[llvm-bugs] [Bug 45398] New: -Wunused-lambda-capture: false positive on `if constexpr`
via llvm-bugs
llvm-bugs at lists.llvm.org
Thu Apr 2 01:38:57 PDT 2020
https://bugs.llvm.org/show_bug.cgi?id=45398
Bug ID: 45398
Summary: -Wunused-lambda-capture: false positive on `if
constexpr`
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: C++17
Assignee: unassignedclangbugs at nondot.org
Reporter: gribozavr at gmail.com
CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk
-Wunused-lambda-capture produces a false positive on lambdas that use captured
variables within inactive branches of `if constexpr`.
$ ./bin/clang++ --version
clang version 11.0.0 (git at github.com:llvm/llvm-project.git
cdce2fe561eb6e63d77d1d589e521a8e2afb1eef)
...
$ cat /tmp/unused-lambda-capture-false-positive.cc
template <int>
struct A {};
struct B {
template <typename T, bool flag>
void Top(T t, int n) {
auto lambda = [t, n, this](auto arg) {
if constexpr (flag) {
UndefinedFunction(t, n);
} else {
TopImpl<A<arg>>();
}
};
}
template <typename U>
void TopImpl() {}
};
void Test(B* b) {
b->Top<int, false>(0, 0);
}
$ ./bin/clang++ -c -std=gnu++17 -Wunused-lambda-capture
/tmp/unused-lambda-capture-false-positive.cc
/tmp/unused-lambda-capture-false-positive.cc:6:20: warning: lambda capture 't'
is not used [-Wunused-lambda-capture]
auto lambda = [t, n, this](auto arg) {
^
/tmp/unused-lambda-capture-false-positive.cc:18:6: note: in instantiation of
function template specialization 'B::Top<int, false>' requested here
b->Top<int, false>(0, 0);
^
/tmp/unused-lambda-capture-false-positive.cc:6:23: warning: lambda capture 'n'
is not used [-Wunused-lambda-capture]
auto lambda = [t, n, this](auto arg) {
^
/tmp/unused-lambda-capture-false-positive.cc:6:26: warning: lambda capture
'this' is not used [-Wunused-lambda-capture]
auto lambda = [t, n, this](auto arg) {
^
3 warnings generated.
ANALYSIS
The false positive on `this` might be a dup of
https://bugs.llvm.org/show_bug.cgi?id=36880, however false positives on `t` and
`n` seem diffirent.
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