Clarify @main error message for invalid return type

[huangyxi]

When the @main function returns a value that is not nothing or convertible to Cint, the current error message may be a bit unclear or unhelpful.

For example, running the script below results in an error message that doesn’t clearly point to the underlying cause:

#!/usr/bin/env julia

# Lengthy, potentially flawed code

function @main(args)
	# Lengthy, potentially flawed code
	[] # Valid in regular functions, but not in `@main`
end

In this case, returning an empty array ([]) is perfectly fine in a normal function, but it’s not allowed in a @main context. The current error message doesn’t clearly emphasize this, which can make it tricky for users to understand and resolve the issue:

ERROR: MethodError: no method matching Int32(::Vector{Any})
The type `Int32` exists, but no method is defined for this combination of argument types when trying to construct it.

Closest candidates are:
  Int32(::Float64)
   @ Base float.jl:895
  Int32(::Float32)
   @ Base float.jl:919
  Int32(::Float16)
   @ Base float.jl:919
  ...

Stacktrace:
 [1] _start()
   @ Base ./client.jl:568

[huangyxi]

This PR would fix #56328.

[LilithHafner]

Thanks for the PR! I like the change to the error message. Why move it into an else block?

[huangyxi]

Thanks for your response.

It's fine to keep it as before. The reason I moved the new try-catch into the else block is that I want the original try-catch to handle only the code that is prone to failure.

[LilithHafner]

I recommend nesting the inner try-catch in the try block of the outer try-catch instead of nesting the inner try-catch in the else block of the outer try-catch. Either way there are nested try-catchs.

[LilithHafner]

Thanks, @huangyxi

[nsajko]

I think this line is unreachable?

Suggested change

Cint(1)

[huangyxi]

Thanks for your comment. Theoretically, @error only prints a message to stderr without throwing an error, so it does not skip the subsequent statements. In practice, changing Cint(1) to Cint(2) makes the program return 2 instead of 1, which proves that Cint(1) is indeed reachable.