diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0567712f11da3..84af60db00c18 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -25499,12 +25499,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
     function getSingleCommonSupertype(types: Type[]) {
         // First, find the leftmost type for which no type to the right is a strict supertype, and if that
-        // type is a strict supertype of all other candidates, return it. Otherwise, return the leftmost type
-        // for which no type to the right is a (regular) supertype.
+        // type is a strict supertype of all other candidates, return it. Otherwise, use asymmetric
+        // assignability as a tiebreaker (i.e. only switch candidates when the assignment is one-way).
         const candidate = reduceLeft(types, (s, t) => isTypeStrictSubtypeOf(s, t) ? t : s)!;
         return every(types, t => t === candidate || isTypeStrictSubtypeOf(t, candidate)) ?
             candidate :
-            reduceLeft(types, (s, t) => isTypeSubtypeOf(s, t) ? t : s)!;
+            reduceLeft(types, (s, t) => isTypeAssignableTo(s, t) && !isTypeAssignableTo(t, s) ? t : s)!;
     }
 
     // Return the leftmost type for which no type to the right is a subtype.
diff --git a/tests/baselines/reference/discriminatedUnionFlatMap.symbols b/tests/baselines/reference/discriminatedUnionFlatMap.symbols
new file mode 100644
index 0000000000000..0bcab917b79c9
--- /dev/null
+++ b/tests/baselines/reference/discriminatedUnionFlatMap.symbols
@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/discriminatedUnionFlatMap.ts] ////
+
+=== discriminatedUnionFlatMap.ts ===
+// https://github.com/microsoft/typescript-go/issues/1057
+
+export type InputOp = { op: "add" } | { op: "remove"; value?: Array<unknown> };
+>InputOp : Symbol(InputOp, Decl(discriminatedUnionFlatMap.ts, 0, 0))
+>op : Symbol(op, Decl(discriminatedUnionFlatMap.ts, 2, 23))
+>op : Symbol(op, Decl(discriminatedUnionFlatMap.ts, 2, 39))
+>value : Symbol(value, Decl(discriminatedUnionFlatMap.ts, 2, 53))
+>Array : Symbol(Array, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --) ... and 2 more)
+
+export type OutputOp = { op: "add" | "remove" };
+>OutputOp : Symbol(OutputOp, Decl(discriminatedUnionFlatMap.ts, 2, 79))
+>op : Symbol(op, Decl(discriminatedUnionFlatMap.ts, 3, 24))
+
+export function f(operations: InputOp[]): OutputOp[] {
+>f : Symbol(f, Decl(discriminatedUnionFlatMap.ts, 3, 48))
+>operations : Symbol(operations, Decl(discriminatedUnionFlatMap.ts, 5, 18))
+>InputOp : Symbol(InputOp, Decl(discriminatedUnionFlatMap.ts, 0, 0))
+>OutputOp : Symbol(OutputOp, Decl(discriminatedUnionFlatMap.ts, 2, 79))
+
+    return operations.flatMap((operation) => {
+>operations.flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>operations : Symbol(operations, Decl(discriminatedUnionFlatMap.ts, 5, 18))
+>flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>operation : Symbol(operation, Decl(discriminatedUnionFlatMap.ts, 6, 31))
+
+        if (operation.op === "remove" && operation.value) {
+>operation.op : Symbol(op, Decl(discriminatedUnionFlatMap.ts, 2, 23), Decl(discriminatedUnionFlatMap.ts, 2, 39))
+>operation : Symbol(operation, Decl(discriminatedUnionFlatMap.ts, 6, 31))
+>op : Symbol(op, Decl(discriminatedUnionFlatMap.ts, 2, 23), Decl(discriminatedUnionFlatMap.ts, 2, 39))
+>operation.value : Symbol(value, Decl(discriminatedUnionFlatMap.ts, 2, 53))
+>operation : Symbol(operation, Decl(discriminatedUnionFlatMap.ts, 6, 31))
+>value : Symbol(value, Decl(discriminatedUnionFlatMap.ts, 2, 53))
+
+            return [].map(() => ({ op: "remove" }));
+>[].map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>map : Symbol(Array.map, Decl(lib.es5.d.ts, --, --))
+>op : Symbol(op, Decl(discriminatedUnionFlatMap.ts, 8, 34))
+
+        } else {
+            return [operation];
+>operation : Symbol(operation, Decl(discriminatedUnionFlatMap.ts, 6, 31))
+        }
+    });
+}
+
diff --git a/tests/baselines/reference/discriminatedUnionFlatMap.types b/tests/baselines/reference/discriminatedUnionFlatMap.types
new file mode 100644
index 0000000000000..6b318796592b7
--- /dev/null
+++ b/tests/baselines/reference/discriminatedUnionFlatMap.types
@@ -0,0 +1,91 @@
+//// [tests/cases/compiler/discriminatedUnionFlatMap.ts] ////
+
+=== discriminatedUnionFlatMap.ts ===
+// https://github.com/microsoft/typescript-go/issues/1057
+
+export type InputOp = { op: "add" } | { op: "remove"; value?: Array<unknown> };
+>InputOp : InputOp
+>        : ^^^^^^^
+>op : "add"
+>   : ^^^^^
+>op : "remove"
+>   : ^^^^^^^^
+>value : unknown[] | undefined
+>      : ^^^^^^^^^^^^^^^^^^^^^
+
+export type OutputOp = { op: "add" | "remove" };
