Provide string completions for in keyword checks

src/services/stringCompletions.ts

@@ -12,6 +12,7 @@ import {
     addToSeen,
     altDirectorySeparator,
     arrayFrom,
+    BinaryExpression,
     CallLikeExpression,
     CancellationToken,
     CaseClause,
@@ -484,7 +485,17 @@ function getStringLiteralCompletionEntries(sourceFile: SourceFile, node: StringL
             const existing = new Set(namedImportsOrExports.elements.map(n => moduleExportNameTextEscaped(n.propertyName || n.name)));
             const uniques = exports.filter(e => e.escapedName !== InternalSymbolName.Default && !existing.has(e.escapedName));
             return { kind: StringLiteralCompletionKind.Properties, symbols: uniques, hasIndexSignature: false };
-
+        case SyntaxKind.BinaryExpression:
+            if ((parent as BinaryExpression).operatorToken.kind === SyntaxKind.InKeyword) {
+                const type = typeChecker.getTypeAtLocation((parent as BinaryExpression).right);
+                const properties = type.isUnion() ? typeChecker.getAllPossiblePropertiesOfTypes(type.types) : type.getApparentProperties();
+                return {
+                    kind: StringLiteralCompletionKind.Properties,
+                    symbols: properties.filter(prop => !prop.valueDeclaration || !isPrivateIdentifierClassElementDeclaration(prop.valueDeclaration)),
+                    hasIndexSignature: false,
+                };
+            }
+            return fromContextualType(ContextFlags.None);
         default:
             return fromContextualType() || fromContextualType(ContextFlags.None);
     }

tests/cases/fourslash/completionInChecks1.ts

@@ -0,0 +1,42 @@
+/// <reference path='fourslash.ts' />
+
+// @target: esnext
+
+//// declare const obj: {
+////   a?: string;
+////   b: number;
+//// };
+////
+//// if ("/*1*/" in obj) {}
+//// if (((("/*2*/"))) in obj) {}
+//// if ("/*3*/" in (((obj)))) {}
+//// if (((("/*4*/"))) in (((obj)))) {}
+////
+//// type MyUnion = { missing: true } | { result: string };
+//// declare const u: MyUnion;
+//// if ("/*5*/" in u) {}
+////
+//// class Cls1 { foo = ''; #bar = 0; }
+//// declare const c1: Cls1;
+//// if ("/*6*/" in c1) {}
+////
+//// class Cls2 { foo = ''; private bar = 0; }
+//// declare const c2: Cls2;
+//// if ("/*7*/" in c2) {}
+
+verify.completions({
+  marker: ["1", "2", "3", "4"],
+  exact: ["a", "b"],
+});
+verify.completions({
+  marker: "5",
+  exact: ["missing", "result"],
+});
+verify.completions({
+  marker: "6",
+  exact: ["foo"],
+});
+verify.completions({
+  marker: "7",
+  exact: ["bar", "foo"],
+});