util: make sure error causes of any type may be inspected

lib/internal/util/inspect.js

@@ -1198,7 +1198,7 @@ function getStackFrames(ctx, err, stack) {
   const frames = stack.split('\n');
 
   // Remove stack frames identical to frames in cause.
-  if (err.cause) {
+  if (err.cause && isError(err.cause)) {
     const causeStack = getStackString(err.cause);
     const causeStackStart = causeStack.indexOf('\n    at');
     if (causeStackStart !== -1) {
@@ -1265,7 +1265,8 @@ function formatError(err, constructor, tag, ctx, keys) {
 
   removeDuplicateErrorKeys(ctx, keys, err, stack);
 
-  if (err.cause && (keys.length === 0 || !keys.includes('cause'))) {
+  if (err.cause !== undefined &&
+      (keys.length === 0 || !keys.includes('cause'))) {
     keys.push('cause');
   }
 

test/message/util-inspect-error-cause.js

@@ -13,6 +13,19 @@ const cause2 = new FoobarError('Individual message', { cause: cause1 });
 cause2.extraProperties = 'Yes!';
 const cause3 = new Error('Stack causes', { cause: cause2 });
 
+const cause4 = new Error('Number error cause', { cause: 42 });
+const cause5 = new Error('Object cause', {
+  cause: {
+    message: 'Unique',
+    name: 'Error',
+    stack: 'Error: Unique\n' +
+           '    at Module._compile (node:internal/modules/cjs/loader:827:30)'
+  }
+});
+
+console.log(cause4);
+console.log(cause5);
+
 process.nextTick(() => {
   const error = new RangeError('New Stack Frames', { cause: cause2 });
   const error2 = new RangeError('New Stack Frames', { cause: cause3 });

test/message/util-inspect-error-cause.out

@@ -1,3 +1,28 @@
+Error: Number error cause
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at * {
+  [cause]: 42
+}
+Error: Object cause
+    at *
+    at *
+    at *
+    at *
+    at *
+    at *
+    at * {
+  [cause]: {
+    message: 'Unique',
+    name: 'Error',
+    stack: 'Error: Unique\n' +
+      '    at Module._compile (node:internal/modules/cjs/loader:827:30)'
+  }
+}
 RangeError: New Stack Frames
     at *
 *[90m    at *[39m {

test/parallel/test-util-inspect.js

@@ -660,6 +660,16 @@ assert.strictEqual(util.inspect(-5e-324), '-5e-324');
   assert(ex.includes('[message]'));
 }
 
+{
+  const falsyCause1 = new Error('', { cause: false });
+  delete falsyCause1.stack;
+  const falsyCause2 = new Error(undefined, { cause: null });
+  falsyCause2.stack = '';
+
+  assert.strictEqual(util.inspect(falsyCause1), '[Error] { [cause]: false }');
+  assert.strictEqual(util.inspect(falsyCause2), '[Error] { [cause]: null }');
+}
+
 {
   const tmp = Error.stackTraceLimit;
   Error.stackTraceLimit = 0;