Clang++ rejects try-catch in discarded if constexpr branch with -fno-exceptions

[zerhud]

Summary:
Clang incorrectly rejects code containing a try-catch block inside a discarded if constexpr branch when compiled with -fno-exceptions. The try-catch block should not be evaluated, as it is excluded at compile-time, but Clang emits an error: cannot use 'try' with exceptions disabled.

Steps to Reproduce:

1. Create a file test.cpp with the following code:

   template<auto enable> void foo(auto&& fnc) {
       if constexpr(enable) try{ fnc(); } catch(...){}
       else fnc();
   }
   
   int main(int, char**) {
       foo<false>([]{});
   }

2. Compile with:

   clang++ -std=c++20 -fno-exceptions -c test.cpp

3. Alternatively, see the issue on Godbolt: https://godbolt.org/z/3ecnde5z4

Actual Results:

test.cpp:2:25: error: cannot use 'try' with exceptions disabled
    if constexpr(enable) try{ fnc(); } catch(...){}
                         ^
1 error generated.

Expected Results:
The code should compile successfully, as the try-catch block is in a discarded if constexpr branch and should not be evaluated.

Additional Information:

• The code compiles successfully with GCC
• A not called template function with try-catch block rejected too (Godbolt)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (zerhud)

**Summary**: Clang incorrectly rejects code containing a `try-catch` block inside a discarded `if constexpr` branch when compiled with `-fno-exceptions`. The `try-catch` block should not be evaluated, as it is excluded at compile-time, but Clang emits an error: `cannot use 'try' with exceptions disabled`.

Steps to Reproduce:

1. Create a file test.cpp with the following code:

   template<auto enable> void foo(auto&& fnc) {
       if constexpr(enable) try{ fnc(); } catch(...){}
       else fnc();
   }
   
   int main(int, char**) {
       foo<false>([]{});
   }

2. Compile with:

   clang++ -std=c++20 -fno-exceptions -c test.cpp

3. Alternatively, see the issue on Godbolt: https://godbolt.org/z/3ecnde5z4

Actual Results:

test.cpp:2:25: error: cannot use 'try' with exceptions disabled
    if constexpr(enable) try{ fnc(); } catch(...){}
                         ^
1 error generated.

Expected Results:
The code should compile successfully, as the try-catch block is in a discarded if constexpr branch and should not be evaluated.

Additional Information:

• The code compiles successfully with GCC
• A not called template function with try-catch block rejected too (Godbolt)

[shafik]

Looks like other implementations accept w/ their various no exceptions flags: https://godbolt.org/z/YhqaxTW3E

[erichkeane]

Technically not a bug/not incorrect, since we define what that flag means as it is non standard:) That said, all of the other implementations seem to diagnose this 'later' than we do so they accept it, and we should too.

I also note we don't document what -fno-exceptions means, though we probably should!

As far as implementation: We currently diagnose this in ActOnCXXTryBlock and BuildCXXThrow.
Those are propbably the correct places here, but what we need to do is change that to ONLY diagnose when it is in a non-dependent context.

Additionally, we'd want to make sure that this doesn't cause us to lose diagnostics in cases like:

template<typename T>
void foo() {
   try{}catch(...){}
   throw 1;
}
void use() {
  foo<int>();
}

If we DO miss those two (which again should only happen because of foo<int>(), so should print the instantiation notes) we might have to move one/both of the diagnostics.

I think I'm going to mark this as a good beginner issue. It might be slightly non-trivial, and if it ends up being, whoever decides to work on it can feel free to reach out to me for advice.