[ty] WIP: Disambiguate legacy typevars

[dcreager]

This addresses @carljm's comment #17956 (review) about wanting to disambiguate legacy typevars that are used in multiple generic contexts.

WIP, not ready for review yet

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

zulip (https://github.com/zulip/zulip)
- zerver/actions/message_send.py:1836:5: error[invalid-assignment] Invalid assignment to data descriptor attribute `type` on type `Message` with custom `__set__` method
- Found 7319 diagnostics
+ Found 7318 diagnostics

Memory usage changes were detected when running on open source projects

trio (https://github.com/python-trio/trio)
-     memo metadata = ~21MB
+     memo metadata = ~22MB

prefect (https://github.com/PrefectHQ/prefect)
- TOTAL MEMORY USAGE: ~541MB
+ TOTAL MEMORY USAGE: ~568MB
-     struct metadata = ~23MB
+     struct metadata = ~25MB
-     memo metadata = ~73MB
+     memo metadata = ~76MB

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #17994 will degrade performances by 6.88%

_{Comparing dcreager/distinct-legacy-typevars (f786216) with main (1d21816)}

Summary

❌ 6 regressions
✅ 35 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	ty_check_file[cold]	116.3 ms	121.3 ms	-4.07%
❌	ty_check_file[incremental]	4.9 ms	5.3 ms	-6.88%
❌	ty_micro[many_tuple_assignments]	116.8 ms	122 ms	-4.27%
❌	anyio	832.8 ms	882.7 ms	-5.65%
❌	attrs	364.8 ms	383.9 ms	-4.99%
❌	hydra-zen	792.1 ms	834.9 ms	-5.12%

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #17994 will degrade performances by 10.88%

_{Comparing dcreager/distinct-legacy-typevars (f786216) with main (1d21816)}

Summary

❌ 7 regressions

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	large[sympy]	43.4 s	47 s	-7.63%
❌	medium[colour-science]	7.2 s	8 s	-9.99%
❌	medium[pandas]	29.1 s	31.9 s	-8.63%
❌	small[altair]	2.4 s	2.7 s	-10.88%
❌	small[freqtrade]	4.1 s	4.4 s	-6.65%
❌	small[pydantic]	2.3 s	2.5 s	-10.01%
❌	small[tanjun]	1.7 s	1.8 s	-8.93%

[dcreager]

superseded by #19604