Generic parameter 'D' could not be inferred

[babbage (Duncan Babbage)]

You're not alone in wanting to do this, but prior to Swift 5.7, Swift cannot infer a type from a default parameter.

If I have understood the proposal correctly (I did not read the entire thing), a Swift Evolution proposal to do this was accepted in April 2022, and implemented in Swift 5.7:

[Accepted] SE-0347: Type inference from default expressions

Hello Swift Community, The review of SE-0347 "Type inference from default expressions" ended on April 5, 2022. The Core Team has decided to accept the proposal. Feedback was light but positive. Two main concerns came up: That this feature might not compose with default arguments for generic parameters, should that feature come in the future. The Core Team agrees with the author's analysis that default arguments for generic parameters could be introduced later without affecting either feature…

The alternative in the meantime is to implement a helper function which omits the default argument:

public static func execute<T: Decodable>(command: some ShellCommand) throws -> T { ... }

From this, call your original function and pass it your default parameter. Not ideal, but works.