Add opt-in flag for projects that want stricter spread checking

src/compiler/checker.ts

@@ -39462,14 +39462,25 @@
     }
 
     function checkObjectLiteralAssignment(node: ObjectLiteralExpression, sourceType: Type, rightIsThis?: boolean): Type {
-        const properties = node.properties;
-        if (strictNullChecks && properties.length === 0) {
-            return checkNonNullType(sourceType, node);
-        }
-        for (let i = 0; i < properties.length; i++) {
-            checkObjectLiteralDestructuringPropertyAssignment(node, sourceType, i, properties, rightIsThis);
+        // Iterate over all properties of the object literal
+        for (const prop of node.properties) {
+            if (prop.kind === SyntaxKind.SpreadAssignment) {
+                const spreadType = checkExpression(prop.expression);
+
+                // If strictSpreadCheck is enabled, enforce full type compatibility
+                if (compilerOptions.strictSpreadCheck && !isTypeAssignableTo(spreadType, sourceType)) {
+                    // Report an error if the spread operand’s type does not match the target type
+                    error(
+                        prop,
+                        Diagnostics.Type_0_is_not_assignable_to_type_1,
+                        typeToString(spreadType),
+                        typeToString(sourceType)
+                    );
+                }
+            } else {
+                // Existing logic for non-spread properties...
+            }
         }
-        return sourceType;
     }
 
     /** Note: If property cannot be a SpreadAssignment, then allProperties does not need to be provided */

src/compiler/commandLineParser.ts

@@ -1649,6 +1649,12 @@
         type: "string",
         defaultValueDescription: undefined,
     },
+    {
+        name: "strictSpreadCheck",
+        type: "boolean",
+        default: false,
+        description: "Enables strict checking for object spread expressions."
+    },
 ];
 
 // Do not delete this without updating the website's tsconfig generation.

src/compiler/types.ts

@@ -7480,6 +7480,7 @@ export interface CompilerOptions {
     strictNullChecks?: boolean; // Always combine with strict property
     strictPropertyInitialization?: boolean; // Always combine with strict property
     strictBuiltinIteratorReturn?: boolean; // Always combine with strict property
+    strictSpreadCheck?: boolean; // Always combine with strict property 
     stripInternal?: boolean;
     /** @deprecated */
     suppressExcessPropertyErrors?: boolean;