[cfe-dev] Class Template Argument Deduction failure

Lorenzo Cavallini via cfe-dev cfe-dev at lists.llvm.org
Mon Apr 29 02:46:17 PDT 2019

Hello everyone,
I'm writing to report what I think is an issue related to Class Template
Argument Deduction (CTAD for short). Compiling this short snippet:

#include <cstdio>
#include <vector>

template<typename T>
void printV(const std::vector<T>& v)
    if (v.size() == 1) {
        printf("%d ", v[0]);
    int middle = v.size()/2;
    printV(std::vector{v.begin(), v.end()+middle});

int main()
    std::vector<int> v = {10,1};
    return 0;

will result in clang generating an infinite error report (at least, I left
it running for several hours, then I stopped the process...), due to (I
guess) wrongly deducted template arguments. If the "printf" line is
removed, no error is generated but clang will compile for what it looks
like forever (the process is constantly at 100% and does not stop after
several hours).
This has been tested also with the following versions of clang: "Apple LLVM
version 10.0.0 (clang-1000.10.44.4)", "clang version 7.0.1 (Fedora
7.0.1-6.fc29)". I also tested clang 8.0 and clang-HEAD on wandbox.org ,
with the same results (on wandbox.org the process gets killed after a
Obviously this only happens when compiling with "--std=c++17" or
Strangely enough, I got more or less the same results with gcc (7, 8.3 and
HEAD), so my guess is that this might be related to some ambiguity either
on the standard or in its interpretation. Given that as far as I know,
clang and gcc are using two completely unrelated parsers and frontends.

Using () instead of {} will properly give a syntax error. So my guess is
that this is related to both CTAD and uniform initialization because of
std::vector having an std::initializer_list constructor. At least form what
I could understand from the huge error output, it looks like the compiler
is recursively instantiating std::vector<T> with T =
std::initializer_list<some iterator type>, but this may be wrong.

If required I can provide part of the huge error output, but it can be
generated by compiling the provided snippet on wandbox.

Thanks to anyone who can sort this out!

Best regards,

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