Test:`in` doesn't narrow types with string indexers

tests/baselines/reference/typeGuardOfFromPropNameInUnionType.js

@@ -89,7 +89,22 @@ class SelfAssert {
         } else {
         }
     }
-}
+}
+
+interface Indexed {
+    [s: string]: any;
+}
+
+function f(i: Indexed) {
+    if ("a" in i) {
+        return i.a;
+    }
+    else if ("b" in i) {
+        return i.b;
+    }
+    return "c" in i && i.c;
+}
+
 
 //// [typeGuardOfFromPropNameInUnionType.js]
 var A = /** @class */ (function () {
@@ -216,3 +231,12 @@ var SelfAssert = /** @class */ (function () {
     };
     return SelfAssert;
 }());
+function f(i) {
+    if ("a" in i) {
+        return i.a;
+    }
+    else if ("b" in i) {
+        return i.b;
+    }
+    return "c" in i && i.c;
+}

tests/baselines/reference/typeGuardOfFromPropNameInUnionType.symbols

@@ -289,3 +289,33 @@ class SelfAssert {
         }
     }
 }
+
+interface Indexed {
+>Indexed : Symbol(Indexed, Decl(typeGuardOfFromPropNameInUnionType.ts, 90, 1))
+
+    [s: string]: any;
+>s : Symbol(s, Decl(typeGuardOfFromPropNameInUnionType.ts, 93, 5))
+}
+
+function f(i: Indexed) {
+>f : Symbol(f, Decl(typeGuardOfFromPropNameInUnionType.ts, 94, 1))
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+>Indexed : Symbol(Indexed, Decl(typeGuardOfFromPropNameInUnionType.ts, 90, 1))
+
+    if ("a" in i) {
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+
+        return i.a;
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+    }
+    else if ("b" in i) {
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+
+        return i.b;
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+    }
+    return "c" in i && i.c;
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+>i : Symbol(i, Decl(typeGuardOfFromPropNameInUnionType.ts, 96, 11))
+}
+

tests/baselines/reference/typeGuardOfFromPropNameInUnionType.types

@@ -316,3 +316,46 @@ class SelfAssert {
         }
     }
 }
+
+interface Indexed {
+>Indexed : Indexed
+
+    [s: string]: any;
+>s : string
+}
+
+function f(i: Indexed) {
+>f : (i: Indexed) => any
+>i : Indexed
+>Indexed : Indexed
+
+    if ("a" in i) {
+>"a" in i : boolean
+>"a" : "a"
+>i : Indexed
+
+        return i.a;
+>i.a : any
+>i : Indexed
+>a : any
+    }
+    else if ("b" in i) {
+>"b" in i : boolean
+>"b" : "b"
+>i : Indexed
+
+        return i.b;
+>i.b : any
+>i : Indexed
+>b : any
+    }
+    return "c" in i && i.c;
+>"c" in i && i.c : any
+>"c" in i : boolean
+>"c" : "c"
+>i : Indexed
+>i.c : any
+>i : Indexed
+>c : any
+}
+

tests/cases/conformance/expressions/typeGuards/typeGuardOfFromPropNameInUnionType.ts

@@ -88,4 +88,18 @@ class SelfAssert {
         } else {
         }
     }
-}
+}
+
+interface Indexed {
+    [s: string]: any;
+}
+
+function f(i: Indexed) {
+    if ("a" in i) {
+        return i.a;
+    }
+    else if ("b" in i) {
+        return i.b;
+    }
+    return "c" in i && i.c;
+}