Why both clang and gcc only give a warning when there is a space after backslash if C standard says that whitespace is forbidden?

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The compiler is allowed to extend the language as long as the document the change which gcc does in their docs in section 6.21 Slightly Looser Rules for Escaped Newlines.

Recently, the preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.

and clang strives to support gcc extensions and points to the gcc docs on them:

this document describes the language extensions provided by Clang. In addition to the language extensions listed here, Clang aims to support a broad range of GCC extensions. Please see the GCC manual for more information on these extensions.

So their obligations with respect to the standard are fulfilled. In fact Linux depends on many gcc extensions. We can see this by looking at the draft C99 standard section 4. Conformance paragraphs 6 which say:

[…]A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behavior of any strictly conforming
program.3)

footnote 3 says:

This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this International Standard.

and paragraph 8:

An implementation shall be accompanied by a document that defines all implementation defined and locale-specific characteristics and all extensions.

gcc also documents that you can use the -pedantic flag to generate a warning when using extensions and you can use -pedantic-errors flag to make it an error:

[…] to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings).

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