all: replace calls to errors.As with errors.AsType

This change replaces most occurrences (in code as well as in comments) of
errors.As with errors.AsType. It leaves the errors package and vendored
code untouched.

Change-Id: I3bde73f318a0b408bdb8f5a251494af15a13118a
GitHub-Last-Rev: 8aaaa36
GitHub-Pull-Request: #75698
Reviewed-on: https://go-review.googlesource.com/c/go/+/708495
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

conn.go

@@ -1645,9 +1645,9 @@ func (c *Conn) handshakeContext(ctx context.Context) (ret error) {
 			// the handshake (RFC 9001, Section 5.7).
 			c.quicSetReadSecret(QUICEncryptionLevelApplication, c.cipherSuite, c.in.trafficSecret)
 		} else {
-			var a alert
 			c.out.Lock()
-			if !errors.As(c.out.err, &a) {
+			a, ok := errors.AsType[alert](c.out.err)
+			if !ok {
 				a = alertInternalError
 			}
 			c.out.Unlock()

handshake_server.go

@@ -948,10 +948,9 @@ func (c *Conn) processCertsFromClient(certificate Certificate) error {
 
 		chains, err := certs[0].Verify(opts)
 		if err != nil {
-			var errCertificateInvalid x509.CertificateInvalidError
-			if errors.As(err, &x509.UnknownAuthorityError{}) {
+			if _, ok := errors.AsType[x509.UnknownAuthorityError](err); ok {
 				c.sendAlert(alertUnknownCA)
-			} else if errors.As(err, &errCertificateInvalid) && errCertificateInvalid.Reason == x509.Expired {
+			} else if errCertificateInvalid, ok := errors.AsType[x509.CertificateInvalidError](err); ok && errCertificateInvalid.Reason == x509.Expired {
 				c.sendAlert(alertCertificateExpired)
 			} else {
 				c.sendAlert(alertBadCertificate)

quic.go

@@ -363,12 +363,11 @@ func quicError(err error) error {
 	if err == nil {
 		return nil
 	}
-	var ae AlertError
-	if errors.As(err, &ae) {
+	if _, ok := errors.AsType[AlertError](err); ok {
 		return err
 	}
-	var a alert
-	if !errors.As(err, &a) {
+	a, ok := errors.AsType[alert](err)
+	if !ok {
 		a = alertInternalError
 	}
 	// Return an error wrapping the original error and an AlertError.