Fix intermittent FS0073 under parallel optimization: leave type definitions untouched in TLR pass 5 - #20879

Fixes #20812.

Cause

Lambda lifting ends with pass 5, RecreateUniqueBounds = copyImplFile g OnlyCloneExprVals. In that mode copyTycon returns each tycon itself, yet copyAndRemapAndBindTyconsAndVals still rebound the tycon's typars and rewrote it in place: entity_typars first, then entity_tycon_repr, the augmentation, attributes, abbreviation and exception info in separate writes.

Under parallel optimization, node (file i, phase p) waits only for (i, p-1) and (i-1, p), so file i's TLR runs while file i+1's first optimization loop reads file i's types. A reader between the two writes sees SeqOps<'C#new> whose field still has type seq<int> -> 'C#old. tyOfExpr of ops.OfSeq cannot substitute 'C, the declaration's typar ends up in an Expr.App type of the member body, and IlxGen does not find it in the method's TypeReprEnv:

• SDK 10.0.4xx: FS0073 Undefined or unsolved type variable: 'C.
• main: the GenTypeAux fallback (Fix internal error FS0073 w/nested inline SRTP and multiple overloads #19710), reached from closure generic arguments since Ship FSharp.Core with a net10.0 target framework #20229, defaults it to obj and emits wrong IL with no error. It is visible with --warnon:3559 as FS3559 at the declaration of 'C.

Under OnlyCloneExprVals the rewrite has no purpose, since tycons and member/module vals are not copied. Its only effect was to swap the declaration's typars for copies on an object every later file references.

Change

Under OnlyCloneExprVals the tycons are left untouched; only the module type is re-pointed at copied vals, as before. The other copy modes are unchanged.

As a side effect, FSharpCheckProjectResults.GetOptimizedAssemblyContents() with --optimize+ no longer rewrites the checked entities that FCS symbols hold.

Testing

• New FCS test Issue 20812 - optimization leaves the type parameters of checked type definitions untouched (ExprTests): it runs single-threaded and needs no timing. It fails before the change and passes after.

• A repro in the shape of the issue (a chain of 12 generic contract classes over a record of functions, --optimize+ --deterministic+), compiled in a loop on the same base commit:

  compiler	parallel builds	output differs from sequential
  main	80	10 (9 with FS3559)
  this PR	100	0

  Sequential output is unchanged.

• With the SDK 10.0.403 compiler, isolating each parallel stage: graph checking alone failed 0 of 100 builds, parallel optimization alone failed 6 of 51.

• After a clean Build.cmd -c Release: ComponentTests EmittedIL 1660/1660 (3 skipped), optimizer tests 36/36, DeterministicTests 26/26, ExprTests 114/114.

Not in this PR

This is one instance of later optimizer phases mutating typed-tree objects that other files' concurrent phases can read. Follow-ups worth considering:

• Narrow the GenTypeAux fallback to the codegen-fresh inference typars of Internal error FS0073 "Undefined or unsolved type variable" with nested inline SRTP and multiple overloads #19709, so this class of bug fails with an internal error again instead of silently producing wrong IL.
• A test-flag checker that fingerprints published entities and vals and asserts they are unchanged after optimization. This catches such writes deterministically, without a stress loop.

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