Fix GeneratorsError for creating FractionField element with domain of PolynomialRing or FractionField

[sylee957]

References to other Issues or PRs

Fixes #19581

Brief description of what is fixed or changed

Fraction field can have domain of fraction field or polynomial ring
and sometimes the coefficient itself can be the element from its domain.

This fixes two issues for constructing
ZZ.frac_field(a, b).frac_field(x)(a+b)
ZZ.poly_ring(a, b).frac_field(x)(a+b)

Other comments

Release Notes

• polys
  • Fixed GeneratorsError for creating some elements of FractionField when its ground domain is FractionField or PolynomialRing.

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• polys
  • Fixed GeneratorsError for creating some elements of FractionField when its ground domain is FractionField or PolynomialRing. (#19713 by @sylee957)

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#### References to other Issues or PRs
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Fixes #19581

#### Brief description of what is fixed or changed

Fraction field can have domain of fraction field or polynomial ring 
and sometimes the coefficient itself can be the element from its domain.

This fixes two issues for constructing
`ZZ.frac_field(a, b).frac_field(x)(a+b)`
`ZZ.poly_ring(a, b).frac_field(x)(a+b)`

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
- polys
  - Fixed `GeneratorsError` for creating some elements of `FractionField` when its ground domain is `FractionField` or `PolynomialRing`.

<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19713 into master will decrease coverage by 0.001%.
The diff coverage is 92.561%.

@@              Coverage Diff              @@
##            master    #19713       +/-   ##
=============================================
- Coverage   75.693%   75.692%   -0.002%     
=============================================
  Files          658       664        +6     
  Lines       171307    172382     +1075     
  Branches     40441     40657      +216     
=============================================
+ Hits        129669    130480      +811     
- Misses       35973     36191      +218     
- Partials      5665      5711       +46     

[oscarbenjamin]

Looks good to me

[jksuom]

Should there be asserts?

[sylee957]

Okay, I'll fix them

[jksuom]

What could be done if self.domain is a FractionField?

[sylee957]

I've fixed them

[jksuom]

Thanks, looks good.