Compilers that required # on the first column?

Were there widely used pre-ANSI C compilers^† that required the # to be on the first column?

_{† I would accept any compiler on this list. If I can find mention of it in the comp.lang.c Usenet newsgroup in a post dated before 1995, I would accept it.}

K&R C did not specify whether whitespace was permitted before the #. From the original The C Programming Language, §12¶1 of the "C Reference Manual" in Appendix A:

The C compiler contains a preprocessor capable of macro substitution, conditional compilation, and inclusion of named files. Lines beginning with # communicate with this preprocessor.

Thus, whether or not whitespace was permitted to precede the # was unspecified. This would mean a pre-ANSI compiler could fail to compile a program if the directive did not begin on the first column.

In ISO C (and in ANSI C before that), the C preprocessing directives were explicitly permitted to be prefixed with whitespace. In ANSI C (C-89):

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character, and is ended by the next new-line character.

ISO C.2011 has similar language, but is clarified even further:

A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following constraints: The first token in the sequence is a # preprocessing token that (at the start of translation phase 4) is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character. The last token in the sequence is the first newline character that follows the first token in the sequence.¹⁶⁵⁾ A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.
_{165) Thus, preprocessing directives are commonly called ‘‘lines’’. These ‘‘lines’’ have no other syntactic
significance, as all white space is equivalent except in certain situations during preprocessing (see the
# character string literal creation operator in 6.10.3.2, for example).}

Solution

Short answer: Yes.

I remember writing things like

#if foo
    /* ... */
#else
#if bar
    /* ... */
#else
 #error "neither foo nor bar specified"
#endif
#endif

so that the various pre-ANSI compilers that I once used wouldn't complain about "unrecognized preprocessor directive '#error'". This would have been with Ritchie's original cc for the pdp11, or pcc (the "portable C compiler" which, IIRC, was the basis for the Vax cc of the 80's or so). Both of those compilers -- more accurately, the preprocessor used with both of those compilers -- definitely required the # to be in the first column. (Actually, although those compilers were very different, they might both have used different variants of basically the same preprocessor, which was always a separate program in those days.)