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[Clang] Use ExtWarn for static local variable in extern inline function (fixes #39524) #166332

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[Clang] Use ExtWarn for static local variable in extern inline function (fixes #39524) #166332

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16414b9
[Clang] Use ExtWarn for static local variable in extern inline functi…
Susikrishna Nov 4, 2025
456e2df
Merge branch 'main' into fix-inline-static-extwarn
Susikrishna Nov 4, 2025
ee314e1
Update clang/test/Sema/inline-static-var.c
Susikrishna Nov 5, 2025
b4abe8e
Update release notes and diagnostics for static local variables in ex…
Susikrishna Nov 5, 2025
d5431b8
Update clang/docs/ReleaseNotes.rst
Susikrishna Nov 5, 2025
544b1ba
Update release note to refer to `inline` functions instead of `extern…
Susikrishna Nov 5, 2025
44c4a48
Update clang/test/Sema/inline-static-var.c
Susikrishna Nov 6, 2025
af464fe
Merge branch 'main' into fix-inline-static-extwarn
Susikrishna Nov 6, 2025
28b67f8
[Clang][Sema] Use Linux triple for inline-static-var.c to ensure cons…
Susikrishna Nov 7, 2025
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4 changes: 4 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -404,6 +404,10 @@ Improvements to Clang's diagnostics
- Clang now emits a diagnostic in case `vector_size` or `ext_vector_type`
attributes are used with a negative size (#GH165463).

- The warning about static local variables declared inside `inline`
functions is now correctly converted to an error if `-pedantic-errors` is
passed (#GH39524).

Improvements to Clang's time-trace
----------------------------------

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2 changes: 1 addition & 1 deletion clang/include/clang/Basic/DiagnosticSemaKinds.td
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Expand Up @@ -6339,7 +6339,7 @@ def warn_c2y_compat_internal_in_extern_inline : Warning<
"using static %select{function|variable}0 %1 in an inline function with "
"external linkage is incompatible with standards before C2y">,
InGroup<CPre2yCompat>, DefaultIgnore;
def warn_static_local_in_extern_inline : Warning<
def warn_static_local_in_extern_inline : ExtWarn<
"non-constant static local variable in inline function may be different "
"in different files">, InGroup<StaticLocalInInline>;
def note_convert_inline_to_static : Note<
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6 changes: 6 additions & 0 deletions clang/test/Sema/inline-static-var.c
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@@ -0,0 +1,6 @@
// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -std=c11 -pedantic-errors -verify %s
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@AaronBallman AaronBallman Nov 7, 2025

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I don't think the test should have a triple; I would expect this diagnostic to be target-agnostic based on the code which emits the diagnostic. Can you hook up a debugger to see why the diagnostic was being silenced on Windows targets in the first place?

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@Susikrishna Susikrishna Nov 7, 2025

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Hi @AaronBallman,

Thanks for the review. I checked it, and it seems the diagnostic is intentionally target-dependent. I'd be grateful if you could sanity-check my reasoning below:

My understanding is that this all comes down to the behavior of isFunctionDefinitionDiscarded() in this block in SemaDecl.cpp:

  • On Windows (MSVC): Clang emulates the MSVC model, where inline acts like C++ and is the "real" external definition. The linker just merges all the identical copies. Because this definition is not "discardable," isFunctionDefinitionDiscarded() returns false, and our diagnostic (correctly) doesn't fire.

  • On Linux (GNU): Clang uses the C99 model, where inline is just a "discardable copy" that's separate from the "real" definition. Because this definition is "discardable," isFunctionDefinitionDiscarded() returns true, which is what triggers our ExtWarn.

So, it seems the CI failed on Windows simply because, under MSVC's rules, this code isn't an error.

If that sounds right, then adding the -triple x86_64-pc-linux-gnu to the test file feels like the correct solution, as it forces the test to check against the GNU/C99 semantics we're actually trying to fix.

I'm not completely sure if this is the standard way to handle a target-specific test, though. Please let me know if there's a better approach!


inline void f(void) { // expected-note {{use 'static' to give inline function 'f' internal linkage}}
static int x; // expected-error {{non-constant static local variable in inline function may be different in different files}}
}