https://github.com/Sirraide commented:
I think this unfortunately isn’t this simple: I can think of a few problems
this runs into and I the solutions I’ve managed to think of are:
First, we could use `Sema::CheckConstexprFunctionDefinition()` (see also the
call to that function in `Sema::BuildLambda()`, which handles making lambdas
implicitly `constexpr`) to make sure a function is even allowed to be
`constexpr`; however, that needs to happen after we parse the function body.
This, however, might have some unfortunate consequences for function
declarations without a body; it would be a bit janky to have to remove the
`constexpr` specifier again once we see the definition and realise it can’t
actually be `constexpr`, but not making it `constexpr` if there is no body runs
into problems with code like:
```c++
int foo();
int bar() { return foo(); }
int foo() { return 3; }
static_assert(bar() == 3);
```
When we encounter `bar()` here, we can only make it implicitly `constexpr` if
we tread `foo()` as `constexpr` as well even though we haven’t seen its body
yet and hence can’t actually check whether it can actually be `constexpr`. GCC
accepts this code if we declare `foo()` and `bar()` as `inline`.
An alternative approach might be to just *not* make functions `constexpr` in
the AST and also never call `CheckConstexprFunctionDefinition()` for such
functions in this mode and instead just allow calling functions that are not
declared `constexpr` if the lang opt is set. That however runs into the problem
that the constant evaluator needs to be able to diagnose any invalid constructs
it runs into w/o crashing, and I’m not sure it’s currently set up for that
given that `CheckConstexprFunctionDefinitions()` rejects e.g. `goto`s before we
ever get to the constant evaluator. So that definitely requires more tests for
all things we might run into there.
https://github.com/llvm/llvm-project/pull/136436
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