Fix Nullable.GetUnderlyingType for MetadataLoadContext types

[AaronRobinsonMSFT]

Summary

Nullable.GetUnderlyingType() returns null for Nullable<T> types loaded via MetadataLoadContext because it uses ReferenceEquals(genericType, typeof(Nullable<>)) to identify nullable types. MLC types are RoType instances (not RuntimeType), so the reference check always fails.

This adds a name-based fallback guarded by is not RuntimeType, so only non-runtime types reach the string comparison. RuntimeType keeps the existing zero-overhead ReferenceEquals fast path.

Bug Details

The ReferenceEquals check on line 111 of Nullable.cs compares a RoType/EcmaDefinitionType against typeof(Nullable<>) (a RuntimeType). These are always different object references, even though they represent the same logical type. Type.Equals() also does not work cross-context because RoType.Equals() checks obj is RoType first, rejecting RuntimeType.

This cascades into NullabilityInfoContext, which calls GetUnderlyingType() at three callsites (lines 352, 565, 600) — causing completely wrong nullability analysis for MLC-loaded types:

• Nullable<int> is reported as NullabilityState.NotNull instead of Nullable
• Index mismatches in NullableAttribute.NullableFlags corrupt results for all subsequent generic type arguments in types like ValueTuple<int, int?, string, string?>

Fix

if (ReferenceEquals(genericType, typeof(Nullable<>))
    || (genericType is not RuntimeType
        && genericType.Namespace == "System"
        && genericType.Name == "Nullable`1"))

• Zero overhead for RuntimeType — ReferenceEquals short-circuits; the is not RuntimeType guard ensures the name check is never reached for runtime types.
• Correct for all Type implementations — MLC types, custom Type subclasses, etc.
• Proven pattern — NullabilityInfoContext already uses name-based matching for attribute detection.

Tests

Added three new tests to NullableTests.cs:

• GetUnderlyingType_MetadataLoadContext_NullableInt_ReturnsUnderlyingType — positive case
• GetUnderlyingType_MetadataLoadContext_NonNullableTypes_ReturnsNull — negative cases (int, string, KeyValuePair)
• GetUnderlyingType_MetadataLoadContext_OpenNullable_ReturnsNull — open generic edge case

All 21 NullableTests and 267 NullabilityInfoContextTests pass.

Fixes #124216

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[Copilot]

Pull request overview

Fixes Nullable.GetUnderlyingType() returning null for Nullable<T> types loaded via MetadataLoadContext by adding a non-RuntimeType fallback that identifies System.Nullable\1` by name/namespace.

Changes:

• Update Nullable.GetUnderlyingType(Type) to recognize Nullable<T> when the generic type definition is not a RuntimeType.
• Add MetadataLoadContext-based regression tests for Nullable.GetUnderlyingType() (positive/negative/open generic cases).
• Reference System.Reflection.MetadataLoadContext from System.Runtime.Tests to enable the new tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Nullable.cs	Adds a name/namespace fallback for non-RuntimeType generic type definitions so MLC-loaded Nullable<T> is detected correctly.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/NullableTests.cs	Adds three new regression tests using MetadataLoadContext to validate the corrected behavior.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System.Runtime.Tests.csproj	Adds a project reference to System.Reflection.MetadataLoadContext for the new tests.

[AaronRobinsonMSFT]

Updated this PR based on review feedback from @jkotas and @MichalStrehovsky.

New approach: Added a public virtual Type.GetNullableUnderlyingType() method following the Enum.GetUnderlyingType() → Type.GetEnumUnderlyingType() pattern. Nullable.GetUnderlyingType() now forwards to this virtual.

API Proposal: #125388

Changes:

• Type.cs — New virtual Type? GetNullableUnderlyingType() with ReferenceEquals default
• Nullable.cs — GetUnderlyingType forwards to the new virtual
• RoType.cs — sealed override using CoreType.NullableT identity comparison
• RuntimeType.Mono.cs — IsNullableOfT updated to use new virtual
• System.Runtime.cs — API added to ref assembly
• NullableTests.cs — Tests for both RuntimeType and MLC paths, ConditionalFact added

All 24 NullableTests + 267 NullabilityInfoContextTests pass.

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