Stole all of Nathan's tests.

tests/cases/conformance/expressions/contextualTyping/implementedPresentPropertyContextualTyping.ts

@@ -0,0 +1,10 @@
+// @strictNullChecks: true
+interface X {
+    n: number
+}
+class C implements X { // error, n: undefined isn't assignable to n: number
+    n = undefined;
+}
+class C2 implements X {
+    n = null;
+}

tests/cases/conformance/expressions/contextualTyping/implementedPropertyContextualTyping1.ts

@@ -0,0 +1,72 @@
+// @noImplicitAny: true
+interface Event {
+    time: number;
+}
+interface Base {
+    superHandle: (e: Event) => number;
+}
+interface Listener extends Base {
+    handle: (e: Event) => void;
+}
+interface Ringer {
+    ring: (times: number) => void;
+}
+interface StringLiteral {
+    literal(): "A";
+    literals: "A" | "B";
+}
+
+abstract class Watcher {
+    abstract watch(e: Event): number;
+}
+
+class Alarm extends Watcher implements Listener, Ringer, StringLiteral {
+    str: string;
+    handle = e => {
+        this.str = e.time; // error
+    }
+    superHandle = e => {
+        this.str = e.time; // error
+        return e.time;
+    }
+    ring(times) {
+        this.str = times; // error
+    }
+    watch(e) {
+        this.str = e.time; // error
+        return e.time;
+    }
+    literal() {
+        return "A"; // ok: "A" is assignable to "A"
+    }
+    literals = "A"; // ok: "A" is assignable to "A" | "B"
+}
+
+interface A {
+    p: string;
+    q(n: string): void;
+    r: string;
+    s: string;
+}
+interface B {
+    p: number;
+    q(n: number): void;
+    r: boolean;
+    s: string;
+}
+class C {
+    r: number;
+}
+class Multiple extends C implements A, B {
+    p = undefined; // error, Multiple.p is implicitly any because A.p and B.p exist
+    q(n) {         // error, n is implicitly any because A.q and B.q exist
+        n.length;
+        n.toFixed;
+    }
+    r = null;     // OK, C.r wins over A.r and B.r
+    s = null;     // OK, A.s and B.s match
+}
+let multiple = new Multiple();
+multiple.r.toFixed; // OK, C.r wins so Multiple.r: number
+multiple.r.length;  // error, Multiple.r: number
+multiple.s.length;   // OK, A.s and B.s match.

tests/cases/conformance/expressions/contextualTyping/implementedPropertyContextualTyping2.ts

@@ -0,0 +1,23 @@
+interface Long {
+    length: number;
+}
+interface Lol {
+    canhaz: string;
+}
+interface Ceiling {
+    location: { [coordinates: string]: [number, number] };
+}
+interface Invisible {
+    invisibles: string[];
+}
+class Cat implements Long, Lol, Ceiling, Invisible {
+    length = undefined;
+    canhaz = null;
+    location = {};
+    invisibles = [];
+}
+const lolCat = new Cat();
+lolCat.length = "wat";
+lolCat.canhaz = false;
+lolCat.location['ceiling'] = -1;
+lolCat.invisibles.push(0);