Reflection.Emit: Allow `ParameterBuilder.SetConstant(null)` for value-typed parameters (#17887)

* Add test for dotnet/corefx#26184

The Roslyn C# compiler encodes an optional, value-typed parameter's
default value of `default(TValueType)` as a null reference constant in
metadata. Add a test that verifies that reflection can do the same
using `ParameterBuilder.SetConstant(null)`.

* Always allow ParameterBuilder.SetConstant(null)

* Remove test project as requested in review

These tests move to CoreFX. See dotnet/corefx#29532.

1 files changed, 3 insertions, 6 deletions

diff --git a/src/mscorlib/src/System/Reflection/Emit/TypeBuilder.cs b/src/mscorlib/src/System/Reflection/Emit/TypeBuilder.cs
index 00d64f6872..f517decc1d 100644
--- a/src/mscorlib/src/System/Reflection/Emit/TypeBuilder.cs
+++ b/src/mscorlib/src/System/Reflection/Emit/TypeBuilder.cs
@@ -398,12 +398,9 @@ namespace System.Reflection.Emit
             }
             else
             {
-                if (destType.IsValueType)
-                {
-                    // nullable types can hold null value.
-                    if (!(destType.IsGenericType && destType.GetGenericTypeDefinition() == typeof(Nullable<>)))
-                        throw new ArgumentException(SR.Argument_ConstantNull);
-                }
+                // A null default value in metadata is permissible even for non-nullable value types.
+                // (See ECMA-335 II.15.4.1.4 "The .param directive" and II.22.9 "Constant" for details.)
+                // This is how the Roslyn compilers generally encode `default(TValueType)` default values.
 
                 SetConstantValue(module.GetNativeHandle(), tk, (int)CorElementType.Class, null);
             }