[SimonCropp]

The HashSet<T>.TryGetValue polyfill is compiled for net461 to net471 and netstandard2.0. On a miss it already returns after a hashed Contains, but on a hit it scans the whole set to find the stored value, so hits are O(n).

HashSet<T>.TryGetValue itself exists in the runtime from .NET Framework 4.7.2 and .NET Core 2.0. Code compiled for these targets very often runs on such a runtime (a net462 app on a machine with .NET Framework 4.8, a netstandard2.0 library on modern .NET). The polyfill now looks the method up once per T, binds it to an open instance delegate held in a static generic class, and calls that. When the method is not there (runtime older than 4.7.2), or binding fails for any reason, the delegate is null and the existing Contains + scan code runs unchanged.

Soundness:

• The lookup checks the shape of the method (public instance, returns bool, parameters (T, out T)) before binding, and is wrapped in a try/catch, so an unexpected runtime can only fall back to the scan.
• typeof(HashSet<T>).GetMethod("TryGetValue", ...) is a pattern the trimmer understands. The code is only compiled for targets where the AOT/trim analyzers do not apply.
• Results are the same either way: the stored value on a hit, default on a miss, null handled, no enumerator invalidation.

Measurements

Compiled for net462, run on .NET Framework 4.8, Release, x64, Stopwatch, best of 7, ns per call. The hit is a lookup of the last item added.

Set size	string hit before	after	int hit before	after	string miss before	after
1	66	32	33	10	68	19
10	233	22	88	11	39	29
1,000	19,083	29	2,179	7	18	13
100,000	568,970	16	194,135	7	15	10

Tests

Added to PolyfillTests_HashSet.cs (run on net462, where the polyfill is active, and against the real method everywhere else): the stored instance is returned rather than the argument; hits and a miss in a 2,000 item set with a case-insensitive comparer; value types (int, int? with null, a struct) including the default out value on a miss; after Remove, re-Add, Clear and on an empty set; calling it while enumerating the set.

Not covered: the scan fallback can not be reached on CI, because the .NET Framework there is 4.8, where the real method exists. That code is unchanged by this PR.