clang hides the type of template arguments

[llvmbot]

Bugzilla Link	11277
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

Consider the following simple code:

#include <vector>

typedef void* hidden_type;

int f(double d);

int main(void)
{
	hidden_type w;
	std::vector<hidden_type> v;
	f(w);
	f(v);
}

This produces as output:

t.cc:11:2: error: no matching function for call to 'f'
        f(w);
        ^
t.cc:5:5: note: candidate function not viable: cannot convert argument of
      incomplete type 'hidden_type' (aka 'void *') to 'double'
int f(double d);
    ^
t.cc:12:2: error: no matching function for call to 'f'
        f(v);
        ^
t.cc:5:5: note: candidate function not viable: no known conversion from
      'std::vector<hidden_type>' to 'double' for 1st argument;
int f(double d);
    ^

Notice how I am told in the first error that 'hidden_type' is void*, but in the second case I am just given std::vector<hidden_type>.

In code I had this actually occur in, the type argument was a much more complicated template monstrosity, and in the end I had to introduce a new error into my code just to get the compiler to tell me what the 'hidden_type' actually was.

As g++ seems to not track template arguments, in this case it produces the more helpful:

t.cc:11:5: error: cannot convert ‘hidden_type {aka void*}’ to ‘double’ for argument ‘1’ to ‘int f(double)’
t.cc:12:5: error: cannot convert ‘std::vector<void*>’ to ‘double’ for argument ‘1’ to ‘int f(double)’

[philnik777]

This has been fixed in Clang 3.8: https://godbolt.org/z/cGb6PMnKe