TypeScript 2.2.1 generates surprising null checking error

[BurtHarris]

TypeScript Version: 2.2.1

Effective type of argument p below seems to not match explicit declaration under tsc 2.2.1

Code

interface RuleContext  {
    _parent: RuleContext | undefined;
};

var  _ctx = { _parent: undefined};

function walk( p: RuleContext | undefined = _ctx ) {
    while (p) {
        p = p._parent; // Error w/ "strictNullChecks": true !
    }
}

Expected behavior:
Compiled w/o error under 2.1
Actual behavior:
Generates error:

index.ts(9,4): error TS2322: Type 'RuleContext | undefined' is not assignable to type 'RuleContext'.
  Type 'undefined' is not assignable to type 'RuleContext'.

[BurtHarris]

Simple workarounds possible, just surprised at the change in behavior.

Another repro case:

var obj: Context = {
  parent: undefined
};

function working(p: Context | undefined) {
  while (p != null) {
    p = p.parent;
  }
}

function broken(p: Context | undefined = obj) {
  while (p != null) {
    p = p.parent;
  }
}

interface Context { parent: Context | undefined; }

[RyanCavanaugh]

We changed the type of defaulted parameters from T | undefined to T in 2.2, but it seems wrong to strip out undefined from an explicitly-written type annotation

[sandersn]

This is an intended result of #12033, but we should revisit whether to use the control-flow-narrowing mechanism instead of the one that shipped, which simply strips off | undefined from default-initialised and optional parameters.