spec: document that type parameters are not permitted as constant declaration types

[gazerro]

The compilation of the following program fails with error ./prog.go:4:10: invalid constant type P

package main

func f[P int]() {
	const y P = 5
}

func main() {}

but this behavior is not documented in the spec.

From the spec 5 is assignable to P because it is an untyped constant representable by a value of type P.

[ianlancetaylor]

CC @griesemer

@gazerro Are you looking at https://tip.golang.org/ref/spec?

[griesemer]

I believe this is not yet fully documented in the in-progress spec. But here it is: type parameters cannot be constant types. While 5 can be stored in a variable of type P, it cannot be the value of a constant of type P. Specifically, and this is documented in the section on conversions, if a constant is converted to a type parameter, say P(5), the result is a non-constant value.

Leaving open until fully documented.

[gazerro]

@gazerro Are you looking at https://tip.golang.org/ref/spec?

Yes.