strictNullChecks false positives in case clause

TypeScript Version: 2.9.0-dev.20180512

Search Terms:

Code

// @strictNullChecks: true

function test1(param?: {prop: string}) {
  switch (param && param.prop) {
    case 'foo':
      return param.prop; // error: 'param' is possibly undefined
  }
}

function test2(param: {prop?: string}) {
  switch (param && param.prop) {
    case 'foo':
      return param.prop.charAt(0); // error: 'param.prop' is possibly undefined
  }
}

Expected behavior:

Compiles without error.

Actual behavior:

strictNullChecks errors as described in the comments above.

Playground Link: https://agentcooper.github.io/typescript-play/?noImplicitReturns=false#code/GYVwdgxgLglg9mABFApgZygRgBQAcCGATvgLYD8AXIgN66Fy5UaExgDmAvgJQ0BQiiNAHcYUCAAtEeIqUQAyOYgLESAOjoMe1fgMQR8aFIgDkwOHGMUduxIRRQQhJMtLr6uANyIA9N8QpCekIqYxcSY0QYNCU4NDQYACMAGwBPRHAAExRgVhQMnQ5eQt5QSFgEZHQoACZpFSpad0pBKBZ2bj4BYVEJKTD5RTC3TU7dfUMTMwsrGwE7ByclGTUNXFUJIgBBKGwABi4vX39AuGCTIdWIqJi4xNT0sCycsDyCoqA

Related Issues:
#23818

Start with the supplied switch examples and the linked Playground reproduction, then compare the behavior with related issue #23818. Investigate how strictNullChecks narrows values in case clauses. Done means both examples compile without the reported possibly-undefined errors while preserving correct narrowing behavior.