Fix bug with unset __name__ of one-line functions

The bug was a regression that I introduced in #1228.

I've created a new special form named `fn*` that works like the old `fn` (that is, it always creates a `FunctionDef`). Since this is intended only for internal use, like `with*`, I haven't documented it.

hy/compiler.py

@@ -2297,9 +2297,13 @@ class HyASTCompiler(object):
         return ret
 
     @builds("fn")
+    @builds("fn*")
+    # The starred version is for internal use (particularly, in the
+    # definition of `defn`). It ensures that a FunctionDef is
+    # produced rather than a Lambda.
     @checkargs(min=1)
     def compile_function_def(self, expression):
-        expression.pop(0)
+        force_functiondef = expression.pop(0) == "fn*"
 
         arglist = expression.pop(0)
         if not isinstance(arglist, HyList):
@@ -2376,7 +2380,7 @@ class HyASTCompiler(object):
             defaults=defaults)
 
         body = self._compile_branch(expression)
-        if not body.stmts:
+        if not force_functiondef and not body.stmts:
             ret += ast.Lambda(
                 lineno=expression.start_line,
                 col_offset=expression.start_column,

hy/core/bootstrap.hy

@@ -46,7 +46,7 @@
     (macro-error name "defn takes a name as first argument"))
   (if (not (isinstance lambda-list hy.HyList))
     (macro-error name "defn takes a parameter list as second argument"))
-  `(setv ~name (fn ~lambda-list ~@body)))
+  `(setv ~name (fn* ~lambda-list ~@body)))
 
 (defmacro if-python2 [python2-form python3-form]
   "If running on python2, execute python2-form, else, execute python3-form"

tests/native_tests/language.hy

@@ -900,6 +900,19 @@
   (assert (= 43 (my-fun 42))))
 
 
+(defn test-defn-dunder-name []
+  "NATIVE: test that defn preserves __name__"
+
+  (defn phooey [x]
+    (+ x 1))
+  (assert (= phooey.__name__ "phooey"))
+
+  (defn mooey [x]
+    (+= x 1)
+    x)
+  (assert (= mooey.__name__ "mooey")))
+
+
 (defn test-mangles []
   "NATIVE: test mangles"
   (assert (= 2 ((fn [] (+ 1 1))))))