case *Basic:
		if !isValid(u) {
			// error is reported elsewhere
			return &emptyInterface
		}
	case *Interface:
		if isTypeParam(bound) {
			// error is reported in Checker.collectTypeParams
			return &emptyInterface
		}
		ityp = u
	}

	// If we don't have an interface, wrap constraint into an implicit interface.
	if ityp == nil {
		ityp = NewInterfaceType(nil, []Type{bound})

	case *Basic:
		if !isValid(u) {
			// error is reported elsewhere
			return &emptyInterface
		}
	case *Interface:
		if isTypeParam(bound) {
			// error is reported in Checker.collectTypeParams
			return &emptyInterface
		}
		ityp = u
	}

	// If we don't have an interface, wrap constraint into an implicit interface.
	if ityp == nil {
		ityp = NewInterfaceType(nil, []Type{bound})