if http11fallback {
		return "", nil
	}
	return "", fmt.Errorf("tls: client requested unsupported application protocols (%s)", clientProtos)
}

// supportsECDHE returns whether ECDHE key exchanges can be used with this
// pre-TLS 1.3 client.
func supportsECDHE(c *Config, version uint16, supportedCurves []CurveID, supportedPoints []uint8) bool {
	supportsCurve := false
	for _, curve := range supportedCurves {

	// only relevant to the AEAD choice, and static RSA does not exist.
	if vers == VersionTLS13 {
		return nil
	}

	// The only signed key exchange we support is ECDHE.
	if !supportsECDHE(config, vers, chi.SupportedCurves, chi.SupportedPoints) {
		return supportsRSAFallback(errors.New("client doesn't support ECDHE, can only use legacy RSA key exchange"))
	}

	var ecdsaCipherSuite bool