https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52953
--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:efafa66c294d261a4d964383674ab9ee51feaf88 commit r14-3714-gefafa66c294d261a4d964383674ab9ee51feaf88 Author: Jakub Jelinek <ja...@redhat.com> Date: Tue Sep 5 17:31:12 2023 +0200 c++: Diagnose [basic.scope.block]/2 violations even for block externs [PR52953] C++17 had in [basic.block.scope]/2 "A parameter name shall not be redeclared in the outermost block of the function definition nor in the outermost block of any handler associated with a function-try-block." and in [basic.block.scope]/4 similar rule for selection/iteration statements. My reading of that is that it applied even for block local externs in all those spots, while they declare something at namespace scope, the redeclaration happens in that outermost block etc. and introduces names into that. Those wordings seemed to have been moved somewhere else in C++20, but what's worse, they were moved back and completely rewritten in P1787R6: Declarations and where to find them which has been applied as a DR (but admittedly, we don't claim yet to implement that). The current wording at https://eel.is/c++draft/basic.scope#block-2 and https://eel.is/c++draft/basic.scope#scope-2.10 seem to imply at least to me that it doesn't apply to extern block local decls because their target scope is the namespace scope and [basic.scope.block]/2 says "and whose target scope is the block scope"... Now, it is unclear if that is actually the intent or not. There seems to be quite large implementation divergence on this as well. Unpatched g++ e.g. on the redeclaration-5.C testcase diagnoses just lines 55,58,67,70 (i.e. where the previous declaration is in for's condition). clang++ trunk diagnoses just lines 8 and 27, i.e. redeclaration in the function body vs. parameter both in normal fn and lambda (but not e.g. function-try-block and others, including ctors, but it diagnoses those for non-extern decls). ICC 19 diagnoses lines 8,32,38,41,45,52,55,58,61,64,67,70,76. And MSCV trunk diagnoses 8,27,32,38,41,45,48,52,55,58,67,70,76,87,100,137 although the last 4 are just warnings. g++ with the patch diagnoses 8,15,27,32,38,41,45,48,52,55,58,61,64,67,70,76,87,100,121,137 as the dg-error directives test. Jason said: > Yes, I suspect that should be > > If a declaration that is not a name-independent declaration and <del>whose > target scope is</del><ins>that binds a name in</ins> the block scope S of a > > which seems to also be needed to prohibit the already-diagnosed > > void f(int i) { union { int i; }; } > void g(int i) { enum { i }; } The following patch diagnoses DECL_EXTERNAL in check_local_shadow like !DECL_EXTERNAL, except that 1) it uses pedwarn instead of errors for those cases 2) it doesn't diagnose shadowing of namespace scope identifiers by block local externs, as they could be not actually shadowing but just redeclaring the same objects 2023-09-05 Jakub Jelinek <ja...@redhat.com> PR c++/52953 * name-lookup.cc (check_local_shadow): Don't punt early for DECL_EXTERNAL decls, instead just disable the shadowing of namespace decls check for those and emit a pedwarn rather than error_at or permerror for those. Formatting fix. * g++.dg/diagnostic/redeclaration-4.C: New test. * g++.dg/diagnostic/redeclaration-5.C: New test. * g++.dg/warn/Wshadow-19.C: New test.
