Support C#7 tuples as default values for optional parameters

[fitdev]

Version Used:
C#7 in VS2017RC3

Steps to Reproduce:
Right now the following code gives a compilation error:

void Foo((int, bool) bar = (3, false))
{    }

Expected Behavior:
It should work as if the type of bar was the primitive type, since both of its constituent parts are clearly compile-time constant: 3 and false

Actual Behavior:

Error CS1736 Default parameter value for 'bar' must be a compile-time constant

[Unknown6656]

AFAIK the tuple (int, bool) is internally compiled as struct System::ValueTuple<int, bool>, meaning that it is not a "primitive" in C#-terms. For this feature to work, one would need a CLI language change to allow "primitive structs" (meaning structures, which are entirely composed of other "primitive structures", pointers and/or primitive fields).

---

Maybe one should consider the implementation of constant or native structure values being allowed to be used as optional parameters. Take a look at the following only mildly related issues:
#16095
#15079
#4982
#5474

[AdamSpeight2008]

Optional Parameter require the default value to be a compile-time constant, as it stored in the metadata.

[gafter]

This appears to be a C# language design proposal. Language design proposals should be opened for discussion on the csharplang mailing list, and if sufficient support is given, it should be submitted as a pull request to add a proposal in markdown at the csharplang repository.

[Unknown6656]

Oh jesus christ, not that mailing list again...
I am sorry, but the migration to the mailing list is one huge back-step in my opinion.
I know nobody will miss me, but I will not be further contributing to the C# language design, if it means writing and receiving tons of emails......

[Joe4evr]

@Unknown6656 Sound your voice in #16916

[Unknown6656]

@Joe4evr : I will -- I just hoped, that @gafter would also see it, if I write it here .....

[jcouv]

I'll go ahead and close this language issue. The current compiler behavior is by-design (tuples don't have a constant value, therefore cannot be used a default values for optional parameters).
Feel free to open a language issue for discussion on the csharplang repo. Thanks