diff --git a/tests/baselines/reference/isomorphicMappedTypeInference.js b/tests/baselines/reference/isomorphicMappedTypeInference.js
index 0ed2285ed04..560c926877a 100644
--- a/tests/baselines/reference/isomorphicMappedTypeInference.js
+++ b/tests/baselines/reference/isomorphicMappedTypeInference.js
@@ -143,7 +143,14 @@ var g1 = applySpec({
 });
 
 // Infers g2: (...args: any[]) => { foo: { bar: { baz: boolean } } }
-var g2 = applySpec({ foo: { bar: { baz: (x: any) => true } } });
+var g2 = applySpec({ foo: { bar: { baz: (x: any) => true } } });
+
+// Repro from #12633
+
+const foo = <T>(object: T, partial: Partial<T>) => object;
+let o = {a: 5, b: 7};
+foo(o, {b: 9});
+o = foo(o, {b: 9});
 
 //// [isomorphicMappedTypeInference.js]
 function box(x) {
@@ -244,6 +251,11 @@ var g1 = applySpec({
 });
 // Infers g2: (...args: any[]) => { foo: { bar: { baz: boolean } } }
 var g2 = applySpec({ foo: { bar: { baz: function (x) { return true; } } } });
+// Repro from #12633
+var foo = function (object, partial) { return object; };
+var o = { a: 5, b: 7 };
+foo(o, { b: 9 });
+o = foo(o, { b: 9 });
 
 
 //// [isomorphicMappedTypeInference.d.ts]
@@ -311,3 +323,8 @@ declare var g2: (...args: any[]) => {
         };
     };
 };
+declare const foo: <T>(object: T, partial: Partial<T>) => T;
+declare let o: {
+    a: number;
+    b: number;
+};
diff --git a/tests/baselines/reference/isomorphicMappedTypeInference.symbols b/tests/baselines/reference/isomorphicMappedTypeInference.symbols
index 92bcbb697ec..93ce825c35d 100644
--- a/tests/baselines/reference/isomorphicMappedTypeInference.symbols
+++ b/tests/baselines/reference/isomorphicMappedTypeInference.symbols
@@ -459,3 +459,31 @@ var g2 = applySpec({ foo: { bar: { baz: (x: any) => true } } });
 >baz : Symbol(baz, Decl(isomorphicMappedTypeInference.ts, 144, 34))
 >x : Symbol(x, Decl(isomorphicMappedTypeInference.ts, 144, 41))
 
+// Repro from #12633
+
+const foo = <T>(object: T, partial: Partial<T>) => object;
+>foo : Symbol(foo, Decl(isomorphicMappedTypeInference.ts, 148, 5))
+>T : Symbol(T, Decl(isomorphicMappedTypeInference.ts, 148, 13))
+>object : Symbol(object, Decl(isomorphicMappedTypeInference.ts, 148, 16))
+>T : Symbol(T, Decl(isomorphicMappedTypeInference.ts, 148, 13))
+>partial : Symbol(partial, Decl(isomorphicMappedTypeInference.ts, 148, 26))
+>Partial : Symbol(Partial, Decl(lib.d.ts, --, --))
+>T : Symbol(T, Decl(isomorphicMappedTypeInference.ts, 148, 13))
+>object : Symbol(object, Decl(isomorphicMappedTypeInference.ts, 148, 16))
+
+let o = {a: 5, b: 7};
+>o : Symbol(o, Decl(isomorphicMappedTypeInference.ts, 149, 3))
+>a : Symbol(a, Decl(isomorphicMappedTypeInference.ts, 149, 9))
+>b : Symbol(b, Decl(isomorphicMappedTypeInference.ts, 149, 14))
+
+foo(o, {b: 9});
+>foo : Symbol(foo, Decl(isomorphicMappedTypeInference.ts, 148, 5))
+>o : Symbol(o, Decl(isomorphicMappedTypeInference.ts, 149, 3))
+>b : Symbol(b, Decl(isomorphicMappedTypeInference.ts, 150, 8))
+
+o = foo(o, {b: 9});
+>o : Symbol(o, Decl(isomorphicMappedTypeInference.ts, 149, 3))
+>foo : Symbol(foo, Decl(isomorphicMappedTypeInference.ts, 148, 5))
+>o : Symbol(o, Decl(isomorphicMappedTypeInference.ts, 149, 3))
+>b : Symbol(b, Decl(isomorphicMappedTypeInference.ts, 151, 12))
+
diff --git a/tests/baselines/reference/isomorphicMappedTypeInference.types b/tests/baselines/reference/isomorphicMappedTypeInference.types
index b4d1383071b..ca20e06e113 100644
--- a/tests/baselines/reference/isomorphicMappedTypeInference.types
+++ b/tests/baselines/reference/isomorphicMappedTypeInference.types
@@ -546,3 +546,42 @@ var g2 = applySpec({ foo: { bar: { baz: (x: any) => true } } });
 >x : any
 >true : true
 
