Better error when using non-existant attributes from cimported modules.

When not imported, this caused was a runtime NameError.  Now an error
is given at compile time.

Cython/Compiler/ExprNodes.py

@@ -1858,6 +1858,9 @@ class NameNode(AtomicExprNode):
             self.entry = env.declare_var(self.name, type, self.pos)
         if self.entry.is_declared_generic:
             self.result_ctype = py_object_type
+        if self.entry.as_module:
+            # cimported modules namespace can shadow actual variables
+            self.entry.is_variable = 1
 
     def analyse_types(self, env):
         self.initialized_check = env.directives['initializedcheck']
@@ -1963,10 +1966,16 @@ class NameNode(AtomicExprNode):
             or entry.is_cpp_class):
                 if self.entry.as_variable:
                     self.entry = self.entry.as_variable
-                else:
+                elif not self.is_cython_module:
                     error(self.pos,
                           "'%s' is not a constant, variable or function identifier" % self.name)
 
+    def is_cimported_module_without_shadow(self, env):
+        if self.is_cython_module or self.cython_attribute:
+            return False
+        entry = self.entry or env.lookup(self.name)
+        return entry.as_module and not entry.is_variable
+
     def is_simple(self):
         #  If it's not a C variable, it'll be in a temp.
         return 1
@@ -6374,6 +6383,9 @@ class AttributeNode(ExprNode):
                     entry.is_cglobal or entry.is_cfunction
                     or entry.is_type or entry.is_const):
                 return self.as_name_node(env, entry, target)
+            if self.is_cimported_module_without_shadow(env):
+                error(self.pos, "cimported module has no attribute '%s'" % self.attribute)
+                return self
         return None
 
     def analyse_as_type_attribute(self, env):
@@ -6610,6 +6622,9 @@ class AttributeNode(ExprNode):
 
     gil_message = "Accessing Python attribute"
 
+    def is_cimported_module_without_shadow(self, env):
+        return self.obj.is_cimported_module_without_shadow(env)
+
     def is_simple(self):
         if self.obj:
             return self.result_in_temp() or self.obj.is_simple()

Cython/Compiler/Symtab.py

@@ -1255,6 +1255,7 @@ class ModuleScope(Scope):
                 return entry
         else:
             entry = self.declare_var(name, py_object_type, pos)
+            entry.is_variable = 0
         entry.as_module = scope
         self.add_imported_module(scope)
         return entry

tests/compile/libc_stdio.pyx

 # mode: compile
 
-cimport libc
+cimport libc.stdio
 from libc cimport stdio
 from libc.stdio cimport printf, puts, fputs, putchar, fputc, putc, stdout
 
 
-libc.stdio.printf("hello %s\n", b"world")
-stdio.printf("hello %s\n", b"world")
-printf("hello %s\n", b"world")
-printf("printf_output %d %d\n", 1, 2)
-puts("puts_output")
-fputs("fputs_output", stdout)
-putchar(b'z')
-fputc(b'x', stdout)
-putc(b'c', stdout)
+with nogil:
+    libc.stdio.printf("hello %s\n", b"world")
+    stdio.printf("hello %s\n", b"world")
+    printf("hello %s\n", b"world")
+    printf("printf_output %d %d\n", 1, 2)
+    puts("puts_output")
+    fputs("fputs_output", stdout)
+    putchar(b'z')
+    fputc(b'x', stdout)
+    putc(b'c', stdout)