from the just-like-Watergate NORMS and CONVENTIONS corner of the twitterverse:

But so far @realdonaldTrump has not been impeached for his indictable crimes. Once that happens, he can not be pardoned.
It's probably why Pelosi is holding that over his head right now and talking about impeaching him for obstruction of justice as laid out in The Mueller Report.

— Cheri Jacobus (@CheriJacobus) December 30, 2019

hmmm
President Ford Pardons Richard Nixon
"A month after taking office, President Gerald Ford, addressed the nation on television [A/V, transcript] to announce that he had decided to pardon Richard Nixon."
BONUS!: Full Text of the Pardon Proclamation"

reference
Ex parte Garland, 71 U.S. 4 Wall. 333 333 (1866)
enemies of the state, excluding impeached officers of the state
CRS, "An Overview of the Presidential Pardoning Power"
Miscellany: Always read the footnotes.