Timothy Bourke authored Jan 23, 2017 and Gwenaël Delaval committed Sep 29, 2017

For a clock (ck on x) to be well-formed in an environment H, we must
have H x = ck, i.e., the clock of x is the same as the clock ck of the
stream being sampled.

This constraint is guaranteed by construction for fully inferred clocks
(by the rules for when and merge), but nothing guarantees that user
declarations be well-formed. This can lead to problems.

For instance, this invalid program is incorrectly accepted:

    node f (x : bool; a : bool :: . on b;
		      b : bool :: . on a) returns (y:bool);
    let
	y = true;
    tel

as is this one:

    node f(a: bool :: . on a; b: bool :: . on a)
    returns (z: bool);
    var w : bool;
    let
	w = a when b;
	z = false fby w;
    tel

This invalid program is incorrectly accepted and leads to an internal
compiler error:

    node f (x : bool) returns (y:bool);
    var a : bool :: . on b;
	b : bool :: . on a;
    let
	y = true;
	a = true;
	b = true;
    tel

This patch enforces the well-formedness constraint. It gives a sensible
error message when the constraint cannot be satisfied.