Dealing with extensions overriding arithmetic operators such as php-decimal

I am working on a code base that makes heavy use of php-decimal[0].
The extension provides a new type "Decimal" which also supports arithemtic and comparion operators.
This causes Phan to issue a bunch of PhanTypeInvalid{Left|Right}OperandOf*Op.

Current Workarounds:

1. Wrap every arithmetic operation in a new Decimal, "return new Decimal($dec1 + $dec2)"
2. add all emitted issues types caused by Decimal to the global suppress_issue_types array.

So the question is: how would one deal with this?

1. Is there a way to tell Phan to treat Decimal just like a regular number?
2. What part of the source would one have to read to extend Phan to make it work with Decimal?

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[0] http://php-decimal.io/#introduction

No source file, test, or entry point is named. Start by locating Phan's checks for invalid arithmetic and comparison operands, then inspect how extension-provided Decimal types are represented; done means a documented, tested way to recognize such types without suppressing unrelated diagnostics.