Try emitting an error when export would be emitted with different unwidened type

src/compiler/checker.ts

@@ -885,6 +885,7 @@ import {
     ObjectLiteralElementLike,
     ObjectLiteralExpression,
     ObjectType,
+    OmittedExpression,
     OptionalChain,
     OptionalTypeNode,
     or,
@@ -2241,6 +2242,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     var potentialReflectCollisions: Node[] = [];
     var potentialUnusedRenamedBindingElementsInTypes: BindingElement[] = [];
     var awaitedTypeStack: number[] = [];
+    var ambiguousVariableDeclarationsWhenWidened: [node: ParameterDeclaration | PropertyDeclaration | PropertySignature | VariableDeclaration | BindingElement, type: Type, widenedLiteralType: Type][] = [];
 
     var diagnostics = createDiagnosticCollection();
     var suggestionDiagnostics = createDiagnosticCollection();
@@ -43141,6 +43143,40 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             }
             checkCollisionsForDeclarationName(node, node.name);
         }
+
+        // TODO(jakebailey): copy pasted from declaration.ts; improve
+        type CanHaveLiteralInitializer = VariableDeclaration | PropertyDeclaration | PropertySignature | ParameterDeclaration;
+        function canHaveLiteralInitializer(node: Node): boolean {
+            switch (node.kind) {
+                case SyntaxKind.PropertyDeclaration:
+                case SyntaxKind.PropertySignature:
+                    return !hasEffectiveModifier(node, ModifierFlags.Private);
+                case SyntaxKind.Parameter:
+                case SyntaxKind.VariableDeclaration:
+                    return true;
+            }
+            return false;
+        }
+
+        function shouldPrintWithInitializer(node: Node) {
+            return canHaveLiteralInitializer(node) && isLiteralConstDeclaration(node as CanHaveLiteralInitializer); // TODO: Make safe
+        }
+
+        function isPrivateDeclaration(node: Node) {
+            return hasEffectiveModifier(node, ModifierFlags.Private) || isPrivateIdentifierClassElementDeclaration(node);
+        }
+
+        if (
+            getEmitDeclarations(compilerOptions)
+            && !(symbol.flags & (SymbolFlags.TypeLiteral | SymbolFlags.Signature))
+            && !shouldPrintWithInitializer(node)
+            && !isPrivateDeclaration(node)
+        ) {
+            const widenedLiteralType = getWidenedLiteralType(type);
+            if (!isTypeIdenticalTo(type, widenedLiteralType)) {
+                ambiguousVariableDeclarationsWhenWidened.push([node, type, widenedLiteralType]);
+            }
+        }
     }
 
     function errorNextVariableOrPropertyDeclarationMustHaveSameType(firstDeclaration: Declaration | undefined, firstType: Type, nextDeclaration: Declaration, nextType: Type): void {
@@ -47243,6 +47279,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             clear(potentialWeakMapSetCollisions);
             clear(potentialReflectCollisions);
             clear(potentialUnusedRenamedBindingElementsInTypes);
+            clear(ambiguousVariableDeclarationsWhenWidened);
 
             forEach(node.statements, checkSourceElement);
             checkSourceElement(node.endOfFileToken);
@@ -47267,6 +47304,26 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             });
 
+            addLazyDiagnostic(() => {
+                function isVisibleExternally(elem: ParameterDeclaration | PropertyDeclaration | PropertySignature | VariableDeclaration | BindingElement | OmittedExpression): boolean {
+                    if (isOmittedExpression(elem)) {
+                        return false;
+                    }
+                    if (isBindingPattern(elem.name)) {
+                        return some(elem.name.elements, isVisibleExternally);
+                    }
+                    else {
+                        return isDeclarationVisible(elem);
+                    }
+                }
+
+                for (const [node, type, widenedLiteralType] of ambiguousVariableDeclarationsWhenWidened) {
+                    if (isVisibleExternally(node)) {
+                        error(node.name, Diagnostics.The_type_of_this_declaration_is_ambiguous_and_may_be_observed_as_either_0_or_1, typeToString(widenedLiteralType), typeToString(type));
+                    }
+                }
+            });
+
             if (isExternalOrCommonJsModule(node)) {
                 checkExternalModuleExports(node);
             }
@@ -47291,6 +47348,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 clear(potentialReflectCollisions);
             }
 
