[clang][ExprConst] Allow non-literal types in C++23 (#100062)
Instead of diagnosing non-literal types in C++23, allow them and later
diagnose them differently, e.g. because they have a non-constexpr
constructor, destructor, etc.
For this test:
```c++
struct NonLiteral {
NonLiteral() {}
};
constexpr int foo() {
NonLiteral L;
return 1;
}
// static_assert(foo() == 1);
```
The current diagnostics with c++20/c++23 are:
```console
~/code/llvm-project/build » clang -c array.cpp -std=c++20
array.cpp:91:14: error: variable of non-literal type 'NonLiteral' cannot be defined in a constexpr function before C++23
91 | NonLiteral L;
| ^
array.cpp:87:8: note: 'NonLiteral' is not literal because it is not an aggregate and has no constexpr constructors other than copy or move constructors
87 | struct NonLiteral {
| ^
1 error generated.
------------------------------------------------------------
~/code/llvm-project/build » clang -c array.cpp -std=c++23
(no output)
```
With the `static_assert` enabled, compiling with `-std=c++23` prints:
```console
array.cpp:95:15: error: static assertion expression is not an integral constant expression
95 | static_assert(foo() == 1);
| ^~~~~~~~~~
array.cpp:91:14: note: non-literal type 'NonLiteral' cannot be used in a constant expression
91 | NonLiteral L;
| ^
array.cpp:95:15: note: in call to 'foo()'
95 | static_assert(foo() == 1);
| ^~~~~
1 error generated.
```
As mentioned in #60311, this is confusing. The output with c++20
suggests that using c++23 will make the problem go away, but it's
diagnosed the same when running the function.
With this commit, the output instead diagnoses _why_ the non-literal
type can't be used:
```console
array.cpp:95:15: error: static assertion expression is not an integral constant expression
95 | static_assert(foo() == 1);
| ^~~~~~~~~~
array.cpp:91:14: note: non-constexpr constructor 'NonLiteral' cannot be used in a constant expression
91 | NonLiteral L;
| ^
array.cpp:95:15: note: in call to 'foo()'
95 | static_assert(foo() == 1);
| ^~~~~
array.cpp:88:3: note: declared here
88 | NonLiteral() {}
| ^
1 error generated.
```
Fixes #60311
llvm-project
dbe308c0 - [clang][ExprConst] Allow non-literal types in C++23 (#100062)
