Gemara

49b (Mishnah): A Nazir shaves (Tiglachas Tum'ah) for these Tum'os:

A Mes; a k'Zayis of a Mes; a k'Zayis of Netzel (fluid that exuded from a Mes); a spoon's volume of a decayed Mes; a spine and (or) skull; a limb that separated from the body, in life or after death, with sufficient flesh on it; half a Kav of bones; and half a Log of blood.

If he touched, moved or was in an Ohel (under a common roof) with any of these, or touched or moved a bone k'Se'orah (the size of a barley seed), he must shave.

53a: The Reisha says 'for these' to hint at something for which he does not shave, i.e. entering an Ohel with a bone k'Se'orah. In the Seifa, these words allude to another such matter, i.e. a Sechuchis stone (one on a Mes or grave).

54a (Mishnah): A Nazir does not shave for the following Tum'os:

Schachos or Pera'os (foliage of trees hanging over the ground, and rocks protruding from a wall); a field in which a grave was lost or plowed; Chutz La'aretz; Golel (a coffin cover or monument), Dofek (its supporting stones), a Revi'is of blood; a quarter Kav of bones; Kelim that touched a Mes...

56b (Mishnah - R. Akiva): A bone k'Se'orah is not Metamei Tum'as Ohel, yet a Nazir shaves for touching or moving it. A Revi'is of blood is Metamei b'Ohel, all the more so a Nazir should shave for touching or moving it!

R. Eliezer: No, you cannot make a Kal va'Chomer from a tradition from Sinai.

R. Yehoshua: R. Akiva would be correct (normally, we can make a Kal va'Chomer from a tradition), but here the tradition opposes it.

Tosefta (Makos 4:17): A Kohen is lashed mid'Oraisa for any Tum'ah for which a Nazir is Megale'ach. If he enters a Beis ha'Peras or goes to Chutz la'Aretz, he is lashed mid'Rabanan.

Rishonim

Rif and Rosh (Hilchos Tum'ah 1a and 1, citing Toras Kohanim): "Lo Yitama Ba'al b'Amav" warns Kohanim only about a (full) Mes. "L'Nefesh" warns about blood. "Lo Yitama" warns about other Tum'os from a Mes.

Rosh: Maseches Semachos (4:21) says that a Kohen is not lashed for any Tum'ah for which a Nazir is not Megale'ach.

Rambam (Hilchos Evel 3:1): A Kohen is lashed for becoming Tamei to a Mes except for relatives for whom he is permitted. This is whether he became Tamei through touching, Ohel or moving a Mes or Tum'os that separated from a Mes.

Rambam (Hilchos Tum'as Mes 3:3): One is liable for Bi'as Makdish for all Tum'os mid'Oraisa. One is liable only for Tum'os from a Mes for which a Nazir shaves. I say that any Tum'ah from a Mes that a Nazir does not shave for is not mid'Oraisa.

R. Meir Simchah (Nazir 54b DH v'Zeh): 'Any Tum'ah for which a Nazir does not shave...' applies to Tum'os from the Mes (Yerushalmi). A Nazir shaves only "Ki Yamus Mes..." (Tum'ah from a Mes), but not for Kelim that touched a Mes, even though one who touches them is liable for Bi'as Makdish.

R. Tam (in Tosfos Shabbos 152b DH Ad): Rashi says that Golel is a coffin cover. This is difficult, for Kohanim used to pass over coffins to greet Nochri kings, and a Golel has Tum'as Ohel! Rather, Golel is a monument over the coffin.

Defense (of Rashi): Kohanim are not warned about Tum'as Golel, because a Nazir does not shave for it.

Rebuttal #1 (R. Tam): The Beraisa of Maseches Semachos is erroneous. Had the Gemara brought the Beraisa, it would have said so! A Nazir does not shave for a Revi'is of blood, but a Kohen is warned about it (Chulin 72a)!

Note: Tana'im (Nazir 47a) argue about whether a Kohen Gadol or Nazir has more Kedushah (regarding a Mes Mitzvah). No one proved that a Nazir is more Kodesh because he is forbidden Tum'os for which he does not shave. It seems that regarding the Isur to become Tamei, a Nazir is like a Kohen.

Defense #1 (Tosfos Berachos 19b DH Medalgin): The Beraisa holds like the early Zekenim who said that a Nazir shaves for a Revi'is of blood (53a).

Defense #2 (Ramban Bava Basra 20a DH Od): A Nazir shaves for a (Chetzi) Log of blood. It does not matter that it is a different Shi'ur! And even if there are exceptions to a Kelal (general rule), we follow it in other cases.

Rebuttal #2 (R. Tam): If a coffin cover were Metamei, it would not block Tum'ah of the Mes from being Metamei one who passed over. (All forbid this to a Kohen!)

Defense (Tosfos ibid. and Rashba Berachos 19b DH Rov): Only the outside of the Golel is Tamei. The inside is Tahor and blocks Tum'ah (Ohalos 15:9). Some say that the coffins were in burial caves and there was a hollow Tefach over them, so there was no Tum'as Golel above the ground. (Note: In any case mid'Rabanan Kohanim should not pass over a coffin, if not to greet kings.)

Tosfos (Sanhedrin 47b DH mishe'Yistom): R. Tam says that the Beraisa teaches only about Kelim that touched a Mes. A Kohen is not warned, for a Nazir does not shave for them.

Ritva (Sukah 23a DH she'Hayah): A Nazir shaves for and Kohanim are warned about Tum'os that come from the Mes itself. This excludes Golel and Dofek.

Poskim

Shulchan Aruch (YD 369:1): A Kohen is commanded not to become Tamei to a Mes or any Tum'ah that separated from a Mes, and not to Golel or Dofek, Ever Min ha'Chai with[out] enough flesh to survive if it were connected, a tree whose branches are separated (or rocks protruding from a wall) and Tum'ah is under one of them, and we do not know which, a field in which a grave was plowed and we do not know where, and Chutz La'aretz.

Beis Yosef (DH u'Mah she'Chosav v'Lo l'Golel): Even though Maseches Semachos says that a Kohen is not lashed for any Tum'ah for which a Nazir is not Megale'ach, he is forbidden.

Hagahos Tur ha'Shalem (6): The Beis Yosef explains the Tur, who brings a verse and says that Kohanim are commanded. This connotes that he rules (like R. Tam) unlike the Beraisa of Maseches Semachos, and holds that a Kohen deserves lashes even when a Nazir does not shave!

Bach (DH Emor): The Tur says that Golel and Dofek are mid'Oraisa, like the Ra'avad, unlike the Rambam

Taz (3) and Shach (1): The Shulchan Aruch says that we do not know under which branch or rock there is Tum'ah. This implies that if we know, a Kohen may go under other branches or rocks.

Rema: Some forbid a Kohen to become Tamei to a sword that became Tamei through a Mes, and some are lenient. The custom is to be lenient.

Beis Yosef (371:4, citing Terumas ha'Deshen 2:24): R. Shalem permitted Kohanim to go through the city gate through which a Mes will leave later. Sof Tum'ah Latzeis is a tradition from Sinai. It is only for taking a Mes out of an Ohel. Also, perhaps a Nazir does not shave for it! However, the Rosh forbids.