Spurious unconstrained const parameter warning in impl

[varkor]

#![feature(const_generics)]

struct S<const X: u32>;

impl<const X: u32> S<{X}> {} //~ ERROR

produces:

error[E0207]: the const parameter `X` is not constrained by the impl trait, self type, or predicates

which is clearly incorrect.

[varkor]

What I believe is happening here is that {X} is no longer treated as a parameter, because it's in brackets. It's a ConstValue::Unevaluated when it gets to

rust/src/librustc_typeck/constrained_generic_params.rs

Lines 81 to 86 in cff1bdb

	fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
	if let ConstValue::Param(data) = c.val {
	self.parameters.push(Parameter::from(data));
	}
	false
	}

rather than a parameter.