Quoting both of you guys here to lump the reply together.
First - The Supreme Court has never ruled on executive privilege as it relates to a conflict between the Executive and the Legislature. US v Nixon was a DOJ special prosecutor, not congress. In that ruling, they opined about executive privilege though they granted nothing to Nixon in that case. Several lower court rulings set the precedent, and they are absolutely nothing like the blanket immunity the Trump admin has asserted. In those cases, the privilege is narrow and weighed against other interests. If you guys can point me to something else please do.
Source here.
Second - I'm not aware of Trump asserting any any executive privilege, as it's normally understood, through this entire Ukraine process. See Corey Lewandowski's testimony for reference. The White House issued a letter describing what Executive Privilege it was asserting and thus areas that he had been instructed not to testify regarding.
Third - The "total immunity" that this and previous White Houses have claimed was related to testimony of close advisors. This was the argument in the McGhan case. It has no case law precedent, but has decades of OLC opinion behind it. Spreading this beyond "close advisors" is, to my knowledge, new ground.
In this case, the WH letter was a blanket denial of congressional oversight at the whim of the Executive. This is not a well crafted and strategic implementation of Executive Privilege as constrained by the courts. There