Why does the statement "2i;" NOT cause a compiler error?

This is a gcc extension, and 2i is the imaginary constant . So you can write a complex number like so:

#include <complex.h>_Complex x = 4 + 5i;

2i is a gcc extension for a complex integer literal, a pure imaginary number twice the square root of -1. This extension is supported by clang as well.

It is somewhat surprising that your compiling with gcc 5.4.0 produces the posted assembly output:

• Compiling on http://gcc.godbolt.org/# I get a compilation error from gcc 5.3.0: http://gcc.godbolt.org/#: error: cannot convert '__complex__ int' to 'int' in return.
• The posted assembly code for function foo is incorrect: it does not return 0. Converting the complex integer constant 2i to int should return its real part 0.

Conversely, with clang 3.7, it compiles without a warning and generates optimum code, but of course not what you expect:

foo(int):                       # @foo(int)    xorl    %eax, %eax    retq

This syntax can be combined with other suffixes in any order. Compiling the code below with clang -Weverything gives me appropriate warnings warning: imaginary constants are a GNU extension [-Wgnu-imaginary-constant]:

#include <stdio.h>int main() {    /* complex integer literals */    printf("sizeof(2i) = %zd\n", sizeof(2i));    printf("sizeof(2ui) = %zd\n", sizeof(2ui));    printf("sizeof(2li) = %zd\n", sizeof(2li));    printf("sizeof(2lli) = %zd\n", sizeof(2lli));    /* complex floating point literals */    printf("sizeof(2.i) = %zd\n", sizeof(2.i));    printf("sizeof(2.fi) = %zd\n", sizeof(2.fi));    printf("sizeof(2e0fi) = %zd\n", sizeof(2e0fi));    printf("sizeof(2e0i) = %zd\n", sizeof(2e0i));    /* alternate order */    printf("sizeof(2il) = %zd\n", sizeof(2il));    printf("sizeof(2ill) = %zd\n", sizeof(2ill));    printf("sizeof(2.if) = %zd\n", sizeof(2.if));    return 0;}

It produces this output in my environment:

sizeof(2i) = 8sizeof(2ui) = 8sizeof(2li) = 16sizeof(2lli) = 16sizeof(2.i) = 16sizeof(2.fi) = 8sizeof(2e0fi) = 8sizeof(2e0i) = 16sizeof(2il) = 16sizeof(2ill) = 16sizeof(2.if) = 8

Try the last one with your syntax coloring editor ;-)