Fix desugaring of return without argument

flisp lowering deals with this in the parser by inserting the `nothing`
there. JuliaSyntax avoids this in order to more faithfully represent the
source, so we need to deal with it in lowering instead.

Author: c42f

src/desugaring.jl

@@ -4236,6 +4236,14 @@ function expand_forms_2(ctx::DesugaringContext, ex::SyntaxTree, docs=nothing)
         throw(LoweringError(ex, "`...` expression outside call"))
     elseif is_leaf(ex)
         ex
+    elseif k == K"return"
+        if numchildren(ex) == 0
+            @ast ctx ex [K"return" "nothing"::K"core"]
+        elseif numchildren(ex) == 1
+            mapchildren(e->expand_forms_2(ctx,e), ctx, ex)
+        else
+            throw(LoweringError(ex, "More than one argument to return"))
+        end
     else
         mapchildren(e->expand_forms_2(ctx,e), ctx, ex)
     end

test/functions_ir.jl

@@ -896,6 +896,28 @@ end
 9   TestMod.f
 10  (return %₉)
 
+########################################
+# Function return without arguments
+function f()
+    return
+    after_return # <- distinguish output from implicit return
+end
+#---------------------
+1   (method TestMod.f)
+2   TestMod.f
+3   (call core.Typeof %₂)
+4   (call core.svec %₃)
+5   (call core.svec)
+6   SourceLocation::1:10
+7   (call core.svec %₄ %₅ %₆)
+8   --- method core.nothing %₇
+    slots: [slot₁/#self#(!read)]
+    1   (return core.nothing)
+    2   TestMod.after_return
+    3   (return %₂)
+9   TestMod.f
+10  (return %₉)
+
 ########################################
 # Binding docs to functions
 """