gsc: keep named-tuple-bearing generic arguments symbolic at every projection gate (ADR-0172)

[DavidObando]

Fixes the remaining systemic failure in the second 2026-08-29 selfmig nightly (run 33260247720, which already included #3630/#3631): 32/52 green against floor 33, with migrated Cs2Gs.Translator failing GS0158 Cannot find member Symbol / GS0159 Cannot find function Contains / GS0116 not indexable at CSharpToGSharpTranslator.Constructors.gs:1537-38 — candidate[0].Symbol and reach[head].Contains(symbol) over List[List[(syntax …, symbol …)]] — cascading into 11 downstream apps.

Root cause

A named tuple nested inside a generic type argument shares its CLR backing with the unnamed shape (ValueTuple<…>), so any keep-symbolic gate that consults only RequiresSymbolicProjection (or a direct is TupleTypeSymbol) collapses the constructed type to its erased CLR form — and the element names vanish, so member access by name fails. The #3622 projection-gate family covered direct named-tuple arguments (List[(a, b)], one level), but not List[List[(a, b)]], List[[](a, b)], or Dictionary[K, (a, b)] reached through construction, indexing, or iteration. Minimal repro (failed before, runs now):

let groups = List[List[(a int32, b string)]]()
…
groups[0][0].a          // was GS0158: Cannot find member a

Fix

Widened six keep-symbolic gates with TypeSymbol.ContainsNamedTupleElements (already recursive through every constructor shape): the generic ctor-call type-argument resolution (the erasure at the root), the constructed-generic receiver form, the indexer element substitution + its #2365 recursive projection filter, the generic-return construction and indexer property projection in MemberLookup, the conversion-classifier parameter substitution, and the type-clause ProjectGenericArgument. Deliberately did not widen ImportedTypeSymbol.HasSubstitutableTypeArgument itself — it feeds conversion/lowering decisions with broader meaning, and widening it regressed working cases during investigation.

Verification

• New NestedNamedTupleProjectionTests (4 end-to-end oracle tests) pin all shapes, including the exact selfmig iteration+index+member chain with an imported interface element type.
• Full Core.Tests: 8218/8218 passed.
• Local selfmig proof over the cs2gs subtree: Cs2Gs.Translator's three real compile errors are gone (the only remaining local failures are the known subtree-proof artifact — GSharpRoundTrip.gs referencing GSharp.Core, which the subtree run excludes from migration; the nightly migrates it).

With this plus #3630/#3631, the next nightly should clear the b7a89d8f22f8/0a665d0d062c/ebe7e1f2f6f8 fingerprint family. The !! ceiling breach (17516 vs 17400) is a separate ratchet question.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Nng28yiBdPVdeML7mSZphs