+// Repro from #12633
+
+const foo = <T>(object: T, partial: Partial<T>) => object;
+>foo : <T>(object: T, partial: Partial<T>) => T
+><T>(object: T, partial: Partial<T>) => object : <T>(object: T, partial: Partial<T>) => T
+>T : T
+>object : T
+>T : T
+>partial : Partial<T>
+>Partial : Partial<T>
+>T : T
+>object : T
+
+let o = {a: 5, b: 7};
+>o : { a: number; b: number; }
+>{a: 5, b: 7} : { a: number; b: number; }
+>a : number
+>5 : 5
+>b : number
+>7 : 7
+
+foo(o, {b: 9});
+>foo(o, {b: 9}) : { a: number; b: number; }
+>foo : <T>(object: T, partial: Partial<T>) => T
+>o : { a: number; b: number; }
+>{b: 9} : { b: number; }
+>b : number
+>9 : 9
+
+o = foo(o, {b: 9});
+>o = foo(o, {b: 9}) : { a: number; b: number; }
+>o : { a: number; b: number; }
+>foo(o, {b: 9}) : { a: number; b: number; }
+>foo : <T>(object: T, partial: Partial<T>) => T
+>o : { a: number; b: number; }
+>{b: 9} : { b: number; }
+>b : number
+>9 : 9
+
diff --git a/tests/baselines/reference/mappedTypeErrors.errors.txt b/tests/baselines/reference/mappedTypeErrors.errors.txt
index 945652bf929..918c6627f66 100644
--- a/tests/baselines/reference/mappedTypeErrors.errors.txt
+++ b/tests/baselines/reference/mappedTypeErrors.errors.txt
@@ -20,9 +20,15 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(60,9): error TS2403: Su
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(61,9): error TS2403: Subsequent variable declarations must have the same type.  Variable 'x' must be of type '{ [P in keyof T]: T[P]; }', but here has type '{ readonly [P in keyof T]: T[P]; }'.
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(62,9): error TS2403: Subsequent variable declarations must have the same type.  Variable 'x' must be of type '{ [P in keyof T]: T[P]; }', but here has type '{ readonly [P in keyof T]?: T[P] | undefined; }'.
 tests/cases/conformance/types/mapped/mappedTypeErrors.ts(67,9): error TS2403: Subsequent variable declarations must have the same type.  Variable 'x' must be of type '{ [P in keyof T]: T[P]; }', but here has type '{ [P in keyof T]: T[P][]; }'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(76,45): error TS2345: Argument of type '{ x: number; }' is not assignable to parameter of type 'Readonly<{ x: number; y: number; }>'.
+  Property 'y' is missing in type '{ x: number; }'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(78,59): error TS2345: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Readonly<{ x: number; y: number; }>'.
+  Object literal may only specify known properties, and 'z' does not exist in type 'Readonly<{ x: number; y: number; }>'.
+tests/cases/conformance/types/mapped/mappedTypeErrors.ts(84,58): error TS2345: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Partial<{ x: number; y: number; }>'.
+  Object literal may only specify known properties, and 'z' does not exist in type 'Partial<{ x: number; y: number; }>'.
 
