diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index 2a4ead5003a89..3fad1e67792ee 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -22636,7 +22636,7 @@ func (c *Checker) cloneTypeParameter(tp *Type) *Type {
 }
 
 func (c *Checker) getHomomorphicTypeVariable(t *Type) *Type {
-	constraintType := c.getConstraintTypeFromMappedType(t)
+	constraintType := c.getActualTypeVariable(c.getConstraintTypeFromMappedType(t))
 	if constraintType.flags&TypeFlagsIndex != 0 {
 		typeVariable := c.getActualTypeVariable(constraintType.AsIndexType().target)
 		if typeVariable.flags&TypeFlagsTypeParameter != 0 {
diff --git a/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.js b/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.js
new file mode 100644
index 0000000000000..d7ab0979e3733
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.js
@@ -0,0 +1,65 @@
+//// [tests/cases/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.ts] ////
+
+//// [mappedTypeHomomorphismWithPriorKeyofTChecks1.ts]
+type SharedUnionFieldsDeep1<T> = [T] extends [unknown[]]
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+    ? {
+        [Key in keyof T]: SharedUnionFieldsDeep1<T[Key]>;
+      }
+    : {}
+  : T;
+
+// distributes
+declare const actual1: SharedUnionFieldsDeep1<
+  { tuple: [number] } | { tuple: [number, string] }
+>;
+
+type SharedUnionFieldsDeep2<T> = [T] extends [unknown[]]
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+    ? keyof T extends infer Keys extends PropertyKey
+      ? {
+          [Key in Keys]: SharedUnionFieldsDeep2<T[Key & keyof T]>;
+        }
+      : never
+    : {}
+  : T;
+
+// doesn't distribute
+declare const actual2: SharedUnionFieldsDeep2<
+  { tuple: [number] } | { tuple: [number, string] }
+>;
+
+// `& unknown` used here just to avoid printing type alias at actual3
+type SharedUnionFieldsDeepMappedType3<T, K extends keyof T = keyof T> = {
+  [Key in K]: SharedUnionFieldsDeep3<T[Key]>;
+} & unknown;
+
+type SharedUnionFieldsDeep3<T> = [T] extends [unknown[]]
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+    ? SharedUnionFieldsDeepMappedType3<T>
+    : {}
+  : T;
+
+// doesn't distribute either
+declare const actual3: SharedUnionFieldsDeep3<
+  { tuple: [number] } | { tuple: [number, string] }
+>;
+
+
+//// [mappedTypeHomomorphismWithPriorKeyofTChecks1.js]
+"use strict";
diff --git a/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.symbols b/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.symbols
new file mode 100644
index 0000000000000..71f97cca66b93
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.symbols
@@ -0,0 +1,158 @@
+//// [tests/cases/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.ts] ////
+
+=== mappedTypeHomomorphismWithPriorKeyofTChecks1.ts ===
+type SharedUnionFieldsDeep1<T> = [T] extends [unknown[]]
+>SharedUnionFieldsDeep1 : Symbol(SharedUnionFieldsDeep1, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+
+    ? [T[0]]
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+
+    : T
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+
+  : keyof T extends infer CommonKeys
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+>CommonKeys : Symbol(CommonKeys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 5, 25))
+
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+>CommonKeys : Symbol(CommonKeys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 5, 25))
+
+    ? {
+        [Key in keyof T]: SharedUnionFieldsDeep1<T[Key]>;
+>Key : Symbol(Key, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 8, 9))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+>SharedUnionFieldsDeep1 : Symbol(SharedUnionFieldsDeep1, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+>Key : Symbol(Key, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 8, 9))
+      }
+    : {}
+  : T;
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 28))
+
+// distributes
+declare const actual1: SharedUnionFieldsDeep1<
+>actual1 : Symbol(actual1, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 14, 13))
+>SharedUnionFieldsDeep1 : Symbol(SharedUnionFieldsDeep1, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 0, 0))
+
+  { tuple: [number] } | { tuple: [number, string] }
+>tuple : Symbol(tuple, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 15, 3))
