Bug: Invalid code generation for preview subobjects

[theunrepentantgeek]

Describe the bug

When a subobject exists only in preview versions of a resource, and that subobject references another object that exists across both preview and stable versions of that resource, we generate invalid conversion code that does not compile.

To Reproduce

Discovered when importing a second preview version of containerservice ManagedCluster.

The subobject ManagedClusterIngressProfileWebAppRouting_STATUS exists only in preview versions of ManagedCluster. It's conversion graph is:

When converting the field ManagedClusterIngressProfileWebAppRouting_STATUS.Identity, of type UserAssignedIdentity_STATUS, we need to convert between versions v20230702ps and v20230202ps.

However, UserAssignedIdentity_STATUS exists in stable versions as well, and has this conversion graph:

Currently the code generator expects to always follow conversions in the same direction, either forwards or backwards.

From v20230702ps it converts forward to v20230701s and then it expects to convert forward from there, which is impossible.

Expected behavior

After conversion to v20230701s, the generated code should work backwards, converting first to v20230201s and then to the final version v20230202ps.

Additional context

Additional bugness: this illegal situation shouldn't have ever generated bad code - it should have returned an error.

[theunrepentantgeek]

Resolved by #3641