proposal: spec: allow having default values for function declaration - Go2

[mostafatalebi]

As of now (go <= 1.17) every function definition needs to declared function parameters as required. We cannot have any default value assigned to the function, for achieving this, the developer needs to adds conditions inside the function, and the user-caller of the function, if not interested in the param, must pass nil as value, and if the parameter is not a pointer type, he is required to pass a zero-literal of that specific type. All of this can be handled by introducing a new feature in go2 that supports specifying a default value in function declaration. The benefits of this:

• Less code for sure, as many conditions inside the beginning of the functions would be eliminated, as they are not necessary anymore
• Compatible with existing code-bases, as it introduces no breaking changes to the current code-bases of go1.*
• More readable codes, less obligatory type-specific zero-values for function invocations
• More welcoming to new developers landing form other languages, as most of them support this feature

Regarding the compile cost, others might have some points.

[robpike]

Duplicate of #21909