 
-==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (14 errors) ====
+==== tests/cases/conformance/types/mapped/mappedTypeErrors.ts (17 errors) ====
     
     interface Shape {
         name: string;
@@ -126,4 +132,30 @@ tests/cases/conformance/types/mapped/mappedTypeErrors.ts(67,9): error TS2403: Su
         var x: { [P in keyof T]: T[P][] };  // Error
             ~
 !!! error TS2403: Subsequent variable declarations must have the same type.  Variable 'x' must be of type '{ [P in keyof T]: T[P]; }', but here has type '{ [P in keyof T]: T[P][]; }'.
+    }
+    
+    // Check that inferences to mapped types are secondary
+    
+    declare function objAndReadonly<T>(primary: T, secondary: Readonly<T>): T;
+    declare function objAndPartial<T>(primary: T, secondary: Partial<T>): T;
+    
+    function f20() {
+        let x1 = objAndReadonly({ x: 0, y: 0 }, { x: 1 });  // Error
+                                                ~~~~~~~~
+!!! error TS2345: Argument of type '{ x: number; }' is not assignable to parameter of type 'Readonly<{ x: number; y: number; }>'.
+!!! error TS2345:   Property 'y' is missing in type '{ x: number; }'.
+        let x2 = objAndReadonly({ x: 0, y: 0 }, { x: 1, y: 1 });
+        let x3 = objAndReadonly({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 });  // Error
+                                                              ~~~~
+!!! error TS2345: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Readonly<{ x: number; y: number; }>'.
+!!! error TS2345:   Object literal may only specify known properties, and 'z' does not exist in type 'Readonly<{ x: number; y: number; }>'.
+    }
+    
+    function f21() {
+        let x1 = objAndPartial({ x: 0, y: 0 }, { x: 1 });
+        let x2 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1 });
+        let x3 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 });  // Error
+                                                             ~~~~
+!!! error TS2345: Argument of type '{ x: number; y: number; z: number; }' is not assignable to parameter of type 'Partial<{ x: number; y: number; }>'.
+!!! error TS2345:   Object literal may only specify known properties, and 'z' does not exist in type 'Partial<{ x: number; y: number; }>'.
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/mappedTypeErrors.js b/tests/baselines/reference/mappedTypeErrors.js
index cb840da32f7..4d2eadbafe9 100644
--- a/tests/baselines/reference/mappedTypeErrors.js
+++ b/tests/baselines/reference/mappedTypeErrors.js
@@ -66,6 +66,23 @@ function f11<T>() {
 function f12<T>() {
     var x: { [P in keyof T]: T[P] };
     var x: { [P in keyof T]: T[P][] };  // Error
+}
+
+// Check that inferences to mapped types are secondary
+
+declare function objAndReadonly<T>(primary: T, secondary: Readonly<T>): T;
+declare function objAndPartial<T>(primary: T, secondary: Partial<T>): T;
+
+function f20() {
+    let x1 = objAndReadonly({ x: 0, y: 0 }, { x: 1 });  // Error
+    let x2 = objAndReadonly({ x: 0, y: 0 }, { x: 1, y: 1 });
+    let x3 = objAndReadonly({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 });  // Error
+}
+
+function f21() {
+    let x1 = objAndPartial({ x: 0, y: 0 }, { x: 1 });
+    let x2 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1 });
+    let x3 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 });  // Error
 }
 
 //// [mappedTypeErrors.js]
@@ -97,6 +114,16 @@ function f12() {
     var x;
     var x; // Error
 }
+function f20() {
+    var x1 = objAndReadonly({ x: 0, y: 0 }, { x: 1 }); // Error
+    var x2 = objAndReadonly({ x: 0, y: 0 }, { x: 1, y: 1 });
+    var x3 = objAndReadonly({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 }); // Error
+}
+function f21() {
+    var x1 = objAndPartial({ x: 0, y: 0 }, { x: 1 });
+    var x2 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1 });
+    var x3 = objAndPartial({ x: 0, y: 0 }, { x: 1, y: 1, z: 1 }); // Error
+}
 
 
 //// [mappedTypeErrors.d.ts]
@@ -137,3 +164,7 @@ declare function f4<T extends keyof Named>(x: T): void;
 declare function f10<T>(): void;
 declare function f11<T>(): void;
 declare function f12<T>(): void;
+declare function objAndReadonly<T>(primary: T, secondary: Readonly<T>): T;
+declare function objAndPartial<T>(primary: T, secondary: Partial<T>): T;
+declare function f20(): void;
+declare function f21(): void;