[llvm-bugs] [Bug 44959] New: Concept causes infinite loops at compile time (-Wstack-exhausted)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44959

            Bug ID: 44959
           Summary: Concept causes infinite loops at compile time
                    (-Wstack-exhausted)
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: llvm at marehr.dialup.fu-berlin.de
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The following code should compile:

```c++
#include <type_traits>

template <typename T>
concept semialphabet = requires(T const &t, T const &u) {t == u;};

template <typename>
concept always_false = false;

template <typename derived_type, int size> struct alphabet_base {
  friend bool operator==(derived_type, derived_type) { return false; };
};

template <int size>
struct semialphabet_any : alphabet_base<semialphabet_any<size>, size> {
  template <typename other_alph_t, typename =
std::enable_if_t<always_false<other_alph_t> && semialphabet<other_alph_t>>>
  operator other_alph_t();
};

template <typename derived_type, int size>
struct aminoacid_base : public alphabet_base<derived_type, size> {
  template <typename other_aa_type, typename =
std::enable_if_t<always_false<other_aa_type> && semialphabet<other_aa_type>>>
  aminoacid_base(other_aa_type);
};

struct aa10li : public aminoacid_base<aa10li, 10> {
  using base_t = aminoacid_base;
  using base_t::base_t;
};

semialphabet_any<10> letter0;

static_assert(!std::is_convertible_v<semialphabet_any<10>, aa10li>);
```

I would expect that the expression `always_false<other_aa_type> &&
semialphabet<other_aa_type>` evaluates from left to right and sees that this
expression is the same as `false && semialphabet<other_aa_type>` and thus does
not evaluate `semialphabet<other_aa_type>`. If I define `semialphabet` via
SFINAE instead of concepts short-circuiting works.


https://godbolt.org/z/KBgLpi

Error message:

```
<source>:14:8: warning: stack nearly exhausted; compilation time may suffer,
and crashes due to stack overflow are likely [-Wstack-exhausted]

struct semialphabet_any : alphabet_base<semialphabet_any<size>, size> {
       ^
<source>:4:58: note: in instantiation of requirement here
concept semialphabet = requires(T const &t, T const &u) {t == u;};
                                                         ^~~~~~
<source>:4:24: note: while substituting template arguments into constraint
expression here
concept semialphabet = requires(T const &t, T const &u) {t == u;};
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:15:94: note: while checking the satisfaction of concept
'semialphabet<semialphabet_any<10> >' requested here
  template <typename other_alph_t, typename =
std::enable_if_t<always_false<other_alph_t> && semialphabet<other_alph_t>>>
                                                                               
             ^~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:16:3: note: in instantiation of default argument for 'operator
type-parameter-0-0<semialphabet_any<10> >' required here
  operator other_alph_t();
  ^~~~~~~~~~~~~~~~~~~~~~~
<source>:4:58: note: while substituting deduced template arguments into
function template 'operator type-parameter-0-0' [with other_alph_t =
semialphabet_any<10>, $1 = (no value)]
concept semialphabet = requires(T const &t, T const &u) {t == u;};
                                                         ^
<source>:4:58: note: (skipping 1516 contexts in backtrace; use
-ftemplate-backtrace-limit=0 to see all)
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/type_traits:1322:2:
note: in instantiation of default argument for '__test<semialphabet_any<10>,
aa10li>' required here
        __test(int);
        ^~~~~~~~~~~
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/type_traits:1329:24:
note: while substituting deduced template arguments into function template
'__test' [with _From1 = semialphabet_any<10>, _To1 = aa10li, $2 = (no value)]
      typedef decltype(__test<_From, _To>(0)) type;
                       ^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/type_traits:1336:14:
note: in instantiation of template class
'std::__is_convertible_helper<semialphabet_any<10>, aa10li, false>' requested
here
    : public __is_convertible_helper<_From, _To>::type
             ^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/type_traits:2964:44:
note: in instantiation of template class
'std::is_convertible<semialphabet_any<10>, aa10li>' requested here
  inline constexpr bool is_convertible_v = is_convertible<_From, _To>::value;
                                           ^
<source>:32:21: note: in instantiation of variable template specialization
'std::is_convertible_v<semialphabet_any<10>, aa10li>' requested here
static_assert(!std::is_convertible_v<semialphabet_any<10>, aa10li>);
                    ^
Compiler returned: 255
```

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