+>OutputOp : OutputOp
+>         : ^^^^^^^^
+>op : "add" | "remove"
+>   : ^^^^^^^^^^^^^^^^
+
+export function f(operations: InputOp[]): OutputOp[] {
+>f : (operations: InputOp[]) => OutputOp[]
+>  : ^          ^^         ^^^^^          
+>operations : InputOp[]
+>           : ^^^^^^^^^
+
+    return operations.flatMap((operation) => {
+>operations.flatMap((operation) => {        if (operation.op === "remove" && operation.value) {            return [].map(() => ({ op: "remove" }));        } else {            return [operation];        }    }) : InputOp[]
+>                                                                                                                                                                                                                 : ^^^^^^^^^
+>operations.flatMap : <U, This = undefined>(callback: (this: This, value: InputOp, index: number, array: InputOp[]) => U | readonly U[], thisArg?: This | undefined) => U[]
+>                   : ^ ^^    ^^^^^^^^^^^^^^        ^^^    ^^^^^^^^     ^^^^^^^^^^^     ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>operations : InputOp[]
+>           : ^^^^^^^^^
+>flatMap : <U, This = undefined>(callback: (this: This, value: InputOp, index: number, array: InputOp[]) => U | readonly U[], thisArg?: This | undefined) => U[]
+>        : ^ ^^    ^^^^^^^^^^^^^^        ^^^    ^^^^^^^^     ^^^^^^^^^^^     ^^      ^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>(operation) => {        if (operation.op === "remove" && operation.value) {            return [].map(() => ({ op: "remove" }));        } else {            return [operation];        }    } : (this: undefined, operation: InputOp) => InputOp[] | { op: "remove"; }[]
+>                                                                                                                                                                                             : ^    ^^^^^^^^^^^^^         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>operation : InputOp
+>          : ^^^^^^^
+
+        if (operation.op === "remove" && operation.value) {
+>operation.op === "remove" && operation.value : false | unknown[] | undefined
+>                                             : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>operation.op === "remove" : boolean
+>                          : ^^^^^^^
+>operation.op : "add" | "remove"
+>             : ^^^^^^^^^^^^^^^^
+>operation : InputOp
+>          : ^^^^^^^
+>op : "add" | "remove"
+>   : ^^^^^^^^^^^^^^^^
+>"remove" : "remove"
+>         : ^^^^^^^^
+>operation.value : unknown[] | undefined
+>                : ^^^^^^^^^^^^^^^^^^^^^
+>operation : { op: "remove"; value?: Array<unknown>; }
+>          : ^^^^^^        ^^^^^^^^^^              ^^^
+>value : unknown[] | undefined
+>      : ^^^^^^^^^^^^^^^^^^^^^
+
+            return [].map(() => ({ op: "remove" }));
+>[].map(() => ({ op: "remove" })) : { op: "remove"; }[]
+>                                 : ^^^^^^^^^^^^^^^^^^^
+>[].map : <U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[]
+>       : ^ ^^          ^^^     ^^^^^^^^^     ^^      ^^     ^^^^^^^^^^^^^^^^^       ^^^   ^^^^^^^^
+>[] : never[]
+>   : ^^^^^^^
+>map : <U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[]
+>    : ^ ^^          ^^^     ^^^^^^^^^     ^^      ^^     ^^^^^^^^^^^^^^^^^       ^^^   ^^^^^^^^
+>() => ({ op: "remove" }) : () => { op: "remove"; }
+>                         : ^^^^^^^^^^^^^^^^^^^^^^^
+>({ op: "remove" }) : { op: "remove"; }
+>                   : ^^^^^^^^^^^^^^^^^
+>{ op: "remove" } : { op: "remove"; }
+>                 : ^^^^^^^^^^^^^^^^^
+>op : "remove"
+>   : ^^^^^^^^
+>"remove" : "remove"
+>         : ^^^^^^^^
+
+        } else {
+            return [operation];
+>[operation] : InputOp[]
+>            : ^^^^^^^^^
+>operation : InputOp
+>          : ^^^^^^^
+        }
+    });
+}
+
diff --git a/tests/cases/compiler/discriminatedUnionFlatMap.ts b/tests/cases/compiler/discriminatedUnionFlatMap.ts
new file mode 100644
index 0000000000000..688e18b4e5313
--- /dev/null
+++ b/tests/cases/compiler/discriminatedUnionFlatMap.ts
@@ -0,0 +1,18 @@
+// @strict: true
+// @noEmit: true
+// @lib: es2019
+
+// https://github.com/microsoft/typescript-go/issues/1057
+
+export type InputOp = { op: "add" } | { op: "remove"; value?: Array<unknown> };
+export type OutputOp = { op: "add" | "remove" };
+
+export function f(operations: InputOp[]): OutputOp[] {
+    return operations.flatMap((operation) => {
+        if (operation.op === "remove" && operation.value) {
+            return [].map(() => ({ op: "remove" }));
+        } else {
+            return [operation];
+        }
+    });
+}