Note on type inference and loop targets.

docs/src/quickstart/cythonize.rst

@@ -137,8 +137,9 @@ in general, a ``nogil`` block may contain only "white" code.
 
 .. figure:: htmlreport.png
 
-Note that Cython deduces the type of local variables based on their assignments,
-which can also cut down on the need to explicitly specify types everywhere.
+Note that Cython deduces the type of local variables based on their assignments
+(including as loop variable targets) which can also cut down on the need to
+explicitly specify types everywhere.
 For example, declaring ``dx`` to be of type double above is unnecessary,
 as is declaring the type of ``s`` in the last version (where the return type
 of ``f`` is known to be a C double.)  A notable exception, however, is

docs/src/reference/compilation.rst

@@ -397,6 +397,9 @@ Cython code.  Here is the list of currently supported directives:
     Infer types of untyped variables in function bodies. Default is
     None, indicating that only safe (semantically-unchanging) inferences
     are allowed.
+    In particular, inferring *integral* types for variables *used in arithmetic
+    expressions* is considered unsafe (due to possible overflow) and must be
+    explicitly requested.
 
 ``language_level`` (2/3)
     Globally set the Python language level to be used for module

docs/src/userguide/wrapping_CPlusPlus.rst

@@ -510,6 +510,9 @@ comprehensions).  For example, one can write::
         f(value)
     return [x*x for x in v if x % 2 == 0]
 
+If the loop target variable is unspecified, an assignment from type
+``*container.begin()`` is used for :ref:`type inference <compiler-directives>`.
+
 
 Simplified wrapping with default constructor
 --------------------------------------------