From 97a061dd2ee66d1b110bce090b2bac6550c4ce89 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Wed, 5 Nov 2025 18:51:28 +0100
Subject: [PATCH 1/2] Widen reverse mapped type properties to fix them being
 treated as EPC-valid sources

---
 src/compiler/checker.ts                       |   2 +-
 ...everseMappedTypeInferenceWidening1.symbols | 181 +++++++++++++
 .../reverseMappedTypeInferenceWidening1.types | 244 ++++++++++++++++++
 .../reverseMappedTypeInferenceWidening1.ts    |  60 +++++
 4 files changed, 486 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/reverseMappedTypeInferenceWidening1.symbols
 create mode 100644 tests/baselines/reference/reverseMappedTypeInferenceWidening1.types
 create mode 100644 tests/cases/compiler/reverseMappedTypeInferenceWidening1.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 480f7189fce31..f84b6cf5cb67f 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -26421,7 +26421,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     function getTypeOfReverseMappedSymbol(symbol: ReverseMappedSymbol): Type {
         const links = getSymbolLinks(symbol);
         if (!links.type) {
-            links.type = inferReverseMappedType(symbol.links.propertyType, symbol.links.mappedType, symbol.links.constraintType) || unknownType;
+            links.type = getWidenedType(inferReverseMappedType(symbol.links.propertyType, symbol.links.mappedType, symbol.links.constraintType) || unknownType);
         }
         return links.type;
     }
diff --git a/tests/baselines/reference/reverseMappedTypeInferenceWidening1.symbols b/tests/baselines/reference/reverseMappedTypeInferenceWidening1.symbols
new file mode 100644
index 0000000000000..635dcd0afed7d
--- /dev/null
+++ b/tests/baselines/reference/reverseMappedTypeInferenceWidening1.symbols
@@ -0,0 +1,181 @@
+//// [tests/cases/compiler/reverseMappedTypeInferenceWidening1.ts] ////
+
+=== reverseMappedTypeInferenceWidening1.ts ===
+// https://github.com/microsoft/TypeScript/issues/62720
+
+type TypeFunction<ReturnType = unknown> = (...args: any[]) => ReturnType;
+>TypeFunction : Symbol(TypeFunction, Decl(reverseMappedTypeInferenceWidening1.ts, 0, 0))
+>ReturnType : Symbol(ReturnType, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 18))
+>args : Symbol(args, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 43))
+>ReturnType : Symbol(ReturnType, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 18))
+
+type Flags = {
+>Flags : Symbol(Flags, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 73))
+
+  [flagName: string]: {
+>flagName : Symbol(flagName, Decl(reverseMappedTypeInferenceWidening1.ts, 4, 3))
+
+    type: TypeFunction;
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 4, 23))
+>TypeFunction : Symbol(TypeFunction, Decl(reverseMappedTypeInferenceWidening1.ts, 0, 0))
+
+    default?: unknown;
+>default : Symbol(default, Decl(reverseMappedTypeInferenceWidening1.ts, 5, 23))
+
+  };
+};
+type TypeFlag<Schemas extends Flags> = {
+>TypeFlag : Symbol(TypeFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 8, 2))
+>Schemas : Symbol(Schemas, Decl(reverseMappedTypeInferenceWidening1.ts, 9, 14))
+>Flags : Symbol(Flags, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 73))
+
+  [flag in keyof Schemas]: Schemas[flag] extends { type: TypeFunction<infer T> }
+>flag : Symbol(flag, Decl(reverseMappedTypeInferenceWidening1.ts, 10, 3))
+>Schemas : Symbol(Schemas, Decl(reverseMappedTypeInferenceWidening1.ts, 9, 14))
+>Schemas : Symbol(Schemas, Decl(reverseMappedTypeInferenceWidening1.ts, 9, 14))
+>flag : Symbol(flag, Decl(reverseMappedTypeInferenceWidening1.ts, 10, 3))
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 10, 50))
+>TypeFunction : Symbol(TypeFunction, Decl(reverseMappedTypeInferenceWidening1.ts, 0, 0))
+>T : Symbol(T, Decl(reverseMappedTypeInferenceWidening1.ts, 10, 75))
+
+    ? T
+>T : Symbol(T, Decl(reverseMappedTypeInferenceWidening1.ts, 10, 75))
+
+    : never;
+};
+
+declare function fn1<Options extends Flags>(
+>fn1 : Symbol(fn1, Decl(reverseMappedTypeInferenceWidening1.ts, 13, 2))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 15, 21))
+>Flags : Symbol(Flags, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 73))
+
+  options: Options,
+>options : Symbol(options, Decl(reverseMappedTypeInferenceWidening1.ts, 15, 44))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 15, 21))
+
+): TypeFlag<Options>;
+>TypeFlag : Symbol(TypeFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 8, 2))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 15, 21))
+
+const result1 = fn1({
