[clang] [Clang] Treat default template argument as constant expressions (PR #107073)

[Younan Zhang via cfe-commits]

================
@@ -960,6 +960,8 @@ Parser::ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position) {
       EnterExpressionEvaluationContext ConstantEvaluated(
           Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
       DefaultArg = Actions.CorrectDelayedTyposInExpr(ParseInitializer());
+      if (DefaultArg.isUsable())
+        DefaultArg = Actions.ActOnConstantExpression(DefaultArg);
----------------
zyn0217 wrote:

If you look at `Sema::CheckLValueToRValueConversionOperand()`, it would bail out if `E` is of a RecordType.
So the following would still be rejected
```cpp
struct S {};

void foo()
{
    constexpr S x{};
    auto a = []<auto = x>{};
}
```

https://github.com/llvm/llvm-project/pull/107073