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digitalmars.D.learn - I don't understand a const

reply bearophile <bearophileHUGS lycos.com> writes:
This program doesn't compile, dmd prints the errors:
test.d(4): Error: template test.foo(T) does not match any function template
declaration
test.d(4): Error: template test.foo(T) cannot deduce template function from
argument types !()(const(int),int)
test.d(9): Error: template instance test.bar!(int) error instantiating

Are you able to tell me what is the error in this code?


void foo(T)(const T a, out T b) {}

void bar(T)(const T x, out T y) {
    foo(x, y); // line 4
}

void main() {
    int s1, s2;
    bar(s1, s2); // line 9
}


Bye, and thank you,
bearophile
Sep 01 2010
next sibling parent reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
bearophile <bearophileHUGS lycos.com> wrote:


 This program doesn't compile, dmd prints the errors:
 test.d(4): Error: template test.foo(T) does not match any function  
 template declaration
 test.d(4): Error: template test.foo(T) cannot deduce template function  
 from argument types !()(const(int),int)
 test.d(9): Error: template instance test.bar!(int) error instantiating

 Are you able to tell me what is the error in this code?


 void foo(T)(const T a, out T b) {}

 void bar(T)(const T x, out T y) {
     foo(x, y); // line 4
 }

 void main() {
     int s1, s2;
     bar(s1, s2); // line 9
 }


 Bye, and thank you,
 bearophile


Simpler test case:

void bar(T)(const T x, out T y) {}

void main() {
     const int s1;
     int s2;
     bar(s1, s2);
}

It seems DMD is confused by const(int) being such a nice fit for the
first parameter. This might have to do with s2 = s1 being ok.

It is probably worth noting that this works:


import std.traits;

void bar(T)(const T x, out T y) {}

void main() {
     const int s1;
     int s2;
     bar!(CommonType!(s1,s2))(s1, s2);
}


-- 
Simen
Sep 02 2010
parent reply Brad Roberts <braddr puremagic.com> writes:
On 9/2/2010 5:59 AM, Simen kjaeraas wrote:

 bearophile <bearophileHUGS lycos.com> wrote:
 

 This program doesn't compile, dmd prints the errors:
 test.d(4): Error: template test.foo(T) does not match any function template
 declaration
 test.d(4): Error: template test.foo(T) cannot deduce template function from
 argument types !()(const(int),int)
 test.d(9): Error: template instance test.bar!(int) error instantiating

 Are you able to tell me what is the error in this code?


 void foo(T)(const T a, out T b) {}

 void bar(T)(const T x, out T y) {
     foo(x, y); // line 4
 }

 void main() {
     int s1, s2;
     bar(s1, s2); // line 9
 }


 Bye, and thank you,
 bearophile


 
 Simpler test case:
 
 void bar(T)(const T x, out T y) {}
 
 void main() {
     const int s1;
     int s2;
     bar(s1, s2);
 }
 
 It seems DMD is confused by const(int) being such a nice fit for the
 first parameter. This might have to do with s2 = s1 being ok.
 
 It is probably worth noting that this works:
 
 
 import std.traits;
 
 void bar(T)(const T x, out T y) {}
 
 void main() {
     const int s1;
     int s2;
     bar!(CommonType!(s1,s2))(s1, s2);
 }


http://d.puremagic.com/issues/show_bug.cgi?id=4594
Sep 02 2010
next sibling parent Michal Minich <michal.minich gmail.com> writes:
On Thu, 02 Sep 2010 11:11:52 -0700, Brad Roberts wrote:


 On 9/2/2010 5:59 AM, Simen kjaeraas wrote:

 void bar(T)(const T x, out T y) {}
 
 void main() {
     const int s1;
     int s2;
     bar(s1, s2);
 }
 
 It seems DMD is confused by const(int) being such a nice fit for the
 first parameter. This might have to do with s2 = s1 being ok.
 
 It is probably worth noting that this works:
 
 
 import std.traits;
 
 void bar(T)(const T x, out T y) {}
 
 void main() {
     const int s1;
     int s2;
     bar!(CommonType!(s1,s2))(s1, s2);
 }


 
 http://d.puremagic.com/issues/show_bug.cgi?id=4594


Right! its dmd bug in implicit instantiation:

void bar(T)(const T x, T y) {}

void main () {
    const int s1;      
    int s2;

    bar!(const int)(s1, s2); // pass ok
    bar!(int)(s1, s2); // pass ok
    bar(s1, s2); // error   
}
Sep 02 2010
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
Brad Roberts:

 http://d.puremagic.com/issues/show_bug.cgi?id=4594


Thanks for all the answers.

Bye,
bearophile
Sep 02 2010
prev sibling parent Michal Minich <michal.minich gmail.com> writes:
On Wed, 01 Sep 2010 22:22:22 -0400, bearophile wrote:


 This program doesn't compile, dmd prints the errors: test.d(4): Error:
 template test.foo(T) does not match any function template declaration
 test.d(4): Error: template test.foo(T) cannot deduce template function
 from argument types !()(const(int),int) test.d(9): Error: template
 instance test.bar!(int) error instantiating
 
 Are you able to tell me what is the error in this code?
 
 
 void foo(T)(const T a, out T b) {}
 
 void bar(T)(const T x, out T y) {
     foo(x, y); // line 4
 }
 
 void main() {
     int s1, s2;
     bar(s1, s2); // line 9
 }
 
 
 Bye, and thank you,
 bearophile


When foo is defined this way, the original example works.

void foo(T, U)(const T a, out U b) {}

foo must use 2 type parameters, because it is called with two types from 
line 4 x : const(int) and y : int. It is ok to call bar (s1, s2) because 
both variables are int at the call site. the s1 is implicitly converted 
to const after the template function is called.

The important thing here is - compiler does not try to implicitly convert 
type when matching template instantiation. 

Therefore it does not try to convert second argument y to const(int) 
(which would result in template match, which in this example does even 
does not make sense - const out).
Sep 02 2010