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[Clang] incorrect assertion when checking template template parameter of a lambda #138121

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May 2, 2025
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[Clang] incorrect assertion when checking template template parameter of a lambda #138121

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[Clang] incorrect assertion when checking template template parameter…
cor3ntin May 1, 2025
6930f81
add assert
cor3ntin May 1, 2025
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2 changes: 2 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -529,6 +529,8 @@ Bug Fixes to C++ Support
- Clang now issues an error when placement new is used to modify a const-qualified variable
in a ``constexpr`` function. (#GH131432)
- Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
- Fix a crash when checking the template template parameters of a dependent lambda appearing in an alias declaration.
(#GH136432), (#GH137014), (#GH138018)

Bug Fixes to AST Handling
^^^^^^^^^^^^^^^^^^^^^^^^^
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7 changes: 6 additions & 1 deletion clang/lib/AST/DeclCXX.cpp
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Expand Up @@ -1696,7 +1696,11 @@ static NamedDecl* getLambdaCallOperatorHelper(const CXXRecordDecl &RD) {
RD.getASTContext().DeclarationNames.getCXXOperatorName(OO_Call);

DeclContext::lookup_result Calls = RD.lookup(Name);
assert(!Calls.empty() && "Missing lambda call operator!");

// This can happen while building the lambda.
if (Calls.empty())
return nullptr;
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So I'm REALLY concerned about this. I don't think it is wrong, but just that we need to make sure we handle this value correctly. At least getLambdaStaticInvoker doesn't handle the nullptr correctly (though I guess the other two uses DO, so thats fine).

I wouldn't mind a bit of an audit to make sure that uses of getLambdaStaticInvoker (and perhaps the others, but as they already acknowledged they could return null, i'm mildly less concerned) do a reasonable thing.

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I added an assert, which hinges on the fact that we don't look at the static invoker in cases where the lambda is incomplete.
I agree that having these various states in lambdas is a bit unfortunate, but it is a consequence of P2036.
If you have a better solution, I'm all ears!


assert(allLookupResultsAreTheSame(Calls) &&
"More than one lambda call operator!");

Expand Down Expand Up @@ -1750,6 +1754,7 @@ CXXMethodDecl *CXXRecordDecl::getLambdaCallOperator() const {

CXXMethodDecl* CXXRecordDecl::getLambdaStaticInvoker() const {
CXXMethodDecl *CallOp = getLambdaCallOperator();
assert(CallOp && "null call operator");
CallingConv CC = CallOp->getType()->castAs<FunctionType>()->getCallConv();
return getLambdaStaticInvoker(CC);
}
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16 changes: 16 additions & 0 deletions clang/test/SemaCXX/lambda-unevaluated.cpp
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Expand Up @@ -266,3 +266,19 @@ void func() {
}

} // namespace GH88081

namespace GH138018 {

template <typename T> struct vec {};

auto structure_to_typelist(auto) {
return []<template <typename> typename T>(T<int>) {
return 0;
}(vec<int>{});
}

template <typename T> using helper = decltype(structure_to_typelist(T{}));
static_assert(__is_same_as(int, helper<int>));


} // namespace GH138018