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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Call to constexpr static method is misclassified as having side effects"
href="https://bugs.llvm.org/show_bug.cgi?id=52174">52174</a>
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<th>Summary</th>
<td>Call to constexpr static method is misclassified as having side effects
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>12.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>maxidlabs@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>I am not sure if the following code is supposed to compile according to the
*Standard*, but:
* Both GCC and MSVC compile it fine.
* Looking at the snippet I can't see how the code would be ill-formed.
=============================
struct MyStruct {
static constexpr int get_value()
{
return 1 + 7;
}
};
void turbo_func(const MyStruct& varRef)
{
// OK with clang, gcc, msvc
constexpr int a = MyStruct::get_value();
// OK with gcc and msvc, but err_constexpr_var_requires_const_init with clang
constexpr int b = varRef.get_value();
}
=============================
I investigated and made the following observations:
In ExprConstant.cpp: LValueExprEvaluator::VisitMemberExpr() there is a block of
code:
// Handle static member functions.
if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(E->getMemberDecl())) {
if (MD->isStatic()) {
VisitIgnoredBaseExpression(E->getBase());
return Success(MD);
}
}
When 'VisitIgnoredBaseExpression' is called, the function ExprConstant.cpp:
LValueExprEvaluator::VisitVarDecl returns 'false' because the 'varRef' is a
'ParmVarDecl', and this in turn makes the function 'EvaluateIgnoredValue' call
Info.noteSideEffect().
To test this theory, I modified the 'LValueExprEvaluator::VisitMemberExpr' to
look like this:
if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(E->getMemberDecl())) {
if (MD->isStatic()) {
if ( const DeclRefExpr *baseDeclRefExpr = dyn_cast<DeclRefExpr>(
E->getBase() ) )
if ( const VarDecl *baseVarDecl = dyn_cast<VarDecl>(
baseDeclRefExpr->getDecl() ) )
if ( isa<ParmVarDecl>( baseVarDecl ) )
return Success(MD);
VisitIgnoredBaseExpression(E->getBase());
return Success(MD);
}
}
Basically this just (hopefully) tells clang that if base expression is a
function param, then it is fine to compile it.
Using this modified clang I was able to build the mentioned test program, which
had the desired behavior.
Can you please suggest if the *current* clang behavior is expected, given that
the mentioned constexpr call does not seem to have side effects?
Thanks!</pre>
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