[llvm-bugs] [Bug 51406] New: Constraint normalization of a parenthesized atomic constraint yields a different expression than an identical non-parenthesized atomic constraint

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51406

            Bug ID: 51406
           Summary: Constraint normalization of a parenthesized atomic
                    constraint yields a different expression than an
                    identical non-parenthesized atomic constraint
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: matthewjbarichello at gmail.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

Affected versions:
 - trunk
 - 12.0.1
 - 12.0.0
 - 11.0.1
 - 11.0.0
 - 10.0.1
 - 10.0.0
 - 9.0.1
 - 9.0.0

Driver cmdline:
Note: For versions not supporting -std=c++20, -std=c++2a was used.
 clang++ -std=c++20 -Werror -Wall -pedantic

Code:
 template<typename>
 concept B = true;

 template<typename T>
 requires (B<T>)
 struct A;

 template<typename T>
 requires B<T>
 struct A {};

Error:
 <source>:9:10: error: requires clause differs in template redeclaration
 requires B<T>
          ^
 <source>:5:10: note: previous template declaration is here
 requires (B<T>)
          ^
 1 error generated.

Notice that in the example the forward declaration of the template `A<T>` has a
requires clause with a parenthesized atomic constraint, `(B<T>)`, whereas the
definition of `A<T>` has the same requires clause, albeit non-parenthesized.
After constraint normalization, both requires clauses should be equivalent,
however, clang yields an error.

Notes:
 - GCC does not exhibit this issue.
 - MSVC has a similar issue that seems to be, incorrectly, fixed by converting
the redeclaration of `A` into a partially specialized template.

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