Non constexpr call from constexpr template function

Question

I stumbled upon constexpr template functions calling non constexpr functions: The next line of the fragment fails to compile, as expected, due to a call to non constexpr, but foo compiles. Can someone tell me the reason why compiling foo?

template<class T>
void set(T& x){
    x++;
}

template<class T>
constexpr void foo(T& x){
    set<T>(x);
}

constexpr void bar(int& x){
    set<int>(x);
}

void bar(){
    int x = 5;
    foo(x);
    bar(x);
}

The compiler cannot compile with an error:

<source>: In function 'constexpr void bar(int&)':
<source>:12:13: error: call to non-constexpr function 'void set(T&) [with T = int]'
     set<int>(x);
     ~~~~~~~~^~~
Compiler returned: 1

Edit: Added compiler error and rehash question. Side effects are not concentrated here.

Bolov Rekete1111 , . constexpr , constexpr sem constexpr. -Og , constexpr foo , foo2 , constexpr (, of constexpr):

template<class T>
void set(volatile T& x){
    x++;
}

template<class T>
constexpr void foo(T& x){
    set<T>(x);
}

template<class T>
void foo2(T& x){
    set<T>(x);
}

void bar(){
    int x = 5;
    foo(x);
    foo2(x);
}

:

void set<int>(int volatile&):
  ldr r3, [r0]
  add r3, r3, #1
  str r3, [r0]
  bx lr
void foo2<int>(int&):
  push {r4, lr}
  bl void set<int>(int volatile&)
  pop {r4, lr}
  bx lr
bar():
  push {r4, lr}
  sub sp, sp, #8
  add r4, sp, #8
  mov r3, #5
  str r3, [r4, #-4]!
  mov r0, r4
  bl void set<int>(int volatile&)
  mov r0, r4
  bl void foo2<int>(int&)
  add sp, sp, #8
  pop {r4, lr}
  bx lr

Answer 1

, foo bar .

(, bar) constexpr, constexpr ( ) . , .

, , constexpr. . , set<T>(x) constexpr, set, constexpr set, . , foo constexpr. foo, constexpr . foo<int> foo<char> ..

++ , constexpr (). , constexpr, , .

:

auto set(int a) { return a; }
constexpr auto set(char a) { return a; }

template<class T>
constexpr auto foo(T x){
    return set(x);
}

auto test()
{
    auto x = foo(24); // foo<int>  OK, no error
    //constexpr auto cx = foo(24) // foo<int> compiler error

    auto y = foo('a'); // foo<char> OK, no erro
    constexpr auto y = foo('a'); // foo<char> OK
}

§7.1.5 [dcl.constexpr]

6. constexpr - constexpr constexpr, - constexpr constexpr , . constexpr constexpr , , ; .