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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Misleading warning when switch controlling expression is promoted from type narrower than int"
href="https://bugs.llvm.org/show_bug.cgi?id=41306">41306</a>
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<th>Summary</th>
<td>Misleading warning when switch controlling expression is promoted from type narrower than int
</td>
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>7.0
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Frontend
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>cuoq@trust-in-soft.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
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<pre>Consider the C function:
long long X;
void f(unsigned char x)
{
switch(x) {
case -1: X=-1; break;
case 0xff: X=0xff; break;
}
}
Clang 7 emits the following warning for the constant -1:
<source>:6:10: warning: overflow converting case value to switch condition type
(-1 to 255) [-Wswitch]
case -1: X=-1; break;
^
1 warning generated.
(Compiler Explorer link: <a href="https://gcc.godbolt.org/z/cbjQSu">https://gcc.godbolt.org/z/cbjQSu</a> )
This bug is related to C11's 6.8.4.2:5 clause
(<a href="https://port70.net/~nsz/c/c11/n1570.html#6.8.4.2p5">https://port70.net/~nsz/c/c11/n1570.html#6.8.4.2p5</a> ).
Because the controlling expression “x” is promoted before being used as
reference for the type the constants such as “-1” should be converted to, the
warning emitted by Clang is misleading in this case. “-1” will NOT, in fact, be
converted to unsigned char nor be changed to 255. The code generation handles
this situation correctly (the case “-1” can never happen and is correctly
eliminated). The only issue is the warning, which gives the impression that the
value 255 for x would make the first branch taken.
GCC's warning is better worded in this particular case.</pre>
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