This could be done in 2 steps:
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A compiler switch to consider variant expressions as errors, and continue with the current wrong evaluator. Something as “Error: Don’t use Variants in expressions. Convert such values to static types.”
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A new evaluator is created fixing the current bugs. Once released, the Error switch for variant expressions can be demoted to a Warning as “Warning: There’s an expression with a variant type here. Be sure it’s correct.” an the build continues anyway.