struct definition leaks out of function prototype scope

[ebdd6ebc-1612-4e9b-aee1-f7d2b6361d5f]

Bugzilla Link	39925
Version	trunk
OS	All
CC	@zygoloid

Extended Description

The following C11 program (prog.c) should compile without error:

struct T;

void f( struct S { struct T { int a; } *b; } );

struct T { int c; };

int main() { }

When I compile it with clang prog.c -std=c11 -pedantic-errors "-w" i get the following unexpected error:

error: redefinition of 'T'

This is wrong since the struct definition inside the function declaration has function prototype scope and therefore should be independent of the struct definition in the file scope.

Online compiler link:

https://wandbox.org/permlink/UpaD0znT5jkohSNP

Note that gcc does not give any errors.

[AaronBallman]

The issue still reproduces as of Clang 16.0.0: https://gcc.godbolt.org/z/7zeKdEYPT

Interesting to note that removing the forward declaration on line 1 causes the diagnostic to go away, so I suspect the issue has to do with redeclaration merging.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c