Compile-time constant discrepency

Question

Update: This appears to be a compiler red-herring, as the following is actually valid:

const int MyInt = default(int);

The issue lies with DateTime not being a valid const, not the use of default.

The main source of the confusion for me was not realising that default(DateTime) is handled specifically in optional parameters (and I had arrived at a false conclusion that default(DateTime) was being treated as compile-time constant due to the error message omitting the other possible conditions). This is addressed by MarcinJuraszek in his answer.

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Original Question:

_{This is shamelessly ripped from a comment from Marc Gravell from this answer to another question.}

Why is the following valid:

// No compiler errors, default(DateTime) seems to satisfy the compile-time constant requirement.
public static void DoSomething(DateTime date = default(DateTime))
{ 
}

But the following not:

// Compiler error: "Constant initializer must be compile-time constant.
const DateTime MyDate = default(DateTime); 

As both appear to want "compile-time constants" (evident if you attempt to provide something like DateTime.MinValue to the optional parameter, the compiler complains that it isn't compile-time constant):

// Compiler error: Default parameter value for 'date' must be a compile-time constant.
public static void DoSomething(DateTime date = DateTime.MinValue) {}

What is going on behind the scenes that causes the compiler to treat these differently?

Answer 1

That's described in C# specification (10.6.1):

A fixed-parameter with a default-argument is known as an optional parameter, whereas a fixed-parameter without a default-argument is a required parameter. A required parameter may not appear after an optional parameter in a formal-parameter-list.

A ref or outparameter cannot have a default-argument. The expression in a default-argument must be one of the following:

• a constant-expression
• an expression of the form new S() where S is a value type
• an expression of the form default(S) where S is a value type

But you're right, the error message asking for compile-time constant is not good.

Answer 2

Because a const that can only have the value default(TypeOfCost) would probably be quite useless :-)... And you can't even change it later :-)

Note that default(TypeOfConst) is a constant-expression

From the C# specifications (5.0): 7.19 Constant expressions... A constant-expression is an expression that can be fully evaluated at compile-time.... Only the following constructs are permitted in constant expressions:... • **Default value expressions**

The error is that const DateTime is illegal..

10.4 Constants... The type specified in a constant declaration must be sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, string, an enum-type, or a reference-type.