[Bug]: New model UserAssignedIdentities will cause breaking changes for SDK

[XiaofeiCao]

Describe the bug

In 0.42, tsp-arm seems to introduce a new model UserAssignedIdentities:

typespec-azure/packages/typespec-azure-resource-manager/lib/common-types/managed-identity.tsp

Line 37 in ac594eb

model UserAssignedIdentities is Record<UserAssignedIdentity>;

though previously, it's a property whose type is defined as Record:

typespec-azure/packages/typespec-azure-resource-manager/lib/arm.foundations.tsp

Line 398 in 4c7a78a

userAssignedIdentities?: Record<UserAssignedIdentity>;

TCGC will generate a new model UserAssignedIdentities if it's defined is Record, which changes Java's generated ManagedServiceIdentity's userAssignedIdentities property type from Map<String, UserAssignedIdentity> to nowUserAssignedIdentities. This a breaking change. See Azure/autorest.java#2698 (comment)

I wonder why we decide to change the UserAssignedIdenties definition?
BTW, isn't model is Record discouraged in ARM?
https://azure.github.io/typespec-azure/docs/libraries/azure-resource-manager/rules/no-record#-incorrect-1

Reproduction

It happens for SDK generation. In Swagger, it refers to the common model.

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the TypeSpec repo
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.

[msyyc]

Python has similar issue, too.

[weidongxu-microsoft]

My guess is that the discourage of Record is for generic usage of additionalProperties.

I think the easiest fix would be as #825 mentioned to revert it to userAssignedIdentities?: Record<UserAssignedIdentity>.

[XiaofeiCao]

duplicate, discussion in #825