Regression: Extension SRTP evidence lost across files

Summary

Extension-SRTP calls failed across files in the released RC1 compiler-service package with preview mode enabled. Copied constraints lost their definition scope or selected witness identity.

Error Manifestation

#20683 and #20684 report failures on fresh main. The members exist in the library source, but compilation fails in the consumer:

FS0001: None of the types 'int, Library.Marker' support the operator 'Add'
FS0971: Undefined value '*'

Library.add 1 2 should produce 3. Library.A.mul "a" 3 should produce "aaa".

A local check reproduced both errors with released FSharp.Compiler.Service 43.13.101-rc1.26425.128.

The released compiler accepted the #20685 sample with --optimize- and produced 1.

Root Cause

A witness is the member selected to satisfy an SRTP constraint. Equal argument types do not imply equal witnesses.

#19602 captured definition scope without remapping its type and value identities. Its operator replay cache used types and source ranges. A copied body can retain its definition range instead of its caller's range.

Invariant: scope and selected evidence must follow each constraint through copying and code generation.

Metadata can store a selected witness. It does not store the captured type-checking environment (TcEnv).

Why It Escaped

The introducing tests already covered file and assembly boundaries. The cross-file inline test opened Lib.Ext in the consumer and enabled optimization. The definition-scope test used one file.

Coverage inference: those separate tests did not establish that definition-only scope or competing witnesses survive a file boundary.

Fix

#20717 retains selected witnesses on fresh application type-argument cells. It remaps captured scope identities and retrieves selected evidence before type normalization or unification discards those cells.

Quotation translation preserves witness order and built-in operator priority. The type-only specialization cache excludes trait-bearing calls when the feature is enabled. The quotation and cache corrections arose during repair review, not from separate public escape reports.

Timeline

Prevention

Existing regression tests cover definition scope, competing witnesses, and imported calls with equal types.

The path-scoped instruction requires application-owned evidence, remapped scope, and tests across file/import boundaries.