+            if (ambiguousVariableDeclarationsWhenWidened.length) {
+                // forEach(potentialAmbiguousDeclarationsWhenWidened, checkAmbiguousDeclaration);
+                clear(ambiguousVariableDeclarationsWhenWidened);
+            }
+
             links.flags |= NodeCheckFlags.TypeChecked;
         }
     }

src/compiler/diagnosticMessages.json

@@ -3755,6 +3755,10 @@
         "category": "Error",
         "code": 2868
     },
+    "The type of this declaration is ambiguous and may be observed as either '{0}' or '{1}'.": {
+        "category": "Error",
+        "code": 2869
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/ambientConstLiterals.errors.txt

@@ -0,0 +1,30 @@
+ambientConstLiterals.ts(20,7): error TS2869: The type of this declaration is ambiguous and may be observed as either 'string' or '"abc" | "def"'.
+ambientConstLiterals.ts(21,7): error TS2869: The type of this declaration is ambiguous and may be observed as either 'number' or '123 | 456'.
+
+
+==== ambientConstLiterals.ts (2 errors) ====
+    function f<T>(x: T): T {
+        return x;
+    }
+
+    enum E { A, B, C, "non identifier" }
+
+    const c1 = "abc";
+    const c2 = 123;
+    const c3 = c1;
+    const c4 = c2;
+    const c5 = f(123);
+    const c6 = f(-123);
+    const c7 = true;
+    const c8 = E.A;
+    const c8b = E["non identifier"];
+    const c9 = { x: "abc" };
+    const c10 = [123];
+    const c11 = "abc" + "def";
+    const c12 = 123 + 456;
+    const c13 = Math.random() > 0.5 ? "abc" : "def";
+          ~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'string' or '"abc" | "def"'.
+    const c14 = Math.random() > 0.5 ? 123 : 456;
+          ~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'number' or '123 | 456'.

tests/baselines/reference/ambiguousLiteralWideningEmit.js

@@ -0,0 +1,23 @@
+//// [tests/cases/compiler/ambiguousLiteralWideningEmit.ts] ////
+
+//// [ambiguousLiteralWideningEmit.ts]
+declare function pad(n: number | string): string;
+
+export default (dateString: string, type: 'date' | 'month' | 'year'): string => {
+    const [year, month = 1, date = 1] = dateString.split('-')
+    return `${year}-${pad(month)}-${pad(date)}`.substr(0, { date: 10, month: 7, year: 4 }[type])
+}
+
+
+//// [ambiguousLiteralWideningEmit.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.default = (function (dateString, type) {
+    var _a = dateString.split('-'), year = _a[0], _b = _a[1], month = _b === void 0 ? 1 : _b, _c = _a[2], date = _c === void 0 ? 1 : _c;
+    return "".concat(year, "-").concat(pad(month), "-").concat(pad(date)).substr(0, { date: 10, month: 7, year: 4 }[type]);
+});
+
+
+//// [ambiguousLiteralWideningEmit.d.ts]
+declare const _default: (dateString: string, type: 'date' | 'month' | 'year') => string;
+export default _default;