+>tuple : Symbol(tuple, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 15, 25))
+
+>;
+
+type SharedUnionFieldsDeep2<T> = [T] extends [unknown[]]
+>SharedUnionFieldsDeep2 : Symbol(SharedUnionFieldsDeep2, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 16, 2))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+
+    ? [T[0]]
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+
+    : T
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+
+  : keyof T extends infer CommonKeys
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+>CommonKeys : Symbol(CommonKeys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 23, 25))
+
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+>CommonKeys : Symbol(CommonKeys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 23, 25))
+
+    ? keyof T extends infer Keys extends PropertyKey
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+>Keys : Symbol(Keys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 25, 27))
+>PropertyKey : Symbol(PropertyKey, Decl(lib.es5.d.ts, --, --))
+
+      ? {
+          [Key in Keys]: SharedUnionFieldsDeep2<T[Key & keyof T]>;
+>Key : Symbol(Key, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 27, 11))
+>Keys : Symbol(Keys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 25, 27))
+>SharedUnionFieldsDeep2 : Symbol(SharedUnionFieldsDeep2, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 16, 2))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+>Key : Symbol(Key, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 27, 11))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+        }
+      : never
+    : {}
+  : T;
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 18, 28))
+
+// doesn't distribute
+declare const actual2: SharedUnionFieldsDeep2<
+>actual2 : Symbol(actual2, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 34, 13))
+>SharedUnionFieldsDeep2 : Symbol(SharedUnionFieldsDeep2, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 16, 2))
+
+  { tuple: [number] } | { tuple: [number, string] }
+>tuple : Symbol(tuple, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 35, 3))
+>tuple : Symbol(tuple, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 35, 25))
+
+>;
+
+// `& unknown` used here just to avoid printing type alias at actual3
+type SharedUnionFieldsDeepMappedType3<T, K extends keyof T = keyof T> = {
+>SharedUnionFieldsDeepMappedType3 : Symbol(SharedUnionFieldsDeepMappedType3, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 36, 2))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 39, 38))
+>K : Symbol(K, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 39, 40))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 39, 38))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 39, 38))
+
+  [Key in K]: SharedUnionFieldsDeep3<T[Key]>;
+>Key : Symbol(Key, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 40, 3))
+>K : Symbol(K, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 39, 40))
+>SharedUnionFieldsDeep3 : Symbol(SharedUnionFieldsDeep3, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 41, 12))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 39, 38))
+>Key : Symbol(Key, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 40, 3))
+
+} & unknown;
+
+type SharedUnionFieldsDeep3<T> = [T] extends [unknown[]]
+>SharedUnionFieldsDeep3 : Symbol(SharedUnionFieldsDeep3, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 41, 12))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+
+    ? [T[0]]
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+
+    : T
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+
+  : keyof T extends infer CommonKeys
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+>CommonKeys : Symbol(CommonKeys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 48, 25))
+
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+>CommonKeys : Symbol(CommonKeys, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 48, 25))
+
+    ? SharedUnionFieldsDeepMappedType3<T>
+>SharedUnionFieldsDeepMappedType3 : Symbol(SharedUnionFieldsDeepMappedType3, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 36, 2))
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+
+    : {}
+  : T;
+>T : Symbol(T, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 43, 28))
+
+// doesn't distribute either
+declare const actual3: SharedUnionFieldsDeep3<