+>result1 : Symbol(result1, Decl(reverseMappedTypeInferenceWidening1.ts, 19, 5))
+>fn1 : Symbol(fn1, Decl(reverseMappedTypeInferenceWidening1.ts, 13, 2))
+
+  booleanFlag: { type: Boolean },
+>booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 19, 21))
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 20, 16))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+  booleanFlagDefault: {
+>booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 20, 33))
+
+    type: Boolean,
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 21, 23))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    default: false,
+>default : Symbol(default, Decl(reverseMappedTypeInferenceWidening1.ts, 22, 18))
+
+  },
+});
+result1.booleanFlag; // boolean
+>result1.booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 19, 21))
+>result1 : Symbol(result1, Decl(reverseMappedTypeInferenceWidening1.ts, 19, 5))
+>booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 19, 21))
+
+result1.booleanFlagDefault; // boolean
+>result1.booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 20, 33))
+>result1 : Symbol(result1, Decl(reverseMappedTypeInferenceWidening1.ts, 19, 5))
+>booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 20, 33))
+
+declare function fn2<Options extends Flags>(
+>fn2 : Symbol(fn2, Decl(reverseMappedTypeInferenceWidening1.ts, 27, 27))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 29, 21))
+>Flags : Symbol(Flags, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 73))
+
+  options: Readonly<Options>,
+>options : Symbol(options, Decl(reverseMappedTypeInferenceWidening1.ts, 29, 44))
+>Readonly : Symbol(Readonly, Decl(lib.es5.d.ts, --, --))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 29, 21))
+
+): TypeFlag<Options>;
+>TypeFlag : Symbol(TypeFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 8, 2))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 29, 21))
+
+const result2 = fn2({
+>result2 : Symbol(result2, Decl(reverseMappedTypeInferenceWidening1.ts, 33, 5))
+>fn2 : Symbol(fn2, Decl(reverseMappedTypeInferenceWidening1.ts, 27, 27))
+
+  booleanFlag: { type: Boolean },
+>booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 33, 21))
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 34, 16))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+  booleanFlagDefault: {
+>booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 34, 33))
+
+    type: Boolean,
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 35, 23))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    default: false,
+>default : Symbol(default, Decl(reverseMappedTypeInferenceWidening1.ts, 36, 18))
+
+  },
+});
+result2.booleanFlag; // boolean
+>result2.booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 33, 21))
+>result2 : Symbol(result2, Decl(reverseMappedTypeInferenceWidening1.ts, 33, 5))
+>booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 33, 21))
+
+result2.booleanFlagDefault; // boolean
+>result2.booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 34, 33))
+>result2 : Symbol(result2, Decl(reverseMappedTypeInferenceWidening1.ts, 33, 5))
+>booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 34, 33))
+
+declare function fn3<Options extends Flags>(
+>fn3 : Symbol(fn3, Decl(reverseMappedTypeInferenceWidening1.ts, 41, 27))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 43, 21))
+>Flags : Symbol(Flags, Decl(reverseMappedTypeInferenceWidening1.ts, 2, 73))
+
+  options: Readonly<Options>,
+>options : Symbol(options, Decl(reverseMappedTypeInferenceWidening1.ts, 43, 44))
+>Readonly : Symbol(Readonly, Decl(lib.es5.d.ts, --, --))
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 43, 21))
+
+): Options;
+>Options : Symbol(Options, Decl(reverseMappedTypeInferenceWidening1.ts, 43, 21))
+
+const result3 = fn3({
+>result3 : Symbol(result3, Decl(reverseMappedTypeInferenceWidening1.ts, 47, 5))
+>fn3 : Symbol(fn3, Decl(reverseMappedTypeInferenceWidening1.ts, 41, 27))
+
+  booleanFlag: { type: Boolean },
+>booleanFlag : Symbol(booleanFlag, Decl(reverseMappedTypeInferenceWidening1.ts, 47, 21))
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 48, 16))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+  booleanFlagDefault: {
+>booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 48, 33))
+
+    type: Boolean,
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 49, 23))
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    default: false, // no cursed EPC failure here
+>default : Symbol(default, Decl(reverseMappedTypeInferenceWidening1.ts, 50, 18))
+
+  },
+});
+
