Assert and undefined behavior

[John Burton via Digitalmars-d-learn]

The spec says this :-

"As a contract, an assert represents a guarantee that the code 
must uphold. Any failure of this expression represents a logic 
error in the code that must be fixed in the source code. A 
program for which the assert contract is false is, by definition, 
invalid, and therefore has undefined behaviour."

Now I worry about the words "undefined behavior" because in C++ 
compiler writers seem to have decided that these words mean that 
it's ok for the compiler to generate code to do whatever it feels 
like even in unconnected code and even before the undefined 
behavior is invoked because some subsequent code has undefined 
behavior.

From my C++ experience this paragraph tells me that if I use 
"assert" to check my assumptions, and the assertion is false, 
then this could lead to my program failing in unpredictable ways 
unconnected with the actual assertion.

I therefore feel like I ought to not use assert and should 
instead validate my assumptions with an if statement and a throw 
or exit or something.

I feel like a failing assertion should not cause "undefined 
behavior" in the sense it is commonly used in C++ programming 
these days but should have exactly defined behavior that it will 
do nothing if the assert passes and throw the specified exception 
if it fails. Can I safely assume this despite the wording?

I know this might seem like a small or pedantic point, but C++ 
compilers can and do use invoking undefined behavior as an excuse 
to do all kinds of unexpected things in generated code these days 
and I want to write safe code :) I feel that if D is specified in 
the same way then assert is not safe for me to use in a real 
program.