Pack indexing expression is not being instantiated

[Eczbek]

Compiler Explorer: https://godbolt.org/z/TG4hMMPv3

template<int... x>
int y = x...[0];

int main() {
	y<0>;
}

<source>:2:5: error: cannot initialize a variable of type 'int' with an lvalue of type '<dependent type>'
    2 | int y = x...[0];
      |     ^   ~~~~~~~
<source>:5:2: warning: expression result unused [-Wunused-value]
    5 |         y<0>;
      |         ^~~~
1 warning and 1 error generated.
Compiler returned: 1

[Sirraide]

CC @cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Eczbek)

Compiler Explorer: https://godbolt.org/z/TG4hMMPv3

template<int... x>
int y = x...[0];

int main() {
	y<0>;
}

<source>:2:5: error: cannot initialize a variable of type 'int' with an lvalue of type '<dependent type>'
    2 | int y = x...[0];
      |     ^   ~~~~~~~
<source>:5:2: warning: expression result unused [-Wunused-value]
    5 |         y<0>;
      |         ^~~~
1 warning and 1 error generated.
Compiler returned: 1

[zyn0217]

It’s not that we're missing an instantiation for y<0>, but rather that we checked the initialization too early, right after the variable template was constructed. This is because the first PackIndexingExpr was assigned a ValueInstantiation dependency but retained a dependent type, which caused the logic to be skipped.

llvm-project/clang/lib/Sema/SemaInit.cpp

Lines 6431 to 6435 in 48bf0a9

	if (DestType->isDependentType() ||
	Expr::hasAnyTypeDependentArguments(Args)) {
	SequenceKind = DependentSequence;
	return;
	}

llvm-project/clang/lib/AST/ExprCXX.cpp

Lines 1722 to 1725 in 48bf0a9

	if (Index && FullySubstituted && !SubstitutedExprs.empty())
	Type = SubstitutedExprs[*Index]->getType();
	else
	Type = Context.DependentTy;

We should probably set the expr type to PackIdExpr->getType() on line 1725.

This also indicates that a workaround is to convert the NTTP to a dependent type, i.e. the following would bypass the initialization check that happens before we see y<0> and its instantiation

template<auto... x>
int y = x...[0];

int main() {
	y<0>;
}