std_exception.html#enforce: example does not compile

[kdevel]

https://dlang.org/phobos/std_exception.html#enforce and also 
https://dlang.org/library/std/exception/enforce.html present this 
example:

---
auto f = enforce(fopen("data.txt"));
auto line = readln(f);
enforce(line.length, "Expected a non-empty line.");
---

fopen, readln and enforce need imports, fopen needs a mode 
parameter:

enforce.d
---
import core.stdc.stdio : fopen;
import std.stdio : readln;
import std.exception;

void main ()
{
    auto f = enforce(fopen("data.txt", "r"));
    auto line = readln(f);
    enforce(line.length, "Expected a non-empty line.");
}
---

$ dmd enforce
enforce.d(7): Error: function expected before (), not module 
enforce of type void

According to https://tour.dlang.org/tour/en/basics/exceptions I 
changed the import (not knowing why):

enforce.d
---
import core.stdc.stdio : fopen;
import std.stdio : readln;
import std.exception : enforce;

void main ()
{
    auto f = enforce(fopen("data.txt", "r"));
    auto line = readln(f);
    enforce(line.length, "Expected a non-empty line.");
}
---

Now I get:

$ dmd enforce
enforce.d(8): Error: template std.stdio.readln cannot deduce 
function from argument types !()(shared(_IO_FILE)*), candidates 
are:
/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d(3921):        
std.stdio.readln(S = string)(dchar terminator = '\x0a') if 
(isSomeString!S)
/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d(3955):        
std.stdio.readln(C)(ref C[] buf, dchar terminator = '\x0a') if 
(isSomeChar!C && is(Unqual!C == C) && !is(C == enum))
/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d(3962):        
std.stdio.readln(C, R)(ref C[] buf, R terminator) if 
(isSomeChar!C && is(Unqual!C == C) && !is(C == enum) && 
isBidirectionalRange!R && is(typeof(terminator.front == 
(dchar).init)))

What's wrong here? And why is the "selective import" of enforce 
necessary?