How does the compiler recognize Nullable <T> as a "special type"?

Question

Nullable<T> is a structure that has the following definition:

public struct Nullable<T> where T : struct

where structis a type constraint, so T is bounded (according to the specification of §4.4.4):

• structure type or enumeration type
• not a type with a null value.

There Nullable<T>are no special attributes in the source text (expect for [Serializable]), since the compiler can recognize it as a "null type"?

In response to the comments below:

intis an alias for Int32:

(Section 4.1.4) simple types are identified using reserved words, but these reserved words are simply aliases for predefined structure types in the System Namespace

T?is an abbreviation for Nullable<T>:

(§4.1.10) - T?, T - . System.Nullable, .

, , .

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Answer 1

Common Source CLI 2.0, Nullable<T> "" PREDEFTYPEDEF, "System.Nullable" PT_G_OPTIONAL, .

int → System.Int32 ., . "nice name".

sscli20\csharp\inc\predeftype.h:

//         id            full type name       required  simple     numer    AggKind  fund type   elementtype,      nice name,    zero, quasi simple numer, attribute arg size serialization type,  predef attribute, arity, in mscorlib)
PREDEFTYPEDEF(PT_BYTE,   "System.Byte",         1,            1,      1,     Struct,   FT_U1,   ELEMENT_TYPE_U1,      L"byte",      0,                 0,      1,      SERIALIZATION_TYPE_U1,      PA_COUNT, 0, 1)
PREDEFTYPEDEF(PT_SHORT,  "System.Int16",        1,            1,      1,     Struct,   FT_I2,   ELEMENT_TYPE_I2,      L"short",     0,                 0,      2,      SERIALIZATION_TYPE_I2,      PA_COUNT, 0, 1)
PREDEFTYPEDEF(PT_INT,    "System.Int32",        1,            1,      1,     Struct,   FT_I4,   ELEMENT_TYPE_I4,      L"int",       0,                 0,      4,      SERIALIZATION_TYPE_I4,      PA_COUNT, 0, 1)
PREDEFTYPEDEF(PT_LONG,   "System.Int64",        1,            1,      1,     Struct,   FT_I8,   ELEMENT_TYPE_I8,      L"long",      &longZero,         0,      8,      SERIALIZATION_TYPE_I8,      PA_COUNT, 0, 1)
// ... snip ...
// Nullable<T>
PREDEFTYPEDEF(PT_G_OPTIONAL, "System.Nullable",  0,     0,     0,   Struct,   FT_STRUCT,  ELEMENT_TYPE_END,      NULL,          0,                 0,      0,      0,                          PA_COUNT, 1, 1)

:

sscli20\csharp\sccomp\nullable.cpp:

/***************************************************************************************************
    Return true iff the method is the nullable ctor taking one parameter.
***************************************************************************************************/
bool FUNCBREC::IsNubCtor(METHSYM * meth)
{
    return meth && meth->getClass()->isPredefAgg(PT_G_OPTIONAL) && meth->params->size == 1 &&
        meth->params->Item(0)->isTYVARSYM() && meth->isCtor();
}

, , , , CLI 2.0, .