+>actual3 : Symbol(actual3, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 55, 13))
+>SharedUnionFieldsDeep3 : Symbol(SharedUnionFieldsDeep3, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 41, 12))
+
+  { tuple: [number] } | { tuple: [number, string] }
+>tuple : Symbol(tuple, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 56, 3))
+>tuple : Symbol(tuple, Decl(mappedTypeHomomorphismWithPriorKeyofTChecks1.ts, 56, 25))
+
+>;
+
diff --git a/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.types b/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.types
new file mode 100644
index 0000000000000..dae46e92623d3
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.types
@@ -0,0 +1,97 @@
+//// [tests/cases/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.ts] ////
+
+=== mappedTypeHomomorphismWithPriorKeyofTChecks1.ts ===
+type SharedUnionFieldsDeep1<T> = [T] extends [unknown[]]
+>SharedUnionFieldsDeep1 : SharedUnionFieldsDeep1<T>
+
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+>true : true
+>false : false
+>false : false
+
+    ? {
+        [Key in keyof T]: SharedUnionFieldsDeep1<T[Key]>;
+      }
+    : {}
+  : T;
+
+// distributes
+declare const actual1: SharedUnionFieldsDeep1<
+>actual1 : { tuple: [number]; } | { tuple: [number, string]; }
+
+  { tuple: [number] } | { tuple: [number, string] }
+>tuple : [number]
+>tuple : [number, string]
+
+>;
+
+type SharedUnionFieldsDeep2<T> = [T] extends [unknown[]]
+>SharedUnionFieldsDeep2 : SharedUnionFieldsDeep2<T>
+
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+>true : true
+>false : false
+>false : false
+
+    ? keyof T extends infer Keys extends PropertyKey
+      ? {
+          [Key in Keys]: SharedUnionFieldsDeep2<T[Key & keyof T]>;
+        }
+      : never
+    : {}
+  : T;
+
+// doesn't distribute
+declare const actual2: SharedUnionFieldsDeep2<
+>actual2 : { tuple: [number]; }
+
+  { tuple: [number] } | { tuple: [number, string] }
+>tuple : [number]
+>tuple : [number, string]
+
+>;
+
+// `& unknown` used here just to avoid printing type alias at actual3
+type SharedUnionFieldsDeepMappedType3<T, K extends keyof T = keyof T> = {
+>SharedUnionFieldsDeepMappedType3 : { [Key in K]: SharedUnionFieldsDeep3<T[Key]>; }
+
+  [Key in K]: SharedUnionFieldsDeep3<T[Key]>;
+} & unknown;
+
+type SharedUnionFieldsDeep3<T> = [T] extends [unknown[]]
+>SharedUnionFieldsDeep3 : SharedUnionFieldsDeep3<T>
+
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+>true : true
+>false : false
+>false : false
+
+    ? SharedUnionFieldsDeepMappedType3<T>
+    : {}
+  : T;
+
+// doesn't distribute either
+declare const actual3: SharedUnionFieldsDeep3<
+>actual3 : { tuple: [number]; }
+
+  { tuple: [number] } | { tuple: [number, string] }
+>tuple : [number]
+>tuple : [number, string]
+
+>;
+
diff --git a/tsc/testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.symbols b/tsc/testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.symbols
new file mode 100644
index 0000000000000..f863319b058b7
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.symbols
@@ -0,0 +1,39 @@
+//// [tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts] ////
+
+=== mappedTypeNotMistakenlyHomomorphic2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63132
+
+type KeyMap<T> = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;
+>KeyMap : Symbol(KeyMap, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 0, 0))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 12))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 12))
+>PropertyKey : Symbol(PropertyKey, Decl(lib.es5.d.ts, --, --))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 50))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 12))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 50))
+
+type X = KeyMap<number>;
+>X : Symbol(X, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 2, 77))
+>KeyMap : Symbol(KeyMap, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 0, 0))
+
+type U = { a?: number; b: string } | { b: string; readonly c: boolean };
+>U : Symbol(U, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 3, 24))
+>a : Symbol(a, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 10))
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 22))
+>b : Symbol(b, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 38))
