[jogibear9988]

Converting an empty nullable value to an interface can throw InvalidOperationException instead of returning null. Box the nullable itself for reference conversions, retaining runtime casts for interfaces not implemented by the underlying type.

Parameters, constants and locals; null and non-null values; object, interface and value-type base classes. Regressions use strict FEC compilation for both Standard and LightExpression.

Validation

Release solution build: zero warnings/errors.

Runtime	TestX groups	Legacy tests
.NET 10	447 passed	1,756 passed
.NET 8	447 passed	1,755 passed
.NET Framework 4.7.2	426 passed	1,639 passed

Measurements

Current master f1d2fb33 → this change, Release, Windows ARM64, .NET 8.0.28 / 10.0.9. Compilation uses prebuilt trees; runtime measures invocation through the same wrapper. Two fresh process rounds with reversed load order, seven samples per round, about 30 ms calibration, one logical CPU and tiered compilation disabled. Tables use the median of the two per-round medians.

The controls below produce correct results on master for their measured inputs. Newly supported trees have no valid before/after execution comparison because master rejects them, produces incorrect results or emits invalid IL. Timings include machine and JIT/code-placement variation; the unchanged add control helps show that variation. These observations do not prove universal performance neutrality.

net8.0 results (master → change)

API / workload	Compile ns	Invoke ns	Compile B/op	Invoke B/op
Standard / add_control	1151.4 → 1204.9	2.1 → 2.0	872.0 → 872.0	0.0 → 0.0
Standard / nullable_box_object	1473.5 → 1508.3	24.2 → 24.3	976.0 → 976.0	24.0 → 24.0
Standard / nullable_box_interface	1611.5 → 1666.3	27.4 → 26.8	1104.0 → 1072.0	48.0 → 48.0
Light / add_control	1106.1 → 1062.7	2.5 → 2.0	872.0 → 872.0	0.0 → 0.0
Light / nullable_box_object	1525.7 → 1348.1	21.7 → 24.6	976.0 → 976.0	24.0 → 24.0
Light / nullable_box_interface	1376.6 → 1474.8	42.4 → 33.0	1104.0 → 1072.0	48.0 → 48.0

net10.0 results (master → change)

API / workload	Compile ns	Invoke ns	Compile B/op	Invoke B/op
Standard / add_control	4761.6 → 4883.8	2.9 → 2.7	872.1 → 872.1	0.0 → 0.0
Standard / nullable_box_object	4799.6 → 4880.5	17.5 → 17.2	976.0 → 976.0	24.0 → 24.0
Standard / nullable_box_interface	6274.9 → 5765.8	112.7 → 130.8	1104.0 → 1072.0	48.0 → 48.0
Light / add_control	4273.3 → 4269.5	3.0 → 3.1	872.0 → 872.0	0.0 → 0.0
Light / nullable_box_object	5495.8 → 6697.9	18.9 → 18.2	976.0 → 976.0	24.0 → 24.0
Light / nullable_box_interface	4128.5 → 4356.3	99.6 → 98.6	1104.0 → 1072.0	48.0 → 48.0

Emitted IL differs in these measured workloads: nullable_box_interface.
No measured allocation increase above 0.1 B/op in these controls.
nullable_box_interface compile allocation: 32 B/op less than master.