+declare function takeType(arg: { type: unknown }): void;
+>takeType : Symbol(takeType, Decl(reverseMappedTypeInferenceWidening1.ts, 53, 3))
+>arg : Symbol(arg, Decl(reverseMappedTypeInferenceWidening1.ts, 55, 26))
+>type : Symbol(type, Decl(reverseMappedTypeInferenceWidening1.ts, 55, 32))
+
+takeType(result3.booleanFlagDefault);
+>takeType : Symbol(takeType, Decl(reverseMappedTypeInferenceWidening1.ts, 53, 3))
+>result3.booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 48, 33))
+>result3 : Symbol(result3, Decl(reverseMappedTypeInferenceWidening1.ts, 47, 5))
+>booleanFlagDefault : Symbol(booleanFlagDefault, Decl(reverseMappedTypeInferenceWidening1.ts, 48, 33))
+
diff --git a/tests/baselines/reference/reverseMappedTypeInferenceWidening1.types b/tests/baselines/reference/reverseMappedTypeInferenceWidening1.types
new file mode 100644
index 0000000000000..f8001f76ebe68
--- /dev/null
+++ b/tests/baselines/reference/reverseMappedTypeInferenceWidening1.types
@@ -0,0 +1,244 @@
+//// [tests/cases/compiler/reverseMappedTypeInferenceWidening1.ts] ////
+
+=== reverseMappedTypeInferenceWidening1.ts ===
+// https://github.com/microsoft/TypeScript/issues/62720
+
+type TypeFunction<ReturnType = unknown> = (...args: any[]) => ReturnType;
+>TypeFunction : TypeFunction<ReturnType>
+>             : ^^^^^^^^^^^^^^^^^^^^^^^^
+>args : any[]
+>     : ^^^^^
+
+type Flags = {
+>Flags : Flags
+>      : ^^^^^
+
+  [flagName: string]: {
+>flagName : string
+>         : ^^^^^^
+
+    type: TypeFunction;
+>type : TypeFunction<unknown>
+>     : ^^^^^^^^^^^^^^^^^^^^^
+
+    default?: unknown;
+>default : unknown
+>        : ^^^^^^^
+
+  };
+};
+type TypeFlag<Schemas extends Flags> = {
+>TypeFlag : TypeFlag<Schemas>
+>         : ^^^^^^^^^^^^^^^^^
+
+  [flag in keyof Schemas]: Schemas[flag] extends { type: TypeFunction<infer T> }
+>type : TypeFunction<T>
+>     : ^^^^^^^^^^^^^^^
+
+    ? T
+    : never;
+};
+
+declare function fn1<Options extends Flags>(
+>fn1 : <Options extends Flags>(options: Options) => TypeFlag<Options>
+>    : ^       ^^^^^^^^^     ^^       ^^       ^^^^^                 
+
+  options: Options,
+>options : Options
+>        : ^^^^^^^
+
+): TypeFlag<Options>;
+
+const result1 = fn1({
+>result1 : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>fn1({  booleanFlag: { type: Boolean },  booleanFlagDefault: {    type: Boolean,    default: false,  },}) : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>                                                                                                         : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>fn1 : <Options extends Flags>(options: Options) => TypeFlag<Options>
+>    : ^       ^^^^^^^^^     ^^       ^^       ^^^^^                 
+>{  booleanFlag: { type: Boolean },  booleanFlagDefault: {    type: Boolean,    default: false,  },} : { booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: false; }; }
+>                                                                                                    : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  booleanFlag: { type: Boolean },
+>booleanFlag : { type: BooleanConstructor; }
+>            : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>{ type: Boolean } : { type: BooleanConstructor; }
+>                  : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>type : BooleanConstructor
+>     : ^^^^^^^^^^^^^^^^^^
+>Boolean : BooleanConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
+  booleanFlagDefault: {
+>booleanFlagDefault : { type: BooleanConstructor; default: false; }
+>                   : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>{    type: Boolean,    default: false,  } : { type: BooleanConstructor; default: false; }
+>                                          : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    type: Boolean,
+>type : BooleanConstructor
+>     : ^^^^^^^^^^^^^^^^^^
+>Boolean : BooleanConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
+    default: false,
+>default : false
+>        : ^^^^^
+>false : false
+>      : ^^^^^
+
+  },
+});
+result1.booleanFlag; // boolean
+>result1.booleanFlag : boolean
+>                    : ^^^^^^^
+>result1 : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>booleanFlag : boolean
+>            : ^^^^^^^
+
+result1.booleanFlagDefault; // boolean
+>result1.booleanFlagDefault : boolean
+>                           : ^^^^^^^
+>result1 : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>booleanFlagDefault : boolean
+>                   : ^^^^^^^
+
+declare function fn2<Options extends Flags>(
