iam: cannot pass iam.ManagedPolicy or iam.Policy to lambda.Function.grantInvoke

[Tietew]

Describe the bug

Following code fails:

const func = new lambda.Function(this, 'Function', { /* ... */ });
const managedPolicy = new iam.ManagedPolicy(this, 'ManagedPolicy');
func.grantInvoke(managedPolicy);

The error message is:

Cannot use a ManagedPolicy 'MyStack/Function' as the 'Principal' or 'NotPrincipal' in an IAM Policy

Regression Issue

• Select this option if this issue appears to be a regression.

Last Known Working CDK Version

No response

Expected Behavior

Grant lambda:InvokeFunction to ManagedPolicy or Policy.

Current Behavior

grantInvoke() throws an error described above.

Reproduction Steps

See the description above.

Possible Solution

grantInvoke() calls grantee.grantPrincipal.policyFragment.conditions to create a dedupe hash.

aws-cdk/packages/aws-cdk-lib/aws-lambda/lib/function-base.ts

Lines 435 to 442 in 2b2443d

	public grantInvoke(grantee: iam.IGrantable): iam.Grant {
	const hash = createHash('sha256')
	.update(JSON.stringify({
	principal: grantee.grantPrincipal.toString(),
	conditions: grantee.grantPrincipal.policyFragment.conditions,
	}), 'utf8')
	.digest('base64');
	const identifier = `Invoke${hash}`;

But policyFragment getter of ManagedPolicy and Policy throws an error. (see #22712)
It should return a dummy policy fragment like Group.

aws-cdk/packages/aws-cdk-lib/aws-iam/lib/group.ts

Lines 82 to 84 in 2b2443d

	public get policyFragment(): PrincipalPolicyFragment {
	return new ArnPrincipal(this.groupArn).policyFragment;
	}

Group is blocked in PolicyStatement

aws-cdk/packages/aws-cdk-lib/aws-iam/lib/policy-statement.ts

Lines 240 to 244 in 2b2443d

	private validatePolicyPrincipal(principal: IPrincipal) {
	if (principal instanceof Group) {
	throw new Error('Cannot use an IAM Group as the \'Principal\' or \'NotPrincipal\' in an IAM Policy');
	}
	}

Additional Information/Context

No response

CDK CLI Version

2.176.0

Framework Version

2.176.0

Node.js Version

22.13.0

OS

Ubuntu

Language

TypeScript

Language Version

No response

Other information

Related to #32795

[pahud]

We generally func.grantInvoke() on principals, not managed policies.
See Resource-based Policies.

What is your intention and use case?