In case of RIL-RPL merger, any exchange of shares held in RPL (the amalgamating company) will not be considered as a sale and consequently there will be no capital gains/ loss as the transfer is made in consideration for being allotted shares in RIL (the amalgamated company). However post-merger, the treatment of capital gain tax in case of newly acquired shares of RIL is based on the respective holding period of RPL shares. The author Shanbhag has explained this in details in the following article.

RIL-RPL merger: Tax incidence on shareholders
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Source: DNA Money