+>c : Symbol(c, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 49))
+
+type MapStringOnly<T> = keyof T extends string ? { [K in keyof T]: [T[K]] } : never;
+>MapStringOnly : Symbol(MapStringOnly, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 72))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 52))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>T : Symbol(T, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 19))
+>K : Symbol(K, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 52))
+
+type Y = MapStringOnly<U>;
+>Y : Symbol(Y, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 6, 84))
+>MapStringOnly : Symbol(MapStringOnly, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 5, 72))
+>U : Symbol(U, Decl(mappedTypeNotMistakenlyHomomorphic2.ts, 3, 24))
+
diff --git a/tsc/testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.types b/tsc/testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.types
new file mode 100644
index 0000000000000..6cccb9bf1ca67
--- /dev/null
+++ b/tsc/testdata/baselines/reference/compiler/mappedTypeNotMistakenlyHomomorphic2.types
@@ -0,0 +1,24 @@
+//// [tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts] ////
+
+=== mappedTypeNotMistakenlyHomomorphic2.ts ===
+// https://github.com/microsoft/TypeScript/issues/63132
+
+type KeyMap<T> = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;
+>KeyMap : KeyMap<T>
+
+type X = KeyMap<number>;
+>X : number
+
+type U = { a?: number; b: string } | { b: string; readonly c: boolean };
+>U : U
+>a : number | undefined
+>b : string
+>b : string
+>c : boolean
+
+type MapStringOnly<T> = keyof T extends string ? { [K in keyof T]: [T[K]] } : never;
+>MapStringOnly : MapStringOnly<T>
+
+type Y = MapStringOnly<U>;
+>Y : { a?: [number | undefined] | undefined; b: [string]; } | { b: [string]; readonly c: [boolean]; }
+
diff --git a/tsc/testdata/tests/cases/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.ts b/tsc/testdata/tests/cases/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.ts
new file mode 100644
index 0000000000000..c279131823125
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/mappedTypeHomomorphismWithPriorKeyofTChecks1.ts
@@ -0,0 +1,58 @@
+type SharedUnionFieldsDeep1<T> = [T] extends [unknown[]]
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+    ? {
+        [Key in keyof T]: SharedUnionFieldsDeep1<T[Key]>;
+      }
+    : {}
+  : T;
+
+// distributes
+declare const actual1: SharedUnionFieldsDeep1<
+  { tuple: [number] } | { tuple: [number, string] }
+>;
+
+type SharedUnionFieldsDeep2<T> = [T] extends [unknown[]]
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+    ? keyof T extends infer Keys extends PropertyKey
+      ? {
+          [Key in Keys]: SharedUnionFieldsDeep2<T[Key & keyof T]>;
+        }
+      : never
+    : {}
+  : T;
+
+// doesn't distribute
+declare const actual2: SharedUnionFieldsDeep2<
+  { tuple: [number] } | { tuple: [number, string] }
+>;
+
+// `& unknown` used here just to avoid printing type alias at actual3
+type SharedUnionFieldsDeepMappedType3<T, K extends keyof T = keyof T> = {
+  [Key in K]: SharedUnionFieldsDeep3<T[Key]>;
+} & unknown;
+
+type SharedUnionFieldsDeep3<T> = [T] extends [unknown[]]
+  ? // Keep only tuple positions guaranteed across the union.
+    1 extends T["length"]
+    ? [T[0]]
+    : T
+  : keyof T extends infer CommonKeys
+  ? ([CommonKeys] extends [never] ? true : false) extends false
+    ? SharedUnionFieldsDeepMappedType3<T>
+    : {}
+  : T;
+
+// doesn't distribute either
+declare const actual3: SharedUnionFieldsDeep3<
+  { tuple: [number] } | { tuple: [number, string] }
+>;
diff --git a/tsc/testdata/tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts b/tsc/testdata/tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts
new file mode 100644
index 0000000000000..2e4a917153061
--- /dev/null
+++ b/tsc/testdata/tests/cases/compiler/mappedTypeNotMistakenlyHomomorphic2.ts
@@ -0,0 +1,11 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/63132
+
+type KeyMap<T> = keyof T extends PropertyKey ? { [K in keyof T]: K } : never;
+type X = KeyMap<number>;
+
+type U = { a?: number; b: string } | { b: string; readonly c: boolean };
+type MapStringOnly<T> = keyof T extends string ? { [K in keyof T]: [T[K]] } : never;
+type Y = MapStringOnly<U>;