Trac 12555 broke conversion of zero p-adics to PARI

[jdemeyer]

Zero p-adics should have precp=0, so this is wrong:

sage: ZpFM(3,10)(0)._pari_().debug()
[&=00000000040368d0] PADIC(lg=5):0e00000000000005 (precp=10,valp=0):0002a00000000000 00000000040368b8 00000000040368a0 0000000004036890 
    p : [&=00000000040368b8] INT(lg=3):0200000000000003 (+,lgefint=3):4000000000000003 0000000000000003 
  p^l : [&=00000000040368a0] INT(lg=3):0200000000000003 (+,lgefint=3):4000000000000003 000000000000e6a9 
    I : [&=0000000004036890] INT(lg=2):0200000000000002 (0,lgefint=2):0000000000000002 

This causes for example the following discriminant to be wrong:

sage: x = polygen(ZpFM(3,10))
sage: (x^3+x+1)._pari_().poldisc()
2 + 3 + 2*3^2 + 2*3^3 + 2*3^4 + 2*3^5 + 2*3^6 + 2*3^7 + 2*3^8 + 2*3^9 + O(3^10)
sage: (x^3+x+1).discriminant()
2 + 3 + 2*3^2 + 3^3 + 2*3^4 + 2*3^5 + 2*3^6 + 2*3^7 + 2*3^8 + 2*3^9 + O(3^10)

Blocker because this is a serious regression caused by #12555.

CC: @roed314

Component: padics

Author: Jeroen Demeyer

Branch/Commit: u/jdemeyer/ticket/15653 @ 7537427

Reviewer: David Roe

Issue created by migration from https://trac.sagemath.org/ticket/15653

[jdemeyer]

Author: Jeroen Demeyer

[jdemeyer]

Branch: u/jdemeyer/ticket/15653

[jdemeyer]

New commits:

7537427

Fix conversion of zero p-adics to PARI

[jdemeyer]

Commit: 7537427

[roed314]

Reviewer: David Roe

[roed314]

comment:4

Good catch.