[llvm-bugs] [Bug 47569] New: 'const' qualifier on referenced decltype(type) isn't allowed by clang++

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https://bugs.llvm.org/show_bug.cgi?id=47569

            Bug ID: 47569
           Summary: 'const' qualifier on referenced decltype(type) isn't
                    allowed by clang++
           Product: clang
           Version: trunk
          Hardware: PC
                OS: FreeBSD
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yuri at tsoft.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Created attachment 23980
  --> https://bugs.llvm.org/attachment.cgi?id=23980&action=edit
testcase-c++-const-decltype.cpp

The attached testcase adds 'const' and the reference symbol & to
decltype(Cls::field) to construct the return type of a function.

Compilation fails:
> testcase-c++-const-decltype.cpp:10:2: warning: 'const' qualifier on reference type 'decltype(X::u[0][0][0])' (aka 'unsigned int &') has no effect [-Wignored-qualifiers]
>         const decltype(X::u[0][0][0])& getXu(unsigned i1, unsigned i2, unsigned i3) const {
>         ^~~~~~
> testcase-c++-const-decltype.cpp:11:10: error: binding reference of type 'unsigned int' to value of type 'const unsigned int' drops 'const' qualifier
>                 return x.u[i1][i2][i3];
>                        ^~~~~~~~~~~~~~~

Replacing decltype(X::u[0][0][0]) with "unsigned" helps, but
decltype(X::u[0][0][0]) resolves to the same "unsigned" which should make
"const decltype(X::u[0][0][0])&" equivalent to "const unsigned&".

The testcase fails with clang-10, clang-11, clang-11, clang-12.

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