Make the relationship between partial mapped types and the empty object not apply for subtype relationship

src/compiler/checker.ts

@@ -12567,7 +12567,7 @@ namespace ts {
                 }
                 else {
                     // An empty object type is related to any mapped type that includes a '?' modifier.
-                    if (isPartialMappedType(target) && isEmptyObjectType(source)) {
+                    if (relation !== subtypeRelation && isPartialMappedType(target) && isEmptyObjectType(source)) {
                         return Ternary.True;
                     }
                     if (isGenericMappedType(target)) {

tests/baselines/reference/partialTypeNarrowedToByTypeGuard.js

@@ -0,0 +1,42 @@
+//// [partialTypeNarrowedToByTypeGuard.ts]
+type Obj = {} | undefined;
+
+type User = {
+    email: string;
+    name: string;
+};
+
+type PartialUser = Partial<User>;
+
+// type PartialUser = {
+//   email?: string;
+//   name?: string;
+// };
+
+function isUser(obj: Obj): obj is PartialUser {
+    return true;
+}
+
+function getUserName(obj: Obj) {
+    if (isUser(obj)) {
+        return obj.name;
+    }
+
+    return '';
+}
+
+//// [partialTypeNarrowedToByTypeGuard.js]
+"use strict";
+// type PartialUser = {
+//   email?: string;
+//   name?: string;
+// };
+function isUser(obj) {
+    return true;
+}
+function getUserName(obj) {
+    if (isUser(obj)) {
+        return obj.name;
+    }
+    return '';
+}

tests/baselines/reference/partialTypeNarrowedToByTypeGuard.symbols

@@ -0,0 +1,52 @@
+=== tests/cases/compiler/partialTypeNarrowedToByTypeGuard.ts ===
+type Obj = {} | undefined;
+>Obj : Symbol(Obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 0, 0))
+
+type User = {
+>User : Symbol(User, Decl(partialTypeNarrowedToByTypeGuard.ts, 0, 26))
+
+    email: string;
+>email : Symbol(email, Decl(partialTypeNarrowedToByTypeGuard.ts, 2, 13))
+
+    name: string;
+>name : Symbol(name, Decl(partialTypeNarrowedToByTypeGuard.ts, 3, 18))
+
+};
+
+type PartialUser = Partial<User>;
+>PartialUser : Symbol(PartialUser, Decl(partialTypeNarrowedToByTypeGuard.ts, 5, 2))
+>Partial : Symbol(Partial, Decl(lib.es5.d.ts, --, --))
+>User : Symbol(User, Decl(partialTypeNarrowedToByTypeGuard.ts, 0, 26))
+
+// type PartialUser = {
+//   email?: string;
+//   name?: string;
+// };
+
+function isUser(obj: Obj): obj is PartialUser {
+>isUser : Symbol(isUser, Decl(partialTypeNarrowedToByTypeGuard.ts, 7, 33))
+>obj : Symbol(obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 14, 16))
+>Obj : Symbol(Obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 0, 0))
+>obj : Symbol(obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 14, 16))
+>PartialUser : Symbol(PartialUser, Decl(partialTypeNarrowedToByTypeGuard.ts, 5, 2))
+
+    return true;
+}
+
+function getUserName(obj: Obj) {
+>getUserName : Symbol(getUserName, Decl(partialTypeNarrowedToByTypeGuard.ts, 16, 1))
+>obj : Symbol(obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 18, 21))
+>Obj : Symbol(Obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 0, 0))
+
+    if (isUser(obj)) {
+>isUser : Symbol(isUser, Decl(partialTypeNarrowedToByTypeGuard.ts, 7, 33))
+>obj : Symbol(obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 18, 21))
+
+        return obj.name;
+>obj.name : Symbol(name, Decl(partialTypeNarrowedToByTypeGuard.ts, 3, 18))
+>obj : Symbol(obj, Decl(partialTypeNarrowedToByTypeGuard.ts, 18, 21))
+>name : Symbol(name, Decl(partialTypeNarrowedToByTypeGuard.ts, 3, 18))
+    }
+
+    return '';
+}

tests/baselines/reference/partialTypeNarrowedToByTypeGuard.types

@@ -0,0 +1,49 @@
+=== tests/cases/compiler/partialTypeNarrowedToByTypeGuard.ts ===
+type Obj = {} | undefined;
+>Obj : Obj
+
+type User = {
+>User : User
+
+    email: string;
+>email : string
+
+    name: string;
+>name : string
+
+};
+
+type PartialUser = Partial<User>;
+>PartialUser : Partial<User>
+
+// type PartialUser = {
+//   email?: string;
+//   name?: string;
+// };
+
+function isUser(obj: Obj): obj is PartialUser {
+>isUser : (obj: Obj) => obj is Partial<User>
+>obj : Obj
+
+    return true;
+>true : true
+}
+
+function getUserName(obj: Obj) {
+>getUserName : (obj: Obj) => string | undefined
+>obj : Obj
+
+    if (isUser(obj)) {
+>isUser(obj) : boolean
+>isUser : (obj: Obj) => obj is Partial<User>
+>obj : Obj
+
+        return obj.name;
+>obj.name : string | undefined
+>obj : Partial<User>
+>name : string | undefined
+    }
+
+    return '';
+>'' : ""
+}

tests/cases/compiler/partialTypeNarrowedToByTypeGuard.ts

@@ -0,0 +1,26 @@
+// @strict: true
+type Obj = {} | undefined;
+
+type User = {
+    email: string;
+    name: string;
+};
+
+type PartialUser = Partial<User>;
+
+// type PartialUser = {
+//   email?: string;
+//   name?: string;
+// };
+
+function isUser(obj: Obj): obj is PartialUser {
+    return true;
+}
+
+function getUserName(obj: Obj) {
+    if (isUser(obj)) {
+        return obj.name;
+    }
+
+    return '';
+}