PEP 634: expand section on side effects and undefined behavior

pep-0634.rst

@@ -517,8 +517,8 @@ This behavior is roughly equivalent to the following::
           return self
 
 
-Side Effects
-============
+Side Effects and Undefined Behavior
+===================================
 
 The only side-effect produced explicitly by the matching process is
 the binding of names.  However, the process relies on attribute
@@ -530,6 +530,15 @@ proposal intentionally leaves out any specification of what methods
 are called or how many times.  This behavior is therefore undefined
 and user code should not rely on it.
 
+Another undefined behavior is the binding of variables by capture
+patterns that are followed (in the same case block) by another pattern
+that fails.  These may happen earlier or later depending on the
+implementation strategy, the only constraint being that capture
+variables must be set before guards that use them explicitly are
+evaluated.  If a guard consists of an ``and`` clause, evaluation of
+the operands may even be interspersed with pattern matching, as long
+as left-to-right evaluation order is maintained.
+
 
 The Standard Library
 ====================