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