[clang] [libcxx] [Clang] Implement CWG2369 "Ordering between constraints and substitution" (PR #102857)

[Younan Zhang via cfe-commits]

================
@@ -1109,12 +1109,50 @@ bool Sema::EnsureTemplateArgumentListConstraints(
   return false;
 }
 
-bool Sema::CheckInstantiatedFunctionTemplateConstraints(
+static bool CheckFunctionConstraintsWithoutInstantiation(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    FunctionTemplateDecl *Template, ArrayRef<TemplateArgument> TemplateArgs,
+    ConstraintSatisfaction &Satisfaction) {
+  SmallVector<const Expr *, 3> TemplateAC;
+  Template->getAssociatedConstraints(TemplateAC);
+  if (TemplateAC.empty()) {
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  LocalInstantiationScope Scope(SemaRef);
+
+  FunctionDecl *FD = Template->getTemplatedDecl();
+  // Collect the list of template arguments relative to the 'primary'
+  // template. We need the entire list, since the constraint is completely
+  // uninstantiated at this point.
+  MultiLevelTemplateArgumentList MLTAL =
+      SemaRef.getTemplateInstantiationArgs(FD, /*DC=*/nullptr,
+                                           /*Final=*/false,
+                                           /*Innermost=*/TemplateArgs,
+                                           /*RelativeToPrimary=*/true,
+                                           /*ForConstraintInstantiation=*/true);
+
+  std::optional<Sema::CXXThisScopeRAII> ThisScope;
+  if (auto *Method = dyn_cast<CXXMethodDecl>(FD))
+    ThisScope.emplace(SemaRef, /*Record=*/Method->getParent(),
+                      /*ThisQuals=*/Method->getMethodQualifiers());
----------------
zyn0217 wrote:

I tried to contrive a case like

```cpp
template <class T>
struct S {
  void foo() requires(__is_same_as(S&, decltype(*this))) {}
};

void g() {
  S<int>().foo();
}
```

But there would still be no difference if we did the `CXXThisScopeRAII` things away. (It always compiles.)

Maybe I'm running out of ideas right now, so let's just preserve these "strange but plausible" things?

https://github.com/llvm/llvm-project/pull/102857