lambda in unevaluated-context is parsed incorrectedly

[nickhuang99]

Bugzilla Link	52299
Version	trunk
OS	Linux
CC	@AaronBallman,@zygoloid

Extended Description

The following code is accepted incorrectly by clang while GCC and MSVC++ all spot the failure of assertion. The type of "foo" is obviously NOT "void" at all.

template
void foo(decltype(+{}) lambda, T param);

static_assert(__is_same(decltype(foo), void));

[nickhuang99]

This is the link of testing:
https://www.godbolt.org/z/zjxv4KsvP

[AaronBallman]

We form a TypeTraitExpr object because we think the trait is dependent, but then we never go and instantiate the static_assert declaration, so we never re-test the type trait expression on instantiation. We think the trait is dependent because we think the expression argument in decltype(foo<int>) is dependent. The expression argument is actually an unresolved lookup expression by the time we get into Sema::ActOnDecltypeExpression(), which calls Sema::CheckPlaceholderExpr().

I think the issue is in Sema::BuildTemplateIdExpr(), which is where the UnresolvedLookupExpr is created. There's a FIXME comment specifically about how "foo could identify a single function unambiguously", but this does not work currently. However, I stopped feeling confident I knew what the correct fix was and so I've not looked farther.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[tstellar]

/cherry-pick 3784e8c

[llvmbot]

/branch llvmbot/llvm-project/issue51641