interface)
-	return u != nil && u.IsConstraint()
+	return u != nil && !u.IsMethodSet()
 }
 
 func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named) {

src/cmd/compile/internal/types2/interface.go

@@ -98,8 +98,8 @@ func (t *Interface) Empty() bool { return t.typeSet().IsAll() }
 // IsComparable reports whether each type in interface t's type set is comparable.
 func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 
-// IsConstraint reports whether interface t is not just a method set.
+// IsMethodSet reports whether the interface t is fully described by its method set.
-func (t *Interface) IsConstraint() bool { return t.typeSet().IsConstraint() }
+func (t *Interface) IsMethodSet() bool { return t.typeSet().IsMethodSet() }
 
 func (t *Interface) Underlying() Type { return t }
 func (t *Interface) String() string   { return TypeString(t, nil) }

src/cmd/compile/internal/types2/typeset.go

@@ -30,8 +30,8 @@ func (s *_TypeSet) IsAll() bool {
 	return !s.comparable && len(s.methods) == 0 && s.terms.isAll()
 }
 
-// IsConstraint reports whether type set s is not just a set of methods.
+// IsMethodSet reports whether the interface t is fully described by its method set.
-func (s *_TypeSet) IsConstraint() bool { return s.comparable || !s.terms.isAll() }
+func (s *_TypeSet) IsMethodSet() bool { return !s.comparable && s.terms.isAll() }
 
 // IsComparable reports whether each type in the set is comparable.
 func (s *_TypeSet) IsComparable() bool {

src/cmd/compile/internal/types2/typexpr.go

@@ -143,7 +143,7 @@ func (check *Checker) varType(e syntax.Expr) Type {
 		if t := asInterface(typ); t != nil {
 			pos := syntax.StartPos(e)
 			tset := computeInterfaceTypeSet(check, pos, t) // TODO(gri) is this the correct position?
-			if tset.IsConstraint() {
+			if !tset.IsMethodSet() {
 				if tset.comparable {
 					check.softErrorf(pos, "interface is (or embeds) comparable")
 				} else {