+>fn2 : <Options extends Flags>(options: Readonly<Options>) => TypeFlag<Options>
+>    : ^       ^^^^^^^^^     ^^       ^^                 ^^^^^                 
+
+  options: Readonly<Options>,
+>options : Readonly<Options>
+>        : ^^^^^^^^^^^^^^^^^
+
+): TypeFlag<Options>;
+
+const result2 = fn2({
+>result2 : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>fn2({  booleanFlag: { type: Boolean },  booleanFlagDefault: {    type: Boolean,    default: false,  },}) : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>                                                                                                         : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>fn2 : <Options extends Flags>(options: Readonly<Options>) => TypeFlag<Options>
+>    : ^       ^^^^^^^^^     ^^       ^^                 ^^^^^                 
+>{  booleanFlag: { type: Boolean },  booleanFlagDefault: {    type: Boolean,    default: false,  },} : { booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: false; }; }
+>                                                                                                    : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  booleanFlag: { type: Boolean },
+>booleanFlag : { type: BooleanConstructor; }
+>            : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>{ type: Boolean } : { type: BooleanConstructor; }
+>                  : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>type : BooleanConstructor
+>     : ^^^^^^^^^^^^^^^^^^
+>Boolean : BooleanConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
+  booleanFlagDefault: {
+>booleanFlagDefault : { type: BooleanConstructor; default: false; }
+>                   : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>{    type: Boolean,    default: false,  } : { type: BooleanConstructor; default: false; }
+>                                          : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    type: Boolean,
+>type : BooleanConstructor
+>     : ^^^^^^^^^^^^^^^^^^
+>Boolean : BooleanConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
+    default: false,
+>default : false
+>        : ^^^^^
+>false : false
+>      : ^^^^^
+
+  },
+});
+result2.booleanFlag; // boolean
+>result2.booleanFlag : boolean
+>                    : ^^^^^^^
+>result2 : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>booleanFlag : boolean
+>            : ^^^^^^^
+
+result2.booleanFlagDefault; // boolean
+>result2.booleanFlagDefault : boolean
+>                           : ^^^^^^^
+>result2 : TypeFlag<{ booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }>
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>booleanFlagDefault : boolean
+>                   : ^^^^^^^
+
+declare function fn3<Options extends Flags>(
+>fn3 : <Options extends Flags>(options: Readonly<Options>) => Options
+>    : ^       ^^^^^^^^^     ^^       ^^                 ^^^^^       
+
+  options: Readonly<Options>,
+>options : Readonly<Options>
+>        : ^^^^^^^^^^^^^^^^^
+
+): Options;
+
+const result3 = fn3({
+>result3 : { booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>fn3({  booleanFlag: { type: Boolean },  booleanFlagDefault: {    type: Boolean,    default: false, // no cursed EPC failure here  },}) : { booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }
+>                                                                                                                                       : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>fn3 : <Options extends Flags>(options: Readonly<Options>) => Options
+>    : ^       ^^^^^^^^^     ^^       ^^                 ^^^^^       
+>{  booleanFlag: { type: Boolean },  booleanFlagDefault: {    type: Boolean,    default: false, // no cursed EPC failure here  },} : { booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: false; }; }
+>                                                                                                                                  : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  booleanFlag: { type: Boolean },
+>booleanFlag : { type: BooleanConstructor; }
+>            : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>{ type: Boolean } : { type: BooleanConstructor; }
+>                  : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>type : BooleanConstructor
+>     : ^^^^^^^^^^^^^^^^^^
+>Boolean : BooleanConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
+  booleanFlagDefault: {
+>booleanFlagDefault : { type: BooleanConstructor; default: false; }
