[Clang] Duplicate diagnostics in C++20+ mode: incomplete type in nested name specifier

[zwuis]

Clang emits duplicate diagnostics when compiling the following code in C++20+ mode:

struct incomplete;
incomplete::type var;

Output:

<source>:2:1: error: incomplete type 'incomplete' named in nested name specifier
    2 | incomplete::type var;
      | ^~~~~~~~~~~~
<source>:1:8: note: forward declaration of 'incomplete'
    1 | struct incomplete;
      |        ^
<source>:2:1: error: incomplete type 'incomplete' named in nested name specifier
    2 | incomplete::type var;
      | ^~~~~~~~~~~~
<source>:1:8: note: forward declaration of 'incomplete'
    1 | struct incomplete;
      |        ^

Compiler Explorer: https://godbolt.org/z/fPo1W7c1n

[shafik]

Looks like is started in clang-10: https://godbolt.org/z/s4e6GE1f7

[shafik]

Based on this comment it looks like a fix in the works: #147036 (comment)

CC @mizvekov