tests/baselines/reference/ambiguousLiteralWideningEmit.symbols

@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/ambiguousLiteralWideningEmit.ts] ////
+
+=== ambiguousLiteralWideningEmit.ts ===
+declare function pad(n: number | string): string;
+>pad : Symbol(pad, Decl(ambiguousLiteralWideningEmit.ts, 0, 0))
+>n : Symbol(n, Decl(ambiguousLiteralWideningEmit.ts, 0, 21))
+
+export default (dateString: string, type: 'date' | 'month' | 'year'): string => {
+>dateString : Symbol(dateString, Decl(ambiguousLiteralWideningEmit.ts, 2, 16))
+>type : Symbol(type, Decl(ambiguousLiteralWideningEmit.ts, 2, 35))
+
+    const [year, month = 1, date = 1] = dateString.split('-')
+>year : Symbol(year, Decl(ambiguousLiteralWideningEmit.ts, 3, 11))
+>month : Symbol(month, Decl(ambiguousLiteralWideningEmit.ts, 3, 16))
+>date : Symbol(date, Decl(ambiguousLiteralWideningEmit.ts, 3, 27))
+>dateString.split : Symbol(String.split, Decl(lib.es5.d.ts, --, --))
+>dateString : Symbol(dateString, Decl(ambiguousLiteralWideningEmit.ts, 2, 16))
+>split : Symbol(String.split, Decl(lib.es5.d.ts, --, --))
+
+    return `${year}-${pad(month)}-${pad(date)}`.substr(0, { date: 10, month: 7, year: 4 }[type])
+>`${year}-${pad(month)}-${pad(date)}`.substr : Symbol(String.substr, Decl(lib.es5.d.ts, --, --))
+>year : Symbol(year, Decl(ambiguousLiteralWideningEmit.ts, 3, 11))
+>pad : Symbol(pad, Decl(ambiguousLiteralWideningEmit.ts, 0, 0))
+>month : Symbol(month, Decl(ambiguousLiteralWideningEmit.ts, 3, 16))
+>pad : Symbol(pad, Decl(ambiguousLiteralWideningEmit.ts, 0, 0))
+>date : Symbol(date, Decl(ambiguousLiteralWideningEmit.ts, 3, 27))
+>substr : Symbol(String.substr, Decl(lib.es5.d.ts, --, --))
+>date : Symbol(date, Decl(ambiguousLiteralWideningEmit.ts, 4, 59))
+>month : Symbol(month, Decl(ambiguousLiteralWideningEmit.ts, 4, 69))
+>year : Symbol(year, Decl(ambiguousLiteralWideningEmit.ts, 4, 79))
+>type : Symbol(type, Decl(ambiguousLiteralWideningEmit.ts, 2, 35))
+}
+

tests/baselines/reference/ambiguousLiteralWideningEmit.types

@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/ambiguousLiteralWideningEmit.ts] ////
+
+=== ambiguousLiteralWideningEmit.ts ===
+declare function pad(n: number | string): string;
+>pad : (n: number | string) => string
+>n : string | number
+
+export default (dateString: string, type: 'date' | 'month' | 'year'): string => {
+>(dateString: string, type: 'date' | 'month' | 'year'): string => {    const [year, month = 1, date = 1] = dateString.split('-')    return `${year}-${pad(month)}-${pad(date)}`.substr(0, { date: 10, month: 7, year: 4 }[type])} : (dateString: string, type: 'date' | 'month' | 'year') => string
+>dateString : string
+>type : "date" | "month" | "year"
+
+    const [year, month = 1, date = 1] = dateString.split('-')
+>year : string
+>month : string | 1
+>1 : 1
+>date : string | 1
+>1 : 1
+>dateString.split('-') : string[]
+>dateString.split : (separator: string | RegExp, limit?: number | undefined) => string[]
+>dateString : string
+>split : (separator: string | RegExp, limit?: number | undefined) => string[]
+>'-' : "-"
+
+    return `${year}-${pad(month)}-${pad(date)}`.substr(0, { date: 10, month: 7, year: 4 }[type])
+>`${year}-${pad(month)}-${pad(date)}`.substr(0, { date: 10, month: 7, year: 4 }[type]) : string
+>`${year}-${pad(month)}-${pad(date)}`.substr : (from: number, length?: number | undefined) => string
+>`${year}-${pad(month)}-${pad(date)}` : string
+>year : string
+>pad(month) : string
+>pad : (n: string | number) => string
+>month : string | 1
+>pad(date) : string
+>pad : (n: string | number) => string
+>date : string | 1
+>substr : (from: number, length?: number | undefined) => string
+>0 : 0
+>{ date: 10, month: 7, year: 4 }[type] : number
+>{ date: 10, month: 7, year: 4 } : { date: number; month: number; year: number; }
+>date : number
+>10 : 10
+>month : number
+>7 : 7
+>year : number
+>4 : 4
+>type : "date" | "month" | "year"
+}
+

tests/baselines/reference/declarationEmitBindingPatterns2.errors.txt

@@ -0,0 +1,14 @@
+declarationEmitBindingPatterns2.ts(7,16): error TS2869: The type of this declaration is ambiguous and may be observed as either 'number' or '1 | 0'.
+
+
+==== declarationEmitBindingPatterns2.ts (1 errors) ====
+    // https://github.com/microsoft/TypeScript/issues/55439
+
+    function foo(): { y: 1 } {
+      return { y: 1 };
+    }
+
+    export const { y = 0 } = foo();
+                   ~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'number' or '1 | 0'.
+

