in operator shouldn't narrow {} originating in unknown

src/compiler/checker.ts

@@ -25150,7 +25150,7 @@ namespace ts {
 
             function narrowByInKeyword(type: Type, name: __String, assumeTrue: boolean) {
                 if (type.flags & TypeFlags.Union
-                    || type.flags & TypeFlags.Object && declaredType !== type
+                    || type.flags & TypeFlags.Object && declaredType !== type && !(declaredType === unknownType && isEmptyAnonymousObjectType(type))
                     || isThisTypeParameter(type)
                     || type.flags & TypeFlags.Intersection && every((type as IntersectionType).types, t => t.symbol !== globalThisSymbol)) {
                     return filterType(type, t => isTypePresencePossible(t, name, assumeTrue));

tests/baselines/reference/inKeywordAndUnknown.js

@@ -0,0 +1,36 @@
+//// [inKeywordAndUnknown.ts]
+// Repro from #50531
+
+function f(x: {}, y: unknown) {
+    if (!("a" in x)) {
+        return;
+    }
+    x;  // {}
+    if (!y) {
+        return;
+    }
+    y;  // {}
+    if (!("a" in y)) {
+        return;
+    }
+    y;  // {}
+}
+
+
+//// [inKeywordAndUnknown.js]
+"use strict";
+// Repro from #50531
+function f(x, y) {
+    if (!("a" in x)) {
+        return;
+    }
+    x; // {}
+    if (!y) {
+        return;
+    }
+    y; // {}
+    if (!("a" in y)) {
+        return;
+    }
+    y; // {}
+}

tests/baselines/reference/inKeywordAndUnknown.symbols

@@ -0,0 +1,33 @@
+=== tests/cases/compiler/inKeywordAndUnknown.ts ===
+// Repro from #50531
+
+function f(x: {}, y: unknown) {
+>f : Symbol(f, Decl(inKeywordAndUnknown.ts, 0, 0))
+>x : Symbol(x, Decl(inKeywordAndUnknown.ts, 2, 11))
+>y : Symbol(y, Decl(inKeywordAndUnknown.ts, 2, 17))
+
+    if (!("a" in x)) {
+>x : Symbol(x, Decl(inKeywordAndUnknown.ts, 2, 11))
+
+        return;
+    }
+    x;  // {}
+>x : Symbol(x, Decl(inKeywordAndUnknown.ts, 2, 11))
+
+    if (!y) {
+>y : Symbol(y, Decl(inKeywordAndUnknown.ts, 2, 17))
+
+        return;
+    }
+    y;  // {}
+>y : Symbol(y, Decl(inKeywordAndUnknown.ts, 2, 17))
+
+    if (!("a" in y)) {
+>y : Symbol(y, Decl(inKeywordAndUnknown.ts, 2, 17))
+
+        return;
+    }
+    y;  // {}
+>y : Symbol(y, Decl(inKeywordAndUnknown.ts, 2, 17))
+}
+

tests/baselines/reference/inKeywordAndUnknown.types

@@ -0,0 +1,42 @@
+=== tests/cases/compiler/inKeywordAndUnknown.ts ===
+// Repro from #50531
+
+function f(x: {}, y: unknown) {
+>f : (x: {}, y: unknown) => void
+>x : {}
+>y : unknown
+
+    if (!("a" in x)) {
+>!("a" in x) : boolean
+>("a" in x) : boolean
+>"a" in x : boolean
+>"a" : "a"
+>x : {}
+
+        return;
+    }
+    x;  // {}
+>x : {}
+
+    if (!y) {
+>!y : boolean
+>y : unknown
+
+        return;
+    }
+    y;  // {}
+>y : {}
+
+    if (!("a" in y)) {
+>!("a" in y) : boolean
+>("a" in y) : boolean
+>"a" in y : boolean
+>"a" : "a"
+>y : {}
+
+        return;
+    }
+    y;  // {}
+>y : {}
+}
+

tests/cases/compiler/inKeywordAndUnknown.ts

@@ -0,0 +1,18 @@
+// @strict: true
+
+// Repro from #50531
+
+function f(x: {}, y: unknown) {
+    if (!("a" in x)) {
+        return;
+    }
+    x;  // {}
+    if (!y) {
+        return;
+    }
+    y;  // {}
+    if (!("a" in y)) {
+        return;
+    }
+    y;  // {}
+}