msg := check.sprintf(format, args...)
	err := check.newError(UnsupportedFeature)
	err.addf(at, "%s requires %s or later", msg, v)
	err.report()
}

// atPos reports the left (= start) position of at.
func atPos(at poser) syntax.Pos {
	switch x := at.(type) {
	case *operand:
		if x.expr != nil {
			return syntax.StartPos(x.expr)
		}
	case syntax.Node:
		return syntax.StartPos(x)
	}

	msg := check.sprintf(format, args...)
	err := check.newError(UnsupportedFeature)
	err.addf(at, "%s requires %s or later", msg, v)
	err.report()
}

// atPos wraps a token.Pos to implement the positioner interface.
type atPos token.Pos

func (s atPos) Pos() token.Pos {
	return token.Pos(s)
}

// posSpan holds a position range along with a highlighted position within that

		// TODO(gri) We should report exactly what went wrong. At the
		//           moment we don't have the (go/constant) API for that.
		//           See also TODO in go/constant/value.go.
		check.error(atPos(opPos), InvalidConstVal, "constant result is not representable")
		return
	}

	// Typed constants must be representable in
	// their type after each constant operation.
	// x.typ cannot be a type parameter (type

		// TODO(gri) We should report exactly what went wrong. At the
		//           moment we don't have the (go/constant) API for that.
		//           See also TODO in go/constant/value.go.
		check.error(atPos(opPos), InvalidConstVal, "constant result is not representable")
		return
	}

	// Typed constants must be representable in
	// their type after each constant operation.
	// x.typ cannot be a type parameter (type

func hasDots(call *ast.CallExpr) bool { return call.Ellipsis.IsValid() }

// dddErrPos returns the positioner for reporting an invalid ... use in a call.
func dddErrPos(call *ast.CallExpr) positioner { return atPos(call.Ellipsis) }

// argErrPos returns positioner for reporting an invalid argument count.
func argErrPos(call *ast.CallExpr) positioner { return inNode(call, call.Rparen) }

// startPos returns the start position of node n.

		switch obj.Type().(type) {
		default:
			panic("unexpected type")
		case *Named:
			err.addf(atPos(edge.pos), "%s implicitly parameterized by %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
		case *TypeParam:
			err.addf(atPos(edge.pos), "%s instantiated as %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
		}
	}
	err.report()
}

		switch obj.Type().(type) {
		default:
			panic("unexpected type")
		case *Named:
			err.addf(atPos(edge.pos), "%s implicitly parameterized by %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
		case *TypeParam:
			err.addf(atPos(edge.pos), "%s instantiated as %s", obj.Name(), TypeString(edge.typ, qf)) // secondary error, \t indented
		}
	}
	err.report()
}

	return nil // invalid embedded field
}

func (check *Checker) declareInSet(oset *objset, pos token.Pos, obj Object) bool {
	if alt := oset.insert(obj); alt != nil {
		err := check.newError(DuplicateDecl)
		err.addf(atPos(pos), "%s redeclared", obj.Name())
		err.addAltDecl(alt)
		err.report()
		return false
	}
	return true
}

func (check *Checker) tag(t *ast.BasicLit) string {
	if t != nil {

		if ityp.embedPos == nil {
			ityp.embedPos = new([]syntax.Pos)
		}
		*ityp.embedPos = append(*ityp.embedPos, pos)
	}

	for _, f := range iface.MethodList {
		if f.Name == nil {
			addEmbedded(atPos(f.Type), parseUnion(check, f.Type))
			continue
		}
		// f.Name != nil

		// We have a method with name f.Name.
		name := f.Name.Value
		if name == "_" {