> C certainly could have added support for nested comments at any time

After C89 was ratified, adding nested comments to C would have risked breaking existing code. For instance, this is a valid program in C89:

  #include <stdio.h>

  int main() {
      /* /* Comment */
      printf("hello */ world");
      return 0;
  }

However, if a later C standard were to introduce nested comments, it would break the above program because then the following part of the program would be recognised as a comment:

      /* /* Comment */
      printf("hello */

The above text would be ignored. Then the compiler would encounter the following:

      world");

This would lead to errors like undeclared identifier 'world', missing terminating " character, etc.

Given the neighboring thread where I just learned that the lexer runs before the preprocessor, I’m not sure that would be the outcome. There’s no reason to assume the comment terminator wouldn’t be ignored in strings. And even today, you can safely write printf(“hello // world\n”); without risking a compile error, right?

> Given the neighboring thread where I just learned that the lexer runs before the preprocessor, I’m not sure that would be the outcome.

That is precisely why nested comments would end up breaking the C89 code example I provided above. I elaborate this further below.

> There’s no reason to assume the comment terminator wouldn’t be ignored in strings.

There is no notion of "comment terminator in strings" in C. At any point of time, the lexer is reading either a string or a comment but never one within the other. For example, in C89, C99, etc., this is an invalid C program too:

  #include <stdio.h>

  int main() {
      /* Comment
      printf("hello */ world");
      return 0;
  }

In this case, we wouldn't say that the lexer is "honoring the comment terminator in a string" because, at the point the comment terminator '*/' is read, there is no active string. There is only a comment that looks like this:

      /* Comment
      printf("hello */

The double quotation mark within the comment is immaterial. It is simply part of the comment. Once the lexer has read the opening '/*', it looks for the terminating '*/'. This behaviour would hold even if future C standards were to allow nested comments, which is why nested comments would break the C89 example I mentioned in my earlier HN comment.

> And even today, you can safely write printf("hello // world\n"); without risking a compile error, right?

Right. But it is not clear what this has got to do with my concern that nested comments would break valid C89 programs. In this printf() example, we only have an ordinary string, so obviously this compiles fine. Once the lexer has read the opening quotation mark as the beginning of a string, it looks for an unescaped terminating quotation mark. So clearly, everything until the unescaped terminating quotation mark is a string!

AFAIK, C didn't get single line comments until C99. They were a C++ feature originally.

Oh wow, I didn’t remember that, and I did start writing C before 99. I stand corrected. I guess that is a little surprising. ;)

Is true that many languages had single line comments? Maybe I’m forgetting more, but I remember everything else having single line comments… asm, basic, shell. I used Pascal in the 80s and apparently forgot it didn’t have line comments either?

Some C compilers supported it as an unofficial extension well before C99, so that could be why you didn't realise or don't remember. I think that included both Visual Studio (which was really a C++ compiler that could turn off the C++ bits) and GCC with GNU extensions enabled.

That's my recollection, that most languages had single line comments. Some had multi-line comments but C++ is the first I remember having syntaxes for both. That said, I'm not terribly familiar with pre-80s stuff.