Does G++ compile it?

g++ (Gentoo 4.3.4 p1.0, pie-10.1.5) 4.3.4
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

• int main() {
  <%"\?"<:0:>;%>
}

  Yes Once the C99 digraphs are simplified, the line reads

    {"\?"[0];}
  

  which is equivalent to simply '?';.
• int main() {
  char file_pattern[] = "all-??-of-20";
}

  Yes ??- is a C trigraph which, by default, is treated as normal text with a warning.
  

  src/trigraph.cc:2:30: warning: trigraph ??- ignored, use -trigraphs to enable
  

• int main() {
  int x,y=x?:y;
}

  Yes A GCC extension allows omitting the middle operand of a conditional. If the condition is true, the condition is returned.
• #define assert(a) FOO
#include <assert.h>
#undef assert
#include <assert.h>

int main() {
  assert(0);
}

  Yes ISO C99 Standard: 7.2 Diagnostics specifies that assert is redefined every time assert.h is included. Mostly this is to allow defining NDEBUG after the first inclusion of assert.h.
• #define foo bar
#define foo bar
int main() {}

  Yes
• #define b a
#define c b
#define c a
int main() {}

  Yes
  

  src/define_indirect.cc:3:1: warning: "c" redefined
  src/define_indirect.cc:2:1: warning: this is the location of the previous definition
  

• #define a b
#define b a
int main() {
  int a, b;
}

  Yes
• class Bar {};
Bar Bar();
int main() {}

  Yes
• class Bar {};
Bar Bar;
int main() {}

  Yes
• class Bar {};
typedef Bar Bar;
int main() {}

  Yes
• typedef int int;
int main() {}

  No
  

  src/int_typedef.cc:1: error: multiple types in one declaration
  src/int_typedef.cc:1: error: declaration does not declare anything
  

• int main() {
  int i();
  return i;
}

  No For C compatibility, i is interpreted as a function declaration.
  

  src/int_init.cc: In function ‘int main()’:
  src/int_init.cc:3: error: invalid conversion from ‘int (*)()’ to ‘int’
  

• int i = 08;
int main() {}

  No A leading 0 introduces an octal number. 8 is not octal.
  

  src/int_number.cc:1:9: error: invalid digit "8" in octal constant
  

• int i = i;
int main() {}

  Yes
• int main() {
  int i, j;
  (+ +i+++ + ++++j);
}

  Yes Read as +(+(i++)) + +(++++j). The pluses before i are unary + and do nothing. The the third + after i is actually binary addition.
• int main() { int i; i++++; }

  No
  

  src/plus_post2.cc: In function ‘int main()’:
  src/plus_post2.cc:1: error: lvalue required as increment operand
  

• int main() { int i; ++i++; }

  No
  

  src/plus_prepost.cc: In function ‘int main()’:
  src/plus_prepost.cc:1: error: lvalue required as increment operand
  

• #include <algorithm>
int main() {
  int a;
  unsigned int b = 0;
  return std::max(a, b);
}

  No Typical type matching requirement of C++. Use std::max<int> instead.
  

  src/max.cc: In function ‘int main()’:
  src/max.cc:5: error: no matching function for call to ‘max(int&, unsigned int&)’
  

• int main() {
  return main();
}

  Yes
• #include <vector>
template <class T> void Foo(std::vector<T> in) {
  for (std::vector<T>::iterator i = in.begin(); i != in.end(); ++i) {}
}
int main() {}

  No The compiler can't tell that std::vector<T>::iterator is a type. Fix with typename std::vector<T>::iterator.
  

  src/template.cc: In function ‘void Foo(std::vector<T, std::allocator<_Tp1> >)’:
  src/template.cc:3: error: expected `;' before ‘i’
  src/template.cc:3: error: ‘i’ was not declared in this scope
  

• template <unsigned size> unsigned OptimizedStrlen(const char value[size]) {
  return size;
}
int main() {
  OptimizedStrlen("value");
}

  No
  

  src/template_array.cc: In function ‘int main()’:
  src/template_array.cc:5: error: no matching function for call to ‘OptimizedStrlen(const char [6])’
  

• template <unsigned size> unsigned OptimizedStrlen(const char (&value)[size]) {
  return size;
}
int main() {
  OptimizedStrlen("value");
}

  Yes discussion
• template <class S> struct A {
  template <class T> static void f() {}
};
template <class T> void g() {
  A<T>::f<T>();
}
int main() {}

  No
  

  src/template_member.cc: In function ‘void g()’:
  src/template_member.cc:5: error: expected primary-expression before ‘>’ token
  src/template_member.cc:5: error: expected primary-expression before ‘)’ token
  

• template <class S> struct A {
  template <class T> static void f() {}
};
template <class T> void g() {
  A<T>::template f<T>();
}
int main() {}

  Yes
• template <class T = unsigned int> void DoNothing() {}

int main() {}

  No
  

  src/template_function.cc:1: error: default template arguments may not be used in function templates
  

• int main() {
  int a[3];
  2[a];
}

  Yes foo[bar] is equivalent to *(foo + bar).
• class Foo {
  virtual void Bar() = 0;
};
void Foo::Bar() {}
int main() {}

  Yes
• class Foo {
  virtual void Bar() = 0;
  void Bar() {}
};
int main() {}

  No
  

  src/virtual2.cc:3: error: ‘void Foo::Bar()’ cannot be overloaded
  src/virtual2.cc:2: error: with ‘virtual void Foo::Bar()’
  

• int main() {
  char a;
  switch(a) {
    case 0 ... 2:
      break;
  }
}

  Yes Extension
• int main() {
  (1, "foo");
}

  Yes
• #include <vector>
namespace foo {
  struct Bar {};
}
int main() {
  std::vector<::foo::Bar> baz;
}

  No
  

  src/digraph_namespace.cc: In function ‘int main()’:
  src/digraph_namespace.cc:6: error: ‘<::’ cannot begin a template-argument list
  src/digraph_namespace.cc:6: note: ‘<:’ is an alternate spelling for ‘[’. Insert whitespace between ‘<’ and ‘::’
  src/digraph_namespace.cc:6: note: (if you use -fpermissive G++ will accept your code)