Clang: Template deduction failed 'double' vs ''

Question

Consider the following code, which uses a function with variable arguments:

#include <iostream>

// Typedef function type
template<typename... Output>
using Func = void(Output*...);

// Function runner
template<typename... Output>
void run_func(Func<Output...>& func, Output*... output) {
  for (int i=0 ; i < 10 ; ++i) {
    func(output...);
  }
}

void f(double* d) {
  *d *= 2;
};

int main() {
  double value = 1.0;
  run_func(f, &value);
  printf("%f\n", value);
}

Compiling this with g++ 4.7.3 works fine, and running produces 1024.0 as expected.

Compiling using icpc 14.0.2 crashes it...

templ.cc(21): internal error: assertion failed: lower_expr: bad kind (shared/cfe/edgcpfe/lower_il.c, line 18582)

    run_func(f, &value);
    ^

Compiling with clang 3.5.0-1 gives the following error message:

templ.cc:21:3: error: no matching function for call to 'run_func'
  run_func(f, &value);
  ^~~~~~~~
templ.cc:9:6: note: candidate template ignored: deduced conflicting types for parameter 'Output' ('double' vs. <double>)
void run_func(Func<Output...>& func, Output*... output) {
     ^

Is this a bug, or should have g++ not compiled this?

Why is clang deducing these "conflicting" types of double and <double>, is <double> meant to represent an unpacked arglist for example?

Update icpc 14.0.3 does not crash, and the program compiles and runs correctly.

See DPD200244439 at Intel® Composer XE 2013 SP1 Compilers Fixes List

Your last question you could answer yourself by passing run_func(f,&value,&value)... which results in which I don't think is correct... you might want to try with a more recent clang from svn, it smells like a bug — PlasmaHH, May 12 '14 at 12:35
Since replacing the `template using` with the native type makes the problem go away, it feels very much like a bug. — danielschemmel, May 12 '14 at 12:49
@PlasmaHH I tried clang version 3.5.0 (trunk 208562), same problem! — WaelJ, May 12 '14 at 16:08
@WaelJ: In this case, I suggest that you report a bug to the Clang team. 3.5 is near, it would be cool if they managed to fix this before it goes out. — Matthieu M., May 18 '14 at 14:49
@WaelJ please post your update as an answer and accept it to get the question off the unanswered list. — AliciaBytes, May 24 '14 at 04:07

score 1 · Accepted Answer · edited May 23 '17 at 12:01

1

Following the above discussion, it seems that this is indeed a bug in clang.

As pointed out by gha.st, skipping template using and using the native function type directly works:

void run_func(void (&func)(Output*...), Output*... output) {

I have a filed a bug against this here.

edited May 23 '17 at 12:01

Community

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answered May 24 '14 at 17:09

WaelJ

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Clang: Template deduction failed 'double' vs ''

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