+>                   : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>{    type: Boolean,    default: false, // no cursed EPC failure here  } : { type: BooleanConstructor; default: false; }
+>                                                                        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    type: Boolean,
+>type : BooleanConstructor
+>     : ^^^^^^^^^^^^^^^^^^
+>Boolean : BooleanConstructor
+>        : ^^^^^^^^^^^^^^^^^^
+
+    default: false, // no cursed EPC failure here
+>default : false
+>        : ^^^^^
+>false : false
+>      : ^^^^^
+
+  },
+});
+
+declare function takeType(arg: { type: unknown }): void;
+>takeType : (arg: { type: unknown; }) => void
+>         : ^   ^^                  ^^^^^    
+>arg : { type: unknown; }
+>    : ^^^^^^^^       ^^^
+>type : unknown
+>     : ^^^^^^^
+
+takeType(result3.booleanFlagDefault);
+>takeType(result3.booleanFlagDefault) : void
+>                                     : ^^^^
+>takeType : (arg: { type: unknown; }) => void
+>         : ^   ^^                  ^^^^^    
+>result3.booleanFlagDefault : { type: BooleanConstructor; default: boolean; }
+>                           : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>result3 : { booleanFlag: { type: BooleanConstructor; }; booleanFlagDefault: { type: BooleanConstructor; default: boolean; }; }
+>        : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+>booleanFlagDefault : { type: BooleanConstructor; default: boolean; }
+>                   : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
diff --git a/tests/cases/compiler/reverseMappedTypeInferenceWidening1.ts b/tests/cases/compiler/reverseMappedTypeInferenceWidening1.ts
new file mode 100644
index 0000000000000..bbaae1d00e01e
--- /dev/null
+++ b/tests/cases/compiler/reverseMappedTypeInferenceWidening1.ts
@@ -0,0 +1,60 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/62720
+
+type TypeFunction<ReturnType = unknown> = (...args: any[]) => ReturnType;
+type Flags = {
+  [flagName: string]: {
+    type: TypeFunction;
+    default?: unknown;
+  };
+};
+type TypeFlag<Schemas extends Flags> = {
+  [flag in keyof Schemas]: Schemas[flag] extends { type: TypeFunction<infer T> }
+    ? T
+    : never;
+};
+
+declare function fn1<Options extends Flags>(
+  options: Options,
+): TypeFlag<Options>;
+
+const result1 = fn1({
+  booleanFlag: { type: Boolean },
+  booleanFlagDefault: {
+    type: Boolean,
+    default: false,
+  },
+});
+result1.booleanFlag; // boolean
+result1.booleanFlagDefault; // boolean
+
+declare function fn2<Options extends Flags>(
+  options: Readonly<Options>,
+): TypeFlag<Options>;
+
+const result2 = fn2({
+  booleanFlag: { type: Boolean },
+  booleanFlagDefault: {
+    type: Boolean,
+    default: false,
+  },
+});
+result2.booleanFlag; // boolean
+result2.booleanFlagDefault; // boolean
+
+declare function fn3<Options extends Flags>(
+  options: Readonly<Options>,
+): Options;
+
+const result3 = fn3({
+  booleanFlag: { type: Boolean },
+  booleanFlagDefault: {
+    type: Boolean,
+    default: false, // no cursed EPC failure here
+  },
+});
+
+declare function takeType(arg: { type: unknown }): void;
+takeType(result3.booleanFlagDefault);

From 6fd5860b0aea561e8f25ab148962ac846db1eb19 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Wed, 5 Nov 2025 18:58:22 +0100
Subject: [PATCH 2/2] move the fix to `inferReverseMappedTypeWorker`

---
 src/compiler/checker.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index f84b6cf5cb67f..410b2c8913f18 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -26421,7 +26421,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     function getTypeOfReverseMappedSymbol(symbol: ReverseMappedSymbol): Type {
         const links = getSymbolLinks(symbol);
         if (!links.type) {
-            links.type = getWidenedType(inferReverseMappedType(symbol.links.propertyType, symbol.links.mappedType, symbol.links.constraintType) || unknownType);
+            links.type = inferReverseMappedType(symbol.links.propertyType, symbol.links.mappedType, symbol.links.constraintType) || unknownType;
         }
         return links.type;
     }
@@ -26431,7 +26431,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const templateType = getTemplateTypeFromMappedType(target);
         const inference = createInferenceInfo(typeParameter);
         inferTypes([inference], sourceType, templateType);
-        return getTypeFromInference(inference) || unknownType;
+        return getWidenedType(getTypeFromInference(inference) || unknownType);
     }
 
     function inferReverseMappedType(source: Type, target: MappedType, constraint: IndexType): Type | undefined {