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

[Matheus Izvekov via cfe-commits]

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@@ -1144,6 +1164,46 @@ bool Sema::CheckInstantiatedFunctionTemplateConstraints(
                                      PointOfInstantiation, Satisfaction);
 }
 
+bool Sema::CheckFunctionConstraintsWithoutInstantiation(
+    SourceLocation PointOfInstantiation, FunctionTemplateDecl *Template,
+    ArrayRef<TemplateArgument> TemplateArgs,
+    ConstraintSatisfaction &Satisfaction) {
+  FunctionDecl *FD = Template->getTemplatedDecl();
+  SmallVector<const Expr *, 3> TemplateAC;
+  Template->getAssociatedConstraints(TemplateAC);
+  if (TemplateAC.empty()) {
+    Satisfaction.IsSatisfied = true;
+    return false;
+  }
+
+  // Enter the scope of this instantiation. We don't use
+  // PushDeclContext because we don't have a scope.
+  LocalInstantiationScope Scope(*this);
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mizvekov wrote:

I am not sure how `PushDeclContext` is related to the instantiation scope.

`PushDeclContext` would have changed the CurContext. If you need that, you can use `ContextRAII` 

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