[release/10.0] Fix ComInterfaceGenerator collision on same-named methods in disjoint cross-assembly bases

[Copilot]

main PR AaronRobinsonMSFT@55aa5e6

Addresses #122708 in .NET 10.

Description

Cross-assembly [GeneratedComInterface] inheritance could fail generation when disjoint external bases each exposed a same-named method (customer repro: MyMethod), producing CS8785 from a ComMethodInfo dictionary key collision. This PR ports the servicing-safe dedup fix for release/10.0 and adds targeted regression coverage.

• Keying fix (generator-internal only)
  • Add ExternalSymbolId to ComMethodInfo so externally-defined methods (Syntax == null) have stable per-symbol identity in record equality/hash.
  • Populate at the external-method construction site using GetDocumentationCommentId() with fully-qualified fallback.
• Regression tests
  • Add external shared interfaces with same-name methods and customer GUIDs.
  • Add two derived test interfaces in ComInterfaceGenerator.Tests.
  • Add three facts covering:
    • combined implementation of both disjoint bases
    • standalone implementation for each base path.

private static string BuildExternalSymbolId(IMethodSymbol method)
    => method.GetDocumentationCommentId()
        ?? method.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat);

Customer Impact

Without this fix, valid cross-assembly COM interface hierarchies fail to compile under source generation, blocking .NET 10 migration scenarios reported by enterprise customers (Synopsys).

Regression

No. This is a latent key-collision bug in external-interface handling, surfaced by specific same-name cross-assembly method shapes.

Testing

• Reproduced pre-fix failure (CS8785 with duplicate ComMethodInfo key).
• Added regression assets/tests for the customer shape (combined + standalone variants).
• Ran ComInterfaceGenerator.Tests (with standard failing/ignore/outerloop exclusions); new tests and suite pass.

Risk

Low. Change is surgical and confined to generator-internal key identity for externally-defined methods; no public API, generated source contract, or runtime behavior changes.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/interop-contrib
See info in area-owners.md if you want to be subscribed.