Make assignments of None to C typed variables a compile time error.

Always coerce Python arguments as "num_threads" in parallel sections to C integers, also to catch some coercion problems at compile time.

Closes #1957.

Cython/Compiler/ExprNodes.py

@@ -1081,6 +1081,12 @@ class NoneNode(PyConstNode):
     def may_be_none(self):
         return True
 
+    def coerce_to(self, dst_type, env):
+        if not (dst_type.is_pyobject or dst_type.is_error):
+            # Catch this error early and loudly.
+            error(self.pos, "Cannot assign None to %s" % dst_type)
+        return super(NoneNode, self).coerce_to(dst_type, env)
+
 
 class EllipsisNode(PyConstNode):
     #  '...' in a subscript list.

Cython/Compiler/Nodes.py

@@ -8060,7 +8060,7 @@ class ParallelStatNode(StatNode, ParallelNode):
                     self.num_threads.compile_time_value(env) <= 0):
                 error(self.pos, "argument to num_threads must be greater than 0")
 
-            if not self.num_threads.is_simple():
+            if not self.num_threads.is_simple() or self.num_threads.type.is_pyobject:
                 self.num_threads = self.num_threads.coerce_to(
                     PyrexTypes.c_int_type, env).coerce_to_temp(env)
         return self

tests/errors/e_cython_parallel.pyx

@@ -149,6 +149,10 @@ with nogil, cython.parallel.parallel():
     with cython.parallel.parallel():
         pass
 
+with nogil, cython.parallel.parallel(num_threads=None):
+    pass
+
+
 _ERRORS = u"""
 3:8: cython.parallel.parallel is not a module
 4:0: No such directive: cython.parallel.something
@@ -183,4 +187,6 @@ _ERRORS = u"""
 139:62: Chunksize not valid for the schedule runtime
 145:70: Calling gil-requiring function not allowed without gil
 149:33: Nested parallel with blocks are disallowed
+152:49: Cannot assign None to int
+152:49: Coercion from Python not allowed without the GIL
 """