[mono][interp] Preserve nullable boxing for GetType

[steveisok]

Fixes #132461.

The interpreter optimized box T followed by Object.GetType() into a constant type lookup. For Nullable<T>, this bypassed nullable boxing semantics and returned typeof(Nullable<T>) instead of the underlying type.

Exclude Nullable<T> from this optimization so it follows the normal boxing path:

• A populated nullable returns the underlying type.
• A null nullable throws NullReferenceException.

Adds regression coverage for the direct box Nullable<T>; call Object.GetType() IL pattern under the Mono interpreter.

Note

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[Copilot]

Pull request overview

Fixes Mono interpreter nullable boxing semantics for Object.GetType() and adds regression coverage.

Changes:

• Excludes Nullable<T> from the constant GetType() optimization.
• Tests populated and null nullable behavior.

Show a summary per file

File	Description
src/tests/JIT/Directed/nullabletypes/gettype.cs	Adds nullable GetType() regression tests.
src/mono/mono/mini/interp/transform.c	Preserves normal nullable boxing behavior.

Review details

Suppressed comments (2)

src/mono/mono/mini/interp/transform.c:5070

• This condition evaluates mono_class_is_nullable for every box pattern, including boxes whose next instruction is not a call. Since mono_class_is_nullable walks generic-class metadata, this adds transform-time work to unrelated boxing; keep the cheap opcode/method-shape checks first and perform the nullable check only after identifying Object.GetType().

	if (!mono_class_is_nullable (box_class) &&
			*next_ip == CEE_CALL &&
			(cmethod = interp_get_method (method, read32 (next_ip + 1), image, generic_context, error)) &&
			(cmethod->klass == mono_defaults.object_class) &&
			(strcmp (cmethod->name, "GetType") == 0)) {

src/mono/mono/mini/interp/transform.c:5067

• ❌ This optimization only recognizes CEE_CALL, but the affected C# lowering uses CEE_CALLVIRT for box Nullable<T> followed by Object.GetType() (the linked repro verifies this). Consequently the new test will take the normal path and this guard does not change the failing shape; include CEE_CALLVIRT here as well so nullable boxes cannot be folded for either legal call opcode.

	if (!mono_class_is_nullable (box_class) &&
			*next_ip == CEE_CALL &&

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[Copilot]

Copilot review overview

🟢 Approval recommended

The change is narrowly scoped to a correctness fix (nullable boxing semantics) and includes targeted regression coverage for the problematic IL shape.

Review tier: Lite
Findings: None

[steveisok]

/ba-g Helix monitor known issue

[steveisok]

/backport to release/11.0

[github-actions]

Started backporting to release/11.0 (link to workflow run)