BTW, you have also hit upon why APL uses distinct symbols.

When your assignment operator is "←" you are never going to confuse assignment for comparison.

    Assignment:

    a ← 23

    Comparison:

    a == 23

Extending this to something like Python:

    # Assignment
    if x ← 3:
        do_something()


    # Comparison
    if x == 3:
        do_something()

In fact, you could argue that once you have a dedicated assignment operator you no longer need the "==" construct, because "=" is enough, which is precisely how it works in APL. The above examples the, would become:

    # Assignment
    if x ← 3: # x is assigned 3 and then evaluated
        do_something()


    # Comparison
    if x = 3: # Is x equal to 3? It even reads exactly as it is meant
        do_something()

So, if we want to reduce error and this kind of ambiguity the solution isn't to now have two different assignment operators using easy to mistype characters but rather to have distinct symbols that cannot be confused. Hence the genius of APL, where the notation is, as Iverson put it, a tool for though and the abomination of J, where notation was tossed out the window in favor of ASCII soup.

The ambiguity could be resolved easily by adopting notation that makes sense. Why do we use "A*B" for multiplication when we can use a proper multiplication symbol "A×B"? Or division? "A÷B" Not to mention the logical comparisons; Or:"A∨B", And: "A∧B" , Nor: "A⍱B", Nand: "A⍲B" (these are real symbols used in Logic, BTW, as well as APL).

Throw a warning? C compilers require additional parentheses of you want to use the value of assignment as a conditional. It's impossible to make a mistake there unless you're dumb on purpose that is disabling or ignoring compiler warnings. That being said I think it matters little if it's a separate operator or not. It solves a real issue I'm the language caused by assignment not being an expression. Better later than never I guess.

There is no ambiguity. This example is invalid syntax.

I don't understand, what is wrong with extending "=" such that in the example you gave "if(x=3)" it stores 3 in x and then makes the comparison?

More accurately, in Python it would be:

    if x = 3: # x is assigned 3 and then tested
        do_something_here()

This is different from:

    if x == 3: # x is compared to 3
        do_something_here()

Very clear difference between the two statements.

What if we want to assign and then compare?

    if (x = 3) == y:
        do_something_here()

or...

    if x = 3; x == y: #Python evaluates left to right...
        do_something_here()

In fact, you could...

    if x = 3; x == y; x = x + 1: #Python evaluates left to right...
        do_something_here()

Not super elegant, but it makes sense. I wouldn't want to write code like that in any language unless it was very well justified.

What if we want to compare and then assign?

    if x == y; x = 3: #Python evaluates left to right...
        do_something_here()

Side note: Why, oh why, don't we have pre and post increment/decrement (++/--) in Python? Another religious decision.

I guess I am having trouble seeing where this ambiguity might be. What is ambiguous about usage that is currently not legal and is later defined as a new way to make assignments?

In other words, nobody is using it that way today because it doesn't work. Tomorrow we say: From now on, you can make assignments inside of several conditional statements.

What's ambiguous about that?

This isn't foreign at all, is it? I mean, we have been able to do this in C for decades:

    int i;
    for(i=1; i<=3; i++)
    {
        printf("%d\n", i);
    }

In fact, you can do this:

    int i;
    for(i=1; i<=3; i=i+1)
    {
        printf("%d\n", i);
    }

That's TWO assignments, not just one. No special operator or syntax. Last I checked no airplanes have crashed, MRI machines stopped working and nobody died because of this. Forgive me but, I just don't see the ambiguity at all. Once you define the extension of functionality the rest is not a problem.

That said, I am eager to learn. I've only been designing hardware and software since about 1982 or so and have programmed in nearly every mainstream language in existence on platforms ranging from embedded to Silicon Graphics supercomputers and even the web. It is quite possible I am confused or just don't understand something. I do have a very pragmatic view of this stuff. By which I mean to say: Simple is usually better and there has to be a very, very good reason to reinvent any wheel.