Move logic from macroexpand_1 to macroexpand

By ending macro-expansion immediately when appropriate, this change fixes a bug arising from the fact that NaN != NaN.

NEWS.rst

@@ -42,6 +42,7 @@ Bug Fixes
 * Fixed a case where `->` and `->>` duplicated an argument
 * Fixed bugs that caused `defclass` to drop statements or crash
 * Fixed a REPL crash caused by illegle unicode escape string inputs
+* `NaN` can no longer create an infinite loop during macro-expansion
 
 Misc. Improvements
 ----------------------------

hy/macros.py

@@ -156,24 +156,16 @@ def make_empty_fn_copy(fn):
     return empty_fn
 
 
-def macroexpand(tree, compiler):
+def macroexpand(tree, compiler, once=False):
     """Expand the toplevel macros for the `tree`.
 
     Load the macros from the given `module_name`, then expand the (top-level)
-    macros in `tree` until it stops changing.
+    macros in `tree` until we no longer can.
 
     """
     load_macros(compiler.module_name)
-    old = None
-    while old != tree:
-        old = tree
-        tree = macroexpand_1(tree, compiler)
-    return tree
+    while True:
 
-
-def macroexpand_1(tree, compiler):
-    """Expand the toplevel macro from `tree` once, in the context of
-    `module_name`."""
         if not isinstance(tree, HyExpression) or tree == []:
             return tree
 
@@ -207,7 +199,16 @@ def macroexpand_1(tree, compiler):
         except Exception as e:
             msg = "expanding `" + str(tree[0]) + "': " + repr(e)
             raise HyMacroExpansionError(tree, msg)
-    return replace_hy_obj(obj, tree)
+        tree = replace_hy_obj(obj, tree)
+
+        if once:
+            return tree
+
+
+def macroexpand_1(tree, compiler):
+    """Expand the toplevel macro from `tree` once, in the context of
+    `compiler`."""
+    return macroexpand(tree, compiler, once=True)
 
 
 def tag_macroexpand(tag, tree, compiler):

tests/macros/test_macro_processor.py

@@ -5,7 +5,7 @@
 from hy.macros import macro, macroexpand
 from hy.lex import tokenize
 
-from hy.models import HyString, HyList, HySymbol, HyExpression
+from hy.models import HyString, HyList, HySymbol, HyExpression, HyFloat
 from hy.errors import HyMacroExpansionError
 
 from hy.compiler import HyASTCompiler
@@ -53,3 +53,12 @@ def test_preprocessor_exceptions():
     except HyMacroExpansionError as e:
         assert "_hy_anon_fn_" not in str(e)
         assert "TypeError" not in str(e)
+
+
+def test_macroexpand_nan():
+   # https://github.com/hylang/hy/issues/1574
+   import math
+   NaN = float('nan')
+   x = macroexpand(HyFloat(NaN), HyASTCompiler(__name__))
+   assert type(x) is HyFloat
+   assert math.isnan(x)