ValidateFunc should support validation of two fields dependent on each other that are defined in resource schema

[saloneerege]

Current Terraform Version

Terraform v0.12.20

Use-cases

I am writing a custom provider in which I have 2 fields defined in the schema namely : host_instance_type and storage_capacity. What is the best way to write a validation function for storage_capacity whose behavior is different for different host_instance_type ? In this the storage_capacity, must be specified only for a specific host_instance_type (say A). But storage_capacity should not be specified for a host_instance_type (say B).

Attempted Solutions

Currently there seems to be no solution for performing such a validation in the terraform plan stage. I can perform such a validation in terraform apply which might not be the most efficient way to do it.

Proposal

ValidateFunc should support a method where two fields of the resource can be compared with a custom validate function.

References

[paultyng]

I think this is the same issue as #233

Until a better way to support this is added, you do have a Plan time hook with the CustomizeDiff method. You can look at other providers who have utilized it to write more sophisticated multi-attribute validation at Plan time.

[ghost]

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