This is both an answer and an extended comment to No, this isn't legal C since 1999. No decent modern C compiler allows for this.

Yes, auto a=1; is illegal in C1999 (and also C2011). Just because this is now illegal does not mean that a modern C compiler should reject code that contains such constructs. I would argue exactly the opposite, that a decent modern C compiler must still allow for this.

Both clang and gcc do just that when compiling the sample code in the question against the 1999 or 2011 versions of the standard. Both compilers issue a diagnostic and then carry on as if the objectionable statement had been auto int a=1; .

In my opinion, this is what a decent compiler should do. By issuing a diagnostic, clang and gcc are full compliant with the standard. The standard does not say that a compiler must reject illegal code. The standard merely says that a conforming implementation must produce at least one diagnostic message if a translation unit contains a violation of any syntax rule or constraint (5.1.1.3).

Given code that contains illegal constructs, any decent compiler will try to make sense of the illegal code so that the compiler can find the next error in the code. A compiler that stops at the first error isn't a very good compiler. There is a way to make sense out of auto a=1 , which is to apply the "implicit int" rule. This rule forces the compiler to interpret auto a=1 as if it were auto int a=1 when the compiler is used in C90 or K&R mode.