https://bugs.llvm.org/show_bug.cgi?id=48666
Bug ID: 48666
Summary: Spurious "expression with side effects has no effect
in an unevaluated context" warning when VLA
Product: clang
Version: 11.0
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: C
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Consider the C compilation unit:
int Y, Z;
void f(int n) {
long u[n];
Y = sizeof *(Z=1, &u);
}
Clang warns that the side-effect “Z=1” will have no effect because it's an
unevaluated context, but actually, u is a VLA, and in this case side-effects
happen. Note that the code generated by Clang is correct, it is only the
warning that should not be emitted:
<source>:5:14: warning: expression with side effects has no effect in an
unevaluated context [-Wunevaluated-expression]
Y = sizeof *(Z=1, &u);
^
1 warning generated.
Compiler returned: 0
Generated code with -O:
f: # @f
movl $1, Z(%rip)
shll $3, %edi
movl %edi, Y(%rip)
retq
Compiler Explorer link: https://gcc.godbolt.org/z/PG9cd9
The relevant C17 clause: https://cigix.me/c17#6.5.3.4.p2
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