Enable temps for C functions by using function pointers instead.

Closes GH-3418.

CHANGES.rst

@@ -21,6 +21,9 @@ Bugs fixed
   implemented functions.
   Patch by David Woods.  (Github issue #3384)
 
+* Using C functions as temporary values lead to invalid C code.
+  Original patch by David Woods.  (Github issue #3418)
+
 * Fix an unhandled C++ exception in comparisons.
   Patch by David Woods.  (Github issue #3361)
 

Cython/Compiler/Code.py

@@ -832,6 +832,9 @@ class FunctionState(object):
             type = type.const_base_type
         elif type.is_reference and not type.is_fake_reference:
             type = type.ref_base_type
+        elif type.is_cfunction:
+            from . import PyrexTypes
+            type = PyrexTypes.c_ptr_type(type)  # A function itself isn't an l-value
         if not type.is_pyobject and not type.is_memoryviewslice:
             # Make manage_ref canonical, so that manage_ref will always mean
             # a decref is needed.

tests/run/if_else_expr.pyx

@@ -55,3 +55,15 @@ def test_syntax():
     y = 0 if 1.0else 1
     z = 0 if 1.else 1
     return x, y, z
+
+
+from libc cimport math
+
+def test_cfunc_ptrs(double x, bint round_down):
+    """
+    >>> test_cfunc_ptrs(2.5, round_down=True)
+    2.0
+    >>> test_cfunc_ptrs(2.5, round_down=False)
+    3.0
+    """
+    return (math.floor if round_down else math.ceil)(x)