[llvm-bugs] [Bug 52174] New: Call to constexpr static method is misclassified as having side effects

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Oct 14 03:29:57 PDT 2021


            Bug ID: 52174
           Summary: Call to constexpr static method is misclassified as
                    having side effects
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: maxidlabs at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

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

// Handle static member functions.
if (const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(E->getMemberDecl())) {
  if (MD->isStatic()) {
    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

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);    

      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?


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