[cfe-dev] CheckConstantExpression issue

[Serge Preis via cfe-dev]

Hello,

I encountered the following issue in lib/AST/ExprConstant.cpp:

In function CheckConstantExpr there is a code:

  if (Value.isUnion() && Value.getUnionField()) {
    return CheckConstantExpression(Info, DiagLoc,
                                   Value.getUnionField()->getType(),
                                   Value.getUnionValue());
  }
  if (Value.isStruct()) {
    RecordDecl *RD = Type->castAs<RecordType>()->getDecl();           ////      <<<<<<    The problematic line
    if (const CXXRecordDecl *CD = dyn_cast<CXXRecordDecl>(RD)) {
      unsigned BaseIndex = 0;
      for (CXXRecordDecl::base_class_const_iterator I = CD->bases_begin(),
             End = CD->bases_end(); I != End; ++I, ++BaseIndex) {
        if (!CheckConstantExpression(Info, DiagLoc, I->getType(),
                                     Value.getStructBase(BaseIndex)))
          return false;


The problematic line causes a crash because Type cannot be cast to RecordType.

This looks like an issue in clang to me, but the question is what's wrong: Value.isStruct() returning true for constant of template parameter type (see below) or absence of check for Type to be RecordType.
I will definitely impelement workaround in my code, but I would also like to investingate and fix this issue in clang. 

Regards,
Serge.


----- useful info ----

The code which triggering this behavior looks like the following:
...
    template<class T>
    void DoTestMax() {
        const T max = Max();
...

I am trying to access and print constant initializer for 'max' variable under following checks:
            } else if (auto* varDecl = llvm::dyn_cast<clang::VarDecl>(decl)) {
                if (!clang::isa<clang::ParmVarDecl>(varDecl) && (varDecl->isConstexpr() || varDecl->getType().isConstQualified())) {
                    const clang::Expr* init = varDecl->getAnyInitializer();
                    if (init != nullptr &&
                        !init->getType().isNull() &&
                        !init->isValueDependent() &&
                        varDecl->ensureEvaluatedStmt() != nullptr &&
                        varDecl->ensureEvaluatedStmt()->Value != nullptr)
                    {
                        const clang::APValue* val = varDecl->evaluateValue();  ///  <<< Problem is here

The relevant stack for debug clang (trunk) looks like below: 
#3  0x00007ffff6bb2ca2 in __GI___assert_fail (assertion=0x2661ae0 "isa<X>(Val) && \"cast<Ty>() argument of incompatible type!\"",  file=0x2661aa0 "/home/spreis/LLVM/latest/llvm/include/llvm/Support/Casting.h", line=241, 
    function=0x2663de0 <std::enable_if<!llvm::is_simple_type<clang::QualType>::value, llvm::cast_retty<clang::RecordType, clang::QualType const>::ret_type>::type llvm::cast<clang::RecordType, clang::QualType>(clang::QualType const&)::__PRETTY_FUNCTION__> "typename std::enable_if<(! llvm::is_simple_type<Y>::value), typename llvm::cast_retty<X, const Y>::ret_type>::type llvm::cast(const Y&) [with X = clang::RecordType; Y = clang::QualType; typename std::"...) 
    at assert.c:101
#4  0x00000000010cf75e in llvm::cast<clang::RecordType, clang::QualType> (Val=...)
    at /home/spreis/LLVM/latest/llvm/include/llvm/Support/Casting.h:241
#5  0x00000000010ce4fa in clang::Type::castAs<clang::RecordType> (this=0x554a880)
    at /home/spreis/LLVM/latest/llvm/tools/clang/include/clang/AST/TypeNodes.def:122
#6  0x0000000001c0b810 in CheckConstantExpression (Info=..., DiagLoc=...,   Type=..., Value=...)
    at /home/spreis/LLVM/latest/llvm/tools/clang/lib/AST/ExprConstant.cpp:1737
#7  0x0000000001c2f1b0 in clang::Expr::EvaluateAsInitializer (this=0x554ab50, Value=..., Ctx=..., VD=0x554a9d8, Notes=...)
    at /home/spreis/LLVM/latest/llvm/tools/clang/lib/AST/ExprConstant.cpp:10079
#8  0x0000000001b923fa in clang::VarDecl::evaluateValue (this=0x554a9d8, Notes=...)
    at /home/spreis/LLVM/latest/llvm/tools/clang/lib/AST/Decl.cpp:2172
#9  0x0000000001b9229c in clang::VarDecl::evaluateValue (this=0x554a9d8)
    at /home/spreis/LLVM/latest/llvm/tools/clang/lib/AST/Decl.cpp:2146

The var:
(gdb) frame 9
2146      return evaluateValue(Notes);
(gdb) call this->dump()
VarDecl 0x554a9d8 <..snip..snip> referenced max 'const T' cinit
`-CallExpr 0x554ab50 <col:23, col:27> '::NPrivate::TMax':'struct NPrivate::TMax'
  `-ImplicitCastExpr 0x554ab38 <col:23> '::NPrivate::TMax (*)(void) noexcept' <FunctionToPointerDecay>
    `-DeclRefExpr 0x554aab8 <col:23> '::NPrivate::TMax (void) noexcept' lvalue Function 0x3f82c28 'Max' '::NPrivate::TMax (void) noexcept'

The type:
(gdb) frame 6
1737        RecordDecl *RD = Type->castAs<RecordType>()->getDecl();
(gdb) call Type.dump()
QualType 0x554a881 'const T' const
`-TemplateTypeParmType 0x554a880 'T' dependent depth 0 index 0
  `-TemplateTypeParm 0x554a840 'T'