Fix IndexOutOfRangeException with value objects implicit conversions

[Copilot]

Fixes System.IndexOutOfRangeException (and InvalidProgramException for non-abstract classes) thrown by CompileFast when using implicit conversion operators declared on an abstract or non-abstract base class within a closure.

Root Cause

In TryEmitConvert, the else if (methodParamType != sourceType) branch unconditionally assumed that whenever the conversion operator's parameter type differs from the source type, it must be a Nullable<sourceType> wrapping case. It called GetNullableConstructor() → type.GetConstructors()[0], but abstract classes have no public constructors, so this returned an empty array → IndexOutOfRangeException. For non-abstract base classes it emitted a wrong newobj instruction → InvalidProgramException.

Fix

In src/FastExpressionCompiler/FastExpressionCompiler.cs, updated the else if (methodParamType != sourceType) branch to only emit Newobj when Nullable.GetUnderlyingType(methodParamType) == sourceType. For the polymorphic base class case (methodParamType is a base type/interface of sourceType), no additional emit is needed — the value already on the stack is assignment-compatible with the conversion method's parameter.

Test

Added regression test Issue500_IndexOutOfRangeException_with_value_objects_implicit_conversions (registered for both FastExpressionCompiler and FastExpressionCompiler.LightExpression) covering:

1. Abstract base class + closure — the exact case from the original report
2. Non-abstract base class + closure — previously caused InvalidProgramException
3. Direct parameter with explicit method reference — exercises the same code path without a closure

All 1672 tests pass.

[dadhi]

@copilot please proceed

[Copilot]

@copilot please proceed

Done. The root cause was in TryEmitConvert (commit ffd609d): when an implicit conversion operator is declared on an abstract/base class (e.g., PrimitiveValueObject<string, MyPrimitive>), the parameter type differs from the source type (MyPrimitive). The code incorrectly assumed this always meant a Nullable<sourceType> wrapping case and called GetNullableConstructor() → GetConstructors()[0] → IndexOutOfRangeException for abstract classes (no public constructors).

Fix: In the else if (methodParamType != sourceType) branch, only emit Newobj when Nullable.GetUnderlyingType(methodParamType) == sourceType. For the polymorphic base class case, no extra emit is needed since the value on the stack is already assignment-compatible.

Added regression test Issue500 with 3 cases: abstract base class closure, non-abstract base class closure, and direct parameter with explicit method. All 1672 tests pass.