Create account link on login page incorrectly appends register to URL

[ThoreKr]

Steps to reproduce

1. A synapse server with a single oidc_provider configured as only identity provider.
2. The .well-known options for m.authentication also point to the same identity provider.
3. Instead of "Continue with click on "Create account" below.
4. A redirect to the following URL happens: https://[...]/_matrix/client/r0/login/sso/redirect/register?redirectUrl=https%3A%2F%2Fc[...]%2F

Outcome

What did you expect?

A redirect to https://[...]/_matrix/client/r0/login/sso/redirect?redirectUrl=https%3A%2F%2Fc[...]%2F
note the missing register in the path.

What happened instead?

Synapse interprets the register as the idp id and fails at this point: https://github.com/matrix-org/synapse/blob/ce00e57a2acc7c06d0e3bf4c9880dc830c6938ef/synapse/handlers/sso.py#L319

returning

{"errcode":"M_NOT_FOUND","error":"Unknown identity provider"}

Operating system

Any

Browser information

Any

URL for webapp

develop.element.io (just as a test if it can be reproduced there)

Application version

Custom deployed: version: 1.11.25 Olm version: 3.2.12

Homeserver

Synapse 1.79.0.

Will you send logs?

No

[felix-two-tone]

Having this same issue. We use a OIDC with Keycloak and get the same incorrect redirect.

[t3chguy]

Regressed by matrix-org/matrix-react-sdk#8681

[t3chguy]

Affects all HSes with an id-less idp (legacy)

[t3chguy]

We should be using const enums instead of string enums to allow Typescript to protect us here.