Templated lambda ends up with dependent type in C++11

[tbaederr]

This is an issue for the bug from #108598

This code:

template<typename T>
constexpr T outer() {
  return []<T x>() { return x; }.template operator()<123>();
}
static_assert(outer<int>() == 123);

ends up with a dependent type that we assert on in codegen.

This starts when we deduce the type of the call operator, which is dependent in C++11 but auto in later versions:

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

Lines 958 to 973 in 7b88e75

	case DeclSpec::TST_unspecified:
	// If this is a missing declspec in a block literal return context, then it
	// is inferred from the return statements inside the block.
	// The declspec is always missing in a lambda expr context; it is either
	// specified with a trailing return type or inferred.
	if (S.getLangOpts().CPlusPlus14 &&
	declarator.getContext() == DeclaratorContext::LambdaExpr) {
	// In C++1y, a lambda's implicit return type is 'auto'.
	Result = Context.getAutoDeductType();
	break;
	} else if (declarator.getContext() == DeclaratorContext::LambdaExpr ||
	checkOmittedBlockReturnType(S, declarator,
	Context.DependentTy)) {
	Result = Context.DependentTy;
	break;
	}

and we never seem to clean this up.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Timm Baeder (tbaederr)

This is an issue for the bug from https://github.com//pull/108598

This code:

template<typename T>
constexpr T outer() {
  return []<T x>() { return x; }.template operator()<123>();
}
static_assert(outer<int>() == 123);

ends up with a dependent type that we assert on in codegen.

This starts when we deduce the type of the call operator, which is dependent in C++11 but auto in later versions:

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

Lines 958 to 973 in 7b88e75

	case DeclSpec::TST_unspecified:
	// If this is a missing declspec in a block literal return context, then it
	// is inferred from the return statements inside the block.
	// The declspec is always missing in a lambda expr context; it is either
	// specified with a trailing return type or inferred.
	if (S.getLangOpts().CPlusPlus14 &&
	declarator.getContext() == DeclaratorContext::LambdaExpr) {
	// In C++1y, a lambda's implicit return type is 'auto'.
	Result = Context.getAutoDeductType();
	break;
	} else if (declarator.getContext() == DeclaratorContext::LambdaExpr ||
	checkOmittedBlockReturnType(S, declarator,
	Context.DependentTy)) {
	Result = Context.DependentTy;
	break;
	}

and we never seem to clean this up.

[cor3ntin]

@mizvekov

[tbaederr]

@mizvekov Any update on this?