On Tue, Jan 20, 2026 at 10:14:35AM +0100, Jakub Jelinek wrote:
> On Mon, Jan 19, 2026 at 03:57:08PM +0100, Josef Melcr wrote:
> > +<!-- introduced in 0eac9cfee8cb0b21de866a04d5d59685ab35208f -->
> > +
> > +<p>
> > +Since GCC 16, <code>-Wunused-but-set-*</code> warning options no longer
> > +consider preincrements and postincrements as uses, which may lead to
> > +compilation failures when using <code>-Werror</code>.
> > +</p>
>
> That IMHO doesn't describe the change correctly (the change wasn't by
> -Wall/-W default only about pre/post increments but also about var @= expr)
> and more importantly, doesn't tell users what they should do.
>
> The PR44677 change simply changed the -Wunused-but-set-* warnings to
> have multiple levels and the old options as well as defaults implied
> by -Wall or -W default to the =3 level, while the GCC 15 and earlier
> provided level was equivalent to the =1 level.
> Usually porting_to.html shows some code snippet, describes what changed
> and suggests what to do.
> I think this porting_to.html entry should refer to
> https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wunused-but-set-variable_003d
> and maybe https://gcc.gnu.org/PR44677 , show the code example
> mentioned in the documentation and suggest users that they can either
> remove the unused but set variables/parameters to fix the warnings,
> or make them used e.g. through (void) casts, or if some project generally
> likes the previous behavior, lower the level to -Wunused-but-set-variable=1
> or similar.
Here is my attempt to do that.
Ok for wwwdocs?
diff --git a/htdocs/gcc-16/porting_to.html b/htdocs/gcc-16/porting_to.html
index 3bd1ff2b..783ec821 100644
--- a/htdocs/gcc-16/porting_to.html
+++ b/htdocs/gcc-16/porting_to.html
@@ -31,9 +31,49 @@ and provide solutions. Let us know if you have suggestions
for improvements!
<!-- introduced in 0eac9cfee8cb0b21de866a04d5d59685ab35208f -->
<p>
-Since GCC 16, <code>-Wunused-but-set-*</code> warning options no longer
-consider preincrements and postincrements as uses, which may lead to
-compilation failures when using <code>-Werror</code>.
+Since GCC 16, <code>-Wunused-but-set-*</code> warning options have been
+extended to have multiple levels of what kind of uses of otherwise
+unused variables disable the warnings. Older versions of GCC only
+implemented what has become <code>-Wunused-but-set-variable=1</code>
+or <code>-Wunused-but-set-parameter=1</code>, while the new default
+for <code>-Wunused-but-set-variable</code> or
+<code>-Wunused-but-set-parameter</code> is <code>=3</code> (also defaulted
+for <code>-Wall</code> resp. <code>-Wextra</code>.
+<code>=2</code> ignores pre/post inc/decrements on the variable,
+<code>=3</code> also compound assignments if the lhs variable is not used
+on the rhs as well.
+<pre><code>
+void foo (void) {
+ int a = 1; // -Wunused-variable warning
+ int b = 0; // Warning for -Wunused-but-set-variable=<i>n</i> <i>n</i> >= 1
+ b = 1; b = 2;
+ int c = 0; // Warning for -Wunused-but-set-variable=<i>n</i> <i>n</i> >= 2
+ ++c; c--; --c; c++;
+ int d = 0; // Warning for -Wunused-but-set-variable=<i>n</i> <i>n</i> >= 3
+ d += 4;
+ int e = 0; // No warning, cast to void
+ (void) e;
+ int f = 0; // No warning, f used
+ int g = f = 5;
+ (void) g;
+ int h = 0; // No warning, preincrement used
+ int i = ++h;
+ (void) i;
+ int j = 0; // No warning, postdecrement used
+ int k = j--;
+ (void) k;
+ int l = 0; // No warning, l used
+ int m = l |= 2;
+ (void) m;
+}
+</code></pre>
+In order to avoid the warnings, one can either remove newly diagnosed
+variables or parameters which aren't used except in pre/post inc/decrements
+or compound assignments, make them used in some way
+including just casting to <code>(void)</code> or lowering the level
+of the warning. See
+<a
href=https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wunused-but-set-variable_003d";
+<code>-Wunused-but-set-*</code></a> documentation for more details.
</p>
</body>
Jakub
