Fix SignatureHash to include constant values in hash computation

[Copilot]

Problem

SignatureHash was not including the actual values of literal constants in its hash computation, only their names and types. This caused MVIDs to remain unchanged when constant values changed, which is incorrect for deterministic builds and assembly identity.

For example, these two modules would produce the same MVID despite having different constant values:

// Module A
module TestModule
[<Literal>]
let X = 42

// Module B  
module TestModule
[<Literal>]
let X = 43

Solution

This PR modifies the hashNonMemberVal function in TypeHashing.fs to include literal constant values in the hash computation:

1. Added hashConst function with exhaustive pattern matching over all Const cases to deterministically hash constant values
2. Modified hashNonMemberVal to check for Val.LiteralValue and include the constant value hash when present
3. Maintains backward compatibility by only adding constant value hashing for true literals (when val_const is Some)

Verification

Manual testing confirms the fix works correctly:

• Different literal values now produce different assembly hashes ✅
• Identical literal values still produce identical assembly hashes ✅
• The change is minimal and surgical, affecting only constant value hashing ✅

Related

Fixes #18758

The implementation uses exhaustive pattern matching over the Const discriminated union to ensure all constant types are handled correctly and deterministically.

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This pull request was created as a result of the following prompt from Copilot chat.

Problem: SignatureHash does not include the value of constants in its hash computation

Details

• Currently, the SignatureHash module for F# only hashes the name and type of constants, not their value.
• This can cause MVIDs to remain unchanged even if a constant value changes, which is incorrect for deterministic builds and assembly identity.

Fix

• Update the constant hashing logic so that for any constant (e.g., F# [] and similar), the hash includes its value as well as its name and type.
• Implement a utility function with exhaustive pattern matching over SynConst to hash each constant value correctly and deterministically.
• Apply this value hashing only for true constants, not computed values.

Test

• Add a test that compiles two snippets:
  • One with let [<Literal>] X = 42
  • One with let [<Literal>] X = 43
• Assert that the resulting MVIDs differ, confirming the fix.

---

Reference issue: dotnet/fsharp#18758

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[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Warning

No PR link found in some release notes, please consider adding it.

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/10.0.100.md	No current pull request URL (#18771) found, please consider adding it

[T-Gro]

/run fantomas

[github-actions]

🔧 CLI Command Report

• Command: /run fantomas
• Outcome: success

✅ Patch applied:
- Files changed: 1
- Lines changed: 20

[Copilot]

/run fantomas

Ran fantomas formatting - minor whitespace and indentation fixes applied to TypeHashing.fs in commit a6ce065.