Use widened type (just like importing using module.exports = in js file)

Fixes microsoft#26429

Author: sheetalkamat

src/compiler/checker.ts

@@ -5092,7 +5092,14 @@ namespace ts {
             // Handle export default expressions
             if (isSourceFile(declaration)) {
                 const jsonSourceFile = cast(declaration, isJsonSourceFile);
-                return jsonSourceFile.statements.length ? checkExpression(jsonSourceFile.statements[0].expression) : emptyObjectType;
+                if (!jsonSourceFile.statements.length) {
+                    return emptyObjectType;
+                }
+                const type = getWidenedLiteralType(checkExpression(jsonSourceFile.statements[0].expression));
+                if (type.flags & TypeFlags.Object) {
+                    return getRegularTypeOfObjectLiteral(type);
+                }
+                return type;
             }
             if (declaration.kind === SyntaxKind.ExportAssignment) {
                 return checkExpression((<ExportAssignment>declaration).expression);

tests/baselines/reference/requireOfJsonFileInJsFile.errors.txt

@@ -1,17 +1,22 @@
 /user.js(2,7): error TS2339: Property 'b' does not exist on type '{ "a": number; }'.
+/user.js(5,7): error TS2322: Type '{ "a": number; }' is not assignable to type '{ b: number; }'.
+  Property 'b' is missing in type '{ "a": number; }'.
 /user.js(9,7): error TS2339: Property 'b' does not exist on type '{ "a": number; }'.
 /user.js(12,7): error TS2322: Type '{ a: number; }' is not assignable to type '{ b: number; }'.
   Property 'b' is missing in type '{ a: number; }'.
 
 
-==== /user.js (3 errors) ====
+==== /user.js (4 errors) ====
     const json0 = require("./json.json");
     json0.b; // Error (good)
           ~
 !!! error TS2339: Property 'b' does not exist on type '{ "a": number; }'.
 
     /** @type {{ b: number }} */
     const json1 = require("./json.json"); // No error (bad)
+          ~~~~~
+!!! error TS2322: Type '{ "a": number; }' is not assignable to type '{ b: number; }'.
+!!! error TS2322:   Property 'b' is missing in type '{ "a": number; }'.
     json1.b; // No error (OK since that's the type annotation)
 
     const js0 = require("./js.js");

tests/baselines/reference/requireOfJsonFileTypes.types

@@ -6,10 +6,10 @@ import c = require('./c.json');
 >c : (string | null)[]
 
 import d = require('./d.json');
->d : "dConfig"
+>d : string
 
 import e = require('./e.json');
->e : -10
+>e : number
 
 import f = require('./f.json');
 >f : number[]
@@ -64,14 +64,14 @@ const stringOrNumberOrNull: string | number | null = c[0];
 >0 : 0
 
 stringLiteral = d;
->stringLiteral = d : "dConfig"
+>stringLiteral = d : string
 >stringLiteral : string
->d : "dConfig"
+>d : string
 
 numberLiteral = e;
->numberLiteral = e : -10
+>numberLiteral = e : number
 >numberLiteral : number
->e : -10
+>e : number
 
 numberLiteral = f[0];
 >numberLiteral = f[0] : number