[clang] [Clang] Implement CWG 2628 "Implicit deduction guides should propagate constraints" (PR #111143)

[Erich Keane via cfe-commits]

================
@@ -446,10 +450,46 @@ struct ConvertConstructorToDeductionGuideTransform {
       return nullptr;
     TypeSourceInfo *NewTInfo = TLB.getTypeSourceInfo(SemaRef.Context, NewType);
 
+    // At this point, the function parameters are already 'instantiated' in the
+    // current scope. Substitute into the constructor's trailing
+    // requires-clause, if any.
+    Expr *FunctionTrailingRC = nullptr;
+    if (Expr *RC = CD->getTrailingRequiresClause()) {
+      MultiLevelTemplateArgumentList Args;
+      Args.setKind(TemplateSubstitutionKind::Rewrite);
+      Args.addOuterTemplateArguments(Depth1Args);
+      Args.addOuterRetainedLevel();
+      if (NestedPattern)
+        Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
+      ExprResult E = SemaRef.SubstConstraintExprWithoutSatisfaction(RC, Args);
+      if (!E.isUsable())
+        return nullptr;
+      FunctionTrailingRC = E.get();
+    }
+
+    // C++ [over.match.class.deduct]p1:
+    // If C is defined, for each constructor of C, a function template with
+    // the following properties:
+    // [...]
+    // - The associated constraints are the conjunction of the associated
+    // constraints of C and the associated constraints of the constructor, if
+    // any.
+    if (OuterRC) {
+      // The outer template parameters are not transformed, so their
+      // associated constraints don't need substitution.
+      if (!FunctionTrailingRC)
+        FunctionTrailingRC = OuterRC;
+      else
+        FunctionTrailingRC = BinaryOperator::Create(
----------------
erichkeane wrote:

This has a certain elegance to it, but I'm not super thrilled with having a BinaryOperator that doesn't have a valid source location.  I wonder if we should instead store a collection and check them that way.  It makes checking them a little more difficult down the road, but keeps source fidelity.

https://github.com/llvm/llvm-project/pull/111143