Handle nullable-unwrapped generic parameter values

[MichalStrehovsky]

Introduce NullableUnwrappedGenericParameterValue to represent a typeof(T) that was unwrapped via Nullable.GetUnderlyingType.

This fixes an issue where typeof(Gen<>).MakeGenericType(Nullable.GetUnderlyingType(typeof(T))) would make ILC precompile Gen<T>, not produce a warning, and then fail at runtime because we actually need Gen<Nullable<T>>.

Cc @dotnet/illink

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[MichalStrehovsky]

After many iterations on this with several models, the only "bug" this has is that:

static Type X<T>() where T : class
    => typeof(Gen<>).MakeGenericType(Nullable.GetUnderlyingType(typeof(T)))

static Type Y<T>() where T : Enum
    => typeof(Gen<>).MakeGenericType(Nullable.GetUnderlyingType(typeof(T)))

Previously didn't generate a warning, now it does generate warning, even though the codepath is actually trim safe (we know GetUnderlyingType will return null). I've decided not to fix this "regression".

[Copilot]

Pull request overview

This PR improves trim/AOT dataflow modeling for the Nullable.GetUnderlyingType(typeof(T)) pattern by introducing a dedicated value kind to represent “nullable-unwrapped generic parameter” results, preventing incorrect generic instantiation assumptions and enabling the appropriate NativeAOT warning behavior.

Changes:

• Add NullableUnwrappedGenericParameterValue to represent typeof(T) after Nullable.GetUnderlyingType unwrapping in the trim analysis value lattice.
• Update trim-analysis logic to recognize constraint-based satisfactions (e.g., new() / Enum) through the new wrapper and to unwrap it for diagnostic formatting.
• Add/linker tests covering the MakeGenericType(Nullable.GetUnderlyingType(typeof(T))) NativeAOT warning corner case and constraint propagation scenarios.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/NullableAnnotations.cs	Adds new constraint-focused test cases for Nullable.GetUnderlyingType(typeof(T)) flows.
src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/MakeGenericDataflowIntrinsics.cs	Adds a regression test expecting NativeAOT-specific IL3050 for the nullable unwrapping + MakeGenericType corner case.
src/tools/illink/src/ILLink.Shared/TrimAnalysis/RequireDynamicallyAccessedMembersAction.cs	Teaches DAM-requirement validation to treat the new wrapper like its underlying GenericParameterValue for constraint-based allowances.
src/tools/illink/src/ILLink.Shared/TrimAnalysis/NullableUnwrappedGenericParameterValue.cs	Introduces the new value type representing a generic parameter after nullable unwrapping.
src/tools/illink/src/ILLink.Shared/TrimAnalysis/HandleCallAction.cs	Produces NullableUnwrappedGenericParameterValue for Nullable.GetUnderlyingType when the input is a GenericParameterValue.
src/tools/illink/src/ILLink.Shared/Annotations.cs	Unwraps the new wrapper when generating annotation-mismatch diagnostics (argument formatting).
src/tools/illink/src/ILLink.RoslynAnalyzer/TrimAnalysis/DiagnosticContext.cs	Unwraps the new wrapper when mapping diagnostics back to Roslyn symbols/locations.

[sbomer]

Could you add the "regression" cases as tests too? Thank you!

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.