lowering: Disallow splatting in non-final default value (#50563)

Pop quiz: Do you know what the following will do?
```
julia> function g1(a=(1,2)..., b...=3)
    b
end

julia> g1()

julia> function g2(a=(1,2)..., b=3, c=4)
    (b, c)
end

julia> g2()

julia> function g3(a=(1,2)..., b=3, c...=4)
    (b, c)
end

julia> g3()

julia> g3(1)
```

I don't either and I don't think it's particularly well defined.
Splatting a default argument makes sense on the last argument, which can
be a vararg, and it is desirable to be able to specify the default for
the whole varargs tuple at once (although arguably that should just be
the non-`...` behavior, but that'd be too breaking a change). Ref
#50518. However, for other arguments, there isn't really a sensible
semantic meaning. This PR disallows this in lowering. This is
technically a minor change, but I doubt anybody is using this. Splatting
in default values wasn't really ever supposed to work anyway, it just
happened to fall out of our lowering.

---------

Co-authored-by: Jeff Bezanson <[email protected]>

Author: Keno

src/julia-syntax.scm

@@ -663,7 +663,10 @@
                       (vals   (list-tail dfl n))
                       (absent (list-tail opt n)) ;; absent arguments
                       (body
-                       (if (any (lambda (defaultv)
+                       (if (any vararg? (butlast vals))
+                         ;; Forbid splat in all but the final default value
+                         (error "invalid \"...\" in non-final positional argument default value")
+                         (if (any (lambda (defaultv)
                                   ;; does any default val expression...
                                   (contains (lambda (e)
                                               ;; contain "e" such that...
@@ -682,7 +685,7 @@
                            ;; otherwise add all
                            `(block
                              ,@prologue
-                             (call ,(arg-name (car req)) ,@(map arg-name (cdr passed)) ,@vals)))))
+                             (call ,(arg-name (car req)) ,@(map arg-name (cdr passed)) ,@vals))))))
                  (method-def-expr- name sp passed body)))
              (iota (length opt)))
       ,(method-def-expr- name sparams overall-argl body rett))))

test/syntax.jl

@@ -3508,6 +3508,26 @@ let x = 1 => 2
     @test_throws "function Base.=> must be explicitly imported to be extended" @eval a => b = 2
 end
 
+# Splatting in non-final default value (Ref #50518)
+for expr in (quote
+    function g1(a=(1,2)..., b...=3)
+        b
+    end
+end,quote
+    function g2(a=(1,2)..., b=3, c=4)
+        (b, c)
+    end
+end,quote
+    function g3(a=(1,2)..., b=3, c...=4)
+        (b, c)
+    end
+end)
+    let exc = try eval(expr); catch exc; exc end
+        @test isa(exc, ErrorException)
+        @test startswith(exc.msg, "syntax: invalid \"...\" in non-final positional argument default value")
+    end
+end
+
 # Test that bad lowering does not segfault (ref #50518)
 @test_throws ErrorException("syntax: Attempted to use slot marked unused") @eval function funused50518(::Float64)
     $(Symbol("#unused#"))