A few weeks ago I came across some code that MSVC and Clang both rejected, but GCC accepted. The error messages are different in MSVC and Clang, so clearly I must have done something wrong, and GCC falsely accepts the code, right? I mostly minimized the code, and ended up with this. (Compiler Explorer link)
struct S{};
template <class T>
void foo(T) {
(void)[]<class U>(U) consteval -> bool {
return requires { 0 * T{}; };
}(0);
}
void bar() {
foo(S{});
}
In this post I will explain how I got here, what I think is going on with all 3 of these compilers, and allude to what I’ll be talking about in the next article after this one.
Getting an expert opinion
I reached out to a compiler developer whose work I really respect, and who I always enjoy speaking with. Normally I wouldn’t do this, I would just file a bug with the relevant compiler and move on. But this one was too weird.
TODO: Add messages.
Alright, so it turns out GCC is actually the correct one here. This code should be accepted. MSVC rejects it for one reason, and Clang rejects it for another. That means that I’ll need to work around each of the compilers individually.
But first, this code looks too contrived. How did I get here?
The general backstory
I like writing compile-time code. For a few years I’ve been working on a compile-time parser generator library using expression templates. The challenge was to make all the parser types entirely empty, with no non-static data members. When I started, I knew significantly less C++ than I do now, and a lot of my learning came from writing the library. If you’re interested to take a look, it’s called tok3n
Because I have this parser generator library that I like very much, most of my effort has actually been focused on testing it. If you write compile-time code, you should also test that code as compile-time, so all of my tests check the compile-time behaviour of my library. This is something I care about deeply, and I’ve given 3 talks on it so far:
Between 2024 and 2026, I entirely changed my testing strategy, based on the amazing idea I encountered when writing my 2024 talk, from the Snitch library: You can store a compile-time condition, and only check the result at run-time. Instead of using static_assert throughout my tests, I want my tests to all compile successfully, even if there are compile-time conditions that fail. Then at run-time, I can have a nice user-defined error message and a nice test printout from the framework. This is nicer than a compiler-generated error message, especially because the printout is compiler-independent, and will look the same regardless.
I spun my testing framework off into its own repo separate from tok3n, and named it k3tchup. Then I started to add more features to it, in an effort to make it a generally usable testing library. This whole story for the past 2 years, with the technical details on many of the features I added, is the topic of my last 2 talks, at C++Now 2026 (link TBD) and ACCU on Sea 2026 (link TBD).
The specific backstory
After many various updates to the k3tchup framework, I updated the k3tchup submodule in tok3n, and encountered approximately 18 quintillion compiler errors, give or take. This was right after ACCU on Sea 2026, so a small amount of the code I showed in my slides is now outdated. Oops.
The short version is this. For any given parser p, you can add a bunch of modifiers onto it. For example, the complete modifier means that the parser will reject any input that has anything leftover after parsing.
constexpr auto p = "abc"_all;
constexpr auto p2 = p % complete; // Or `complete(p)`
static_assert(p.parse("abcd"));
static_assert(!p2.parse("abcd"));
Many of the errors were triggered inside a k3tchup “packet” (a nested callable) in tok3n’s tests, checking what happens when you add the modifiers. The library k3tchup itself doesn’t have any matchers, but tok3n’s tests build up a system of matchers.
For example, this is a simplified version of what happens.
EXPECT_THAT(the_parser<P> | is_modifiable_by<complete>);
Which parser am I checking? Many of them. I have a long list of samples, and I want to check this condition for all of them. So I write something like this.
constexpr auto complete_modifier_tester =
[]<parser P>(P) {
EXPECT_THAT(the_parser<P> | is_modifiable_by<complete>);
};
Then I loop over the list of samples with this tester lambda.
EXPECT_THAT(all_samples.satisfy(complete_modifier_tester));
The issue is happening inside the is_modifiable_by fragment, whose class is defined similar to this.
template <modifier M>
struct is_modifiable_by_fragment {
template <parser P>
void operator()(P) const {
EXPECT_COMPILE_TIME(requires { M{}(P{}); });
EXPECT_COMPILE_TIME(requires { P{} % M{}; });
// etc...
}
};
I minimized the issue, removing all usage of the standard library or my libraries, and ended up with this.
struct S{};
template <class T>
void foo(T) {
(void)[]<class U>(U) consteval -> bool {
return requires { 0 * T{}; };
}(0);
}
void bar() {
foo(S{});
}
What’s going on in MSVC?
Here is the error message I got with the MSVC version in Visual Studio 18.6.3.
<source>(7,29): error C2677: binary '*': no global operator found
which takes type 'S' (or there is no acceptable conversion)
MSVC is complaining that 0 * T{} isn’t a valid expression. Well of course it isn’t valid, I’m using the requires clause to check for the validity. Sometimes the answer will be that it isn’t valid. To make this work in MSVC, I need to make the requires clause dependent on the lambda’s template parameter.
struct S{};
template <class T>
void foo(T) {
(void)[]<class U>(U) consteval -> bool {
return requires { U{0} * T{}; };
}(0);
}
void bar() {
foo(S{});
}
Notice that now says requires { U{0} * T{}; }, instead of requires { 0 * T{}; }. I have a guess, but nothing conclusive. My guess is that MSVC evaluates the requires clause early when it isn’t dependent on U, and it treats it the same as if it was non-dependent. From the perspective of the lambda, it is non-dependent, even though it is actually dependent on the outer template paramter.
This one is actually not that bad to work around. Instead of the test code I showed above, I just factor out those conditions into their own variable templates or concepts. It’s not ideal, but it’s not terrible.
template <modifier M>
struct is_modifiable_by_fragment {
template <parser P>
static constexpr bool call_op = requires { M{}(P{}); };
template <parser P>
static constexpr bool infix = requires { P{} % M{}; };
template <parser P>
void operator()(P) const {
EXPECT_COMPILE_TIME(call_op<P>);
EXPECT_COMPILE_TIME(infix<P>);
// etc...
}
};
Now that that’s sorted…
What’s going on in Clang?
Here is the error message I got with Clang 22.1.3.
<source>:4:11: error: call to consteval function
'foo(S)::(lambda)::operator()<int>' is not a constant expression
5 | (void)[]<class U>(U) consteval -> bool {
| ^
Why is this not a constant expression? It’s just returning the result of a requires clause, which is always a constant expression by definition. I was able to minimize this one much further, and still get the same error.
template <class T = void>
void foo() {
[](auto) consteval -> void {
}(0);
}
int main() {
foo();
}
An empty body is certainly a constant expression, so there’s clearly a bug here. I opened llvm/llvm-project#205218, but it was closed as a duplicate of llvm/llvm-project#35052. This bug has been known since 2017, and a fix hasn’t been managed yet. No blame here; I understand compilers are really difficult. I wouldn’t be able to do it.
Basically, Clang can’t evaluate certain generic lambdas at compile-time inside of another template.
For me here, this issue pops up in the source code not in the tok3n tests, but in the k3tchup framework itself. In certain places in the framework, I need to defensively add an extra template context, and I do that with an immediately-invoked generic lambda. If that lambda is invoked at compile-time, that’s exactly the problem seen here.
I have to work around it though, if I want to keep the same API in k3tchup. Of course I could change my API choices, but I don’t want to. I want to see how far I can take this.
Conclusion
I’ll be writing another article about the antics I used in order to work around this Clang bug. I want my library to be as general-purpose as possible, so I need to work around this.
Thanks for reading!
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