[PATCH] D128750: [C++20] Implement P2113R0: Changes to the Partial Ordering of Constrained Functions

[Matheus Izvekov via Phabricator via cfe-commits]

mizvekov added inline comments.


================
Comment at: clang/include/clang/Sema/SemaConcept.h:48-52
+      if (ArgA.getKind() == TemplateArgument::Expression &&
+          ArgB.getKind() == TemplateArgument::Expression &&
+          ArgA.getAsExpr()->getType()->isUndeducedAutoType() &&
+          ArgB.getAsExpr()->getType()->isUndeducedAutoType())
+        continue;
----------------
Why are looking at only the type of the expression?
The expression can be arbitrarily different, but as long as they are both undeduced auto, that is okay?


================
Comment at: clang/include/clang/Sema/SemaConcept.h:54-68
+      if (ArgA.getKind() == TemplateArgument::Type &&
+          ArgB.getKind() == TemplateArgument::Type)
+        if (const auto *SubstA =
+                ArgA.getAsType()->getAs<SubstTemplateTypeParmType>())
+          if (const auto *SubstB =
+                  ArgB.getAsType()->getAs<SubstTemplateTypeParmType>()) {
+            QualType ReplacementA = SubstA->getReplacementType();
----------------
It's a bit odd to find `SubstTemplateTypeParmType` necessary to implement the semantics of this change.

This is just type sugar we leave behind in the template instantiator to mark where type replacement happened.

There are several potential issues here:
1) This could be marking a substitution that happened at any template depth. Ie this could be marking a substitution from an outer template. Does the level not matter here at all? 
2) If the level does matter, you won't be able to reconstitute that easily without further improvements. See https://github.com/llvm/llvm-project/issues/55886 for example.
3) A substitution can replace a dependent type for another one, and when that other gets replaced, we lose track of that because `SubstTemplateTypeParmType` only holds a canonical underlying type.

----

Leaving