tests/baselines/reference/declarationEmitBindingPatterns2.js

@@ -0,0 +1,16 @@
+//// [tests/cases/compiler/declarationEmitBindingPatterns2.ts] ////
+
+//// [declarationEmitBindingPatterns2.ts]
+// https://github.com/microsoft/TypeScript/issues/55439
+
+function foo(): { y: 1 } {
+  return { y: 1 };
+}
+
+export const { y = 0 } = foo();
+
+
+
+
+//// [declarationEmitBindingPatterns2.d.ts]
+export declare const y: number;

tests/baselines/reference/declarationEmitBindingPatterns2.symbols

@@ -0,0 +1,17 @@
+//// [tests/cases/compiler/declarationEmitBindingPatterns2.ts] ////
+
+=== declarationEmitBindingPatterns2.ts ===
+// https://github.com/microsoft/TypeScript/issues/55439
+
+function foo(): { y: 1 } {
+>foo : Symbol(foo, Decl(declarationEmitBindingPatterns2.ts, 0, 0))
+>y : Symbol(y, Decl(declarationEmitBindingPatterns2.ts, 2, 17))
+
+  return { y: 1 };
+>y : Symbol(y, Decl(declarationEmitBindingPatterns2.ts, 3, 10))
+}
+
+export const { y = 0 } = foo();
+>y : Symbol(y, Decl(declarationEmitBindingPatterns2.ts, 6, 14))
+>foo : Symbol(foo, Decl(declarationEmitBindingPatterns2.ts, 0, 0))
+

tests/baselines/reference/declarationEmitBindingPatterns2.types

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/declarationEmitBindingPatterns2.ts] ////
+
+=== declarationEmitBindingPatterns2.ts ===
+// https://github.com/microsoft/TypeScript/issues/55439
+
+function foo(): { y: 1 } {
+>foo : () => {    y: 1;}
+>y : 1
+
+  return { y: 1 };
+>{ y: 1 } : { y: 1; }
+>y : 1
+>1 : 1
+}
+
+export const { y = 0 } = foo();
+>y : 1 | 0
+>0 : 0
+>foo() : { y: 1; }
+>foo : () => { y: 1; }
+

tests/baselines/reference/declarationEmitSpreadStringlyKeyedEnum.js

@@ -53,3 +53,53 @@ export declare const SpotifyAgeGroupEnum: {
     "60-150": (typeof AgeGroups)["60-150"];
 };
 export {};
+
+
+//// [DtsFileErrors]
+
+
+declarationEmitSpreadStringlyKeyedEnum.d.ts(12,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["0-17"]'.
+declarationEmitSpreadStringlyKeyedEnum.d.ts(13,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["18-22"]'.
+declarationEmitSpreadStringlyKeyedEnum.d.ts(14,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["23-27"]'.
+declarationEmitSpreadStringlyKeyedEnum.d.ts(15,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["28-34"]'.
+declarationEmitSpreadStringlyKeyedEnum.d.ts(16,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["35-44"]'.
+declarationEmitSpreadStringlyKeyedEnum.d.ts(17,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["45-59"]'.
+declarationEmitSpreadStringlyKeyedEnum.d.ts(18,5): error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["60-150"]'.
+
+
+==== declarationEmitSpreadStringlyKeyedEnum.d.ts (7 errors) ====
+    declare enum AgeGroups {
+        "0-17" = 0,
+        "18-22" = 1,
+        "23-27" = 2,
+        "28-34" = 3,
+        "35-44" = 4,
+        "45-59" = 5,
+        "60-150" = 6
+    }
+    export declare const SpotifyAgeGroupEnum: {
+        [x: number]: string;
+        "0-17": (typeof AgeGroups)["0-17"];
+        ~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["0-17"]'.
+        "18-22": (typeof AgeGroups)["18-22"];
+        ~~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["18-22"]'.
+        "23-27": (typeof AgeGroups)["23-27"];
+        ~~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["23-27"]'.
+        "28-34": (typeof AgeGroups)["28-34"];
+        ~~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["28-34"]'.
+        "35-44": (typeof AgeGroups)["35-44"];
+        ~~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["35-44"]'.
+        "45-59": (typeof AgeGroups)["45-59"];
+        ~~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["45-59"]'.
+        "60-150": (typeof AgeGroups)["60-150"];
+        ~~~~~~~~
+!!! error TS2869: The type of this declaration is ambiguous and may be observed as either 'AgeGroups' or '(typeof AgeGroups)["60-150"]'.
+    };
+    export {};
+