[llvm-bugs] [Bug 44674] New: SFINAE leaks through requires expression requirement

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44674

            Bug ID: 44674
           Summary: SFINAE leaks through requires expression requirement
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: cjdb.ns at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Created attachment 23063
  --> https://bugs.llvm.org/attachment.cgi?id=23063&action=edit
Initial preprocessed file

# Description

[iterator.concept.contiguous] depends on `std::to_address`, which (correctly)
fails in the "minimal" repro below. Unfortunately, the requirement seems to
leak an instantiation error, instead of SFINAEing out the overload and the
concept evaluating as false.

# LLVM configuration

```
cmake -GNinja  ../llvm-project/llvm/                                           
     \
   -DCMAKE_C_COMPILER=/usr/bin/clang-9                                         
     \
   -DCMAKE_CXX_COMPILER=/usr/bin/clang++-9                                     
     \
   -DCMAKE_BUILD_TYPE=Release                                                  
     \
   -DLLVM_TARGETS_TO_BUILD=X86                                                 
     \
  
-DLLVM_ENABLE_PROJECTS='clang;clang-tools-extra;compiler-rt;libcxx;libcxxabi;lld'
\
   -DLLVM_ENABLE_LLD=On                                                        
     \
   -DLLVM_ENABLE_LTO=Off                                                       
     \
   -DPYTHON_EXECUTABLE=/usr/bin/python3                                        
     \
   -DLLVM_ENABLE_LIBCXX=On                                                     
     \
   -DLLVM_ENABLE_BINDINGS=Off
```

# Sample code built with

```
-std=c++2a -stdlib=libc++ -save-temps
```

# "Minimal" repo

```
#include <memory>

struct dummy {};

struct iterator {
        using element_type = dummy;
        int* operator->() const;
};

template<typename I>
concept passing = requires(I const& i) {
    std::to_address(i);
};

static_assert(not passing<iterator>);
```

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