error when applying plan when undeclared variables exist

[jbardin]

#35903 doesn't take into account the presence of undeclared variables, so when any automatically applied variable files exist in the configuration directory, apply can't proceed.

Reproduction: apply any configuration which has a terraform.tfvars or *.auto.tfvars* file with a variable assignment not declared in the configuration.

[bschaatsbergen]

In addition to automatically applied variable files, a "Value for undeclared variable" error diagnostic is also thrown when a named file is used. This now happens when there's an undeclared variable declaration: https://github.com/hashicorp/terraform/blob/main/internal/backend/local/backend_apply.go#L272-L285.

We already produce warning diagnostics for undeclared variables when parsing variables values: https://github.com/hashicorp/terraform/blob/main/internal/backend/local/backend_local.go#L194 - which makes me believe we no longer need the first diagnostic I've linked to. However, this is only part of the issue—the error message you’re seeing comes from when a set variable cannot be found in the list of declared variables in the root module configuration, causing this else and error diagnostic to trigger.

[github-actions]

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