Mark libcpp.optional.value as except + (#6190)

std::optional::value() raises a std::bad_optional_access
if it does not contain a value.

Cython/Includes/libcpp/optional.pxd

@@ -13,7 +13,7 @@ cdef extern from "<optional>" namespace "std" nogil:
         optional(optional&) except +
         optional(T&) except +
         bool has_value()
-        T& value()
+        T& value() except +
         T& value_or[U](U& default_value)
         void swap(optional&)
         void reset()

tests/run/cpp_stl_optional.pyx

@@ -13,6 +13,12 @@ def simple_test():
     """
     cdef optional[int] o
     assert(not o.has_value())
+    try:
+        o.value()
+    except Exception as err:
+        pass
+    else:
+        assert False, "value() did not raise a catchable error"
     o = 5
     assert(o.has_value())
     assert(o.value()==5)