RFC: Allow macrocall as function sig

@KristofferC requested that `function @main(args) end` should work.
This is currently a parse error. This PR makes it work as expected
by allowing macrocall as a valid signature in function (needs
to exapand to a call expr). Note that this is only the flisp changes.
If this PR is accepted, an equivalent change would need to be made
in JuliaSyntax.

base/client.jl

@@ -599,7 +599,7 @@ The `@main` macro may be used standalone or as part of the function definition,
 case, parentheses are required. In particular, the following are equivalent:
 
 ```
-function (@main)(args)
+function @main(args)
     println("Hello World")
 end
 ```
@@ -618,7 +618,7 @@ imported into `Main`, it will be treated as an entrypoint in `Main`:
 ```
 module MyApp
     export main
-    (@main)(args) = println("Hello World")
+    @main(args) = println("Hello World")
 end
 using .MyApp
 # `julia` Will execute MyApp.main at the conclusion of script execution
@@ -628,7 +628,7 @@ Note that in particular, the semantics do not attach to the method
 or the name:
 ```
 module MyApp
-    (@main)(args) = println("Hello World")
+    @main(args) = println("Hello World")
 end
 const main = MyApp.main
 # `julia` Will *NOT* execute MyApp.main unless there is a separate `@main` annotation in `Main`
@@ -638,9 +638,6 @@ const main = MyApp.main
     This macro is new in Julia 1.11. At present, the precise semantics of `@main` are still subject to change.
 """
 macro main(args...)
-    if !isempty(args)
-        error("`@main` is expected to be used as `(@main)` without macro arguments.")
-    end
     if isdefined(__module__, :main)
         if Base.binding_module(__module__, :main) !== __module__
             error("Symbol `main` is already a resolved import in module $(__module__). `@main` must be used in the defining module.")
@@ -651,5 +648,9 @@ macro main(args...)
         global main
         global var"#__main_is_entrypoint__#"::Bool = true
     end)
-    esc(:main)
+    if !isempty(args)
+        Expr(:call, esc(:main), map(esc, args)...)
+    else
+        esc(:main)
+    end
 end

src/julia-parser.scm

@@ -1329,13 +1329,12 @@
 
 (define (valid-func-sig? paren sig)
   (and (pair? sig)
-       (or (eq? (car sig) 'call)
-           (eq? (car sig) 'tuple)
+       (or (memq (car sig) '(macrocall call tuple))
            (and paren (eq? (car sig) 'block))
            (and paren (eq? (car sig) '...))
            (and (eq? (car sig) '|::|)
                 (pair? (cadr sig))
-                (eq? (car (cadr sig)) 'call))
+                (memq (car (cadr sig)) '(call macrocall)))
            (and (eq? (car sig) 'where)
                 (valid-func-sig? paren (cadr sig))))))
 

test/syntax.jl

@@ -3904,3 +3904,31 @@ module ExtendedIsDefined
         @test !$(Expr(:isdefined, GlobalRef(@__MODULE__, :x4), false))
     end
 end
+
+# PR# 55040 - Macrocall as function sig
+@test :(function @f()() end) == :(function (@f)() end)
+
+function callme end
+macro callmemacro(args...)
+    Expr(:call, esc(:callme), map(esc, args)...)
+end
+function @callmemacro(a::Int)
+    return 1
+end
+@callmemacro(b::Float64) = 2
+function @callmemacro(a::T, b::T) where T <: Int64
+    return 3
+end
+function @callmemacro(a::Int, b::Int, c::Int)::Float64
+    return 4
+end
+function @callmemacro(d::String)
+    (a, b, c)
+    # ^ Should not be accidentally parsed as an argument list
+    return 4
+end
+
+@test callme(1) === 1
+@test callme(2.0) === 2
+@test callme(3, 3) === 3
+@test callme(4, 4, 4) === 4.0