Fix for polymorphic fields in SqlMapping

[milessabin]

• The resolution of polymorphic field mappings now takes the runtime context into account properly.
• All narrowing operations on cursors yield a value in Result allowing mappings to report internal errors for failed discriminators.
• In SQL mappings, failed discriminators now fail the entire query with an internal error.
• The TypeCase extractor has been removed because it can't take the context into account.

[tpolecat]

Can you summarize what this change does? It looks like it's causing all of our discriminator predicates to fail.

[tpolecat]

👍 this is working fine for us. The problem was on our end and was related to one of the new static methods on ResultT, for which we had a local syntax method with a different type (everything continued to typecheck after the update but some database operations were no longer being executed)

[milessabin]

@tpolecat thanks for the update!

[milessabin]

Rebased.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.13861% with 14 lines in your changes missing coverage. Please review.

Project coverage is 75.59%. Comparing base (3a6b7b8) to head (a1dbc5f).
Report is 89 commits behind head on main.

Files with missing lines	Patch %	Lines
modules/sql/shared/src/main/scala/SqlMapping.scala	88.00%	6 Missing ⚠️
modules/core/src/main/scala/cursor.scala	0.00%	2 Missing ⚠️
modules/core/src/main/scala/mapping.scala	89.47%	2 Missing ⚠️
modules/circe/src/main/scala/circemapping.scala	80.00%	1 Missing ⚠️
modules/core/src/main/scala/queryinterpreter.scala	93.75%	1 Missing ⚠️
modules/core/src/main/scala/valuemapping.scala	80.00%	1 Missing ⚠️
...les/generic/src/main/scala-2/genericmapping2.scala	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #634      +/-   ##
==========================================
+ Coverage   74.22%   75.59%   +1.36%     
==========================================
  Files          32       32              
  Lines        4609     5064     +455     
  Branches     1025     1099      +74     
==========================================
+ Hits         3421     3828     +407     
- Misses       1188     1236      +48     

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