If you compare C89 with C++ then here are a couple of things
No tentative definitions in C++
int n;
int n; // ill-formed: n already defined
int[] and int[N] not compatible (no compatible types in C++)
int a[1];
int (*ap)[] = &a; // ill-formed: a does not have type int[]
No K&R function definition style
int b(a) int a; { } // ill-formed: grammar error
Nested struct has class-scope in C++
struct A { struct B { int a; } b; int c; };
struct B b; // ill-formed: b has incomplete type (*not* A::B)
No default int
auto a; // ill-formed: type-specifier missing
C99 adds a whole lot of other cases
No special handling of declaration specifiers in array dimensions of parameters
// ill-formed: invalid syntax
void f(int p[static 100]) { }
No variable length arrays
// ill-formed: n is not a constant expression
int n = 1;
int an[n];
No flexible array member
// ill-formed: fam has incomplete type
struct A { int a; int fam[]; };
No restrict qualifier for helping aliasing analysis
// ill-formed: two names for one parameter?
void copy(int *restrict src, int *restrict dst);