Support callable instances (#2517)

* Support callable instances

* Clean prettier lint

---------

Co-authored-by: - <->

Author: bojavou

lib/sinon/proxy-invoke.js

@@ -43,7 +43,10 @@ module.exports = function invoke(func, thisValue, args) {
                 concat([thisValue], args)
             ))();
 
-            if (typeof returnValue !== "object") {
+            if (
+                typeof returnValue !== "object" &&
+                typeof returnValue !== "function"
+            ) {
                 returnValue = thisValue;
             }
         } else {

test/proxy-call-test.js

@@ -1246,6 +1246,48 @@ describe("sinonSpy.call", function () {
         });
     });
 
+    describe("constructor return", function () {
+        it("preserves returned object", function () {
+            const customReturn = {};
+            function CustomConstructor() {
+                return customReturn;
+            }
+            const SpiedCustomConstructor = sinonSpy(CustomConstructor);
+            const myInstance = new SpiedCustomConstructor();
+
+            assert(myInstance === customReturn);
+        });
+
+        it("allows explicit returned object", function () {
+            const StubConstructor = sinonStub();
+            const customReturn = {};
+            StubConstructor.returns(customReturn);
+            const myInstance = new StubConstructor();
+
+            assert(myInstance === customReturn);
+        });
+
+        it("preserves returned function", function () {
+            function customReturn() {} // eslint-disable-line no-empty-function
+            function CustomConstructor() {
+                return customReturn;
+            }
+            const SpiedCustomConstructor = sinonSpy(CustomConstructor);
+            const myInstance = new SpiedCustomConstructor();
+
+            assert(myInstance === customReturn);
+        });
+
+        it("allows explicit returned function", function () {
+            const StubConstructor = sinonStub();
+            function customReturn() {} // eslint-disable-line no-empty-function
+            StubConstructor.returns(customReturn);
+            const myInstance = new StubConstructor();
+
+            assert(myInstance === customReturn);
+        });
+    });
+
     describe("functions", function () {
         it("throws if spying on non-existent property", function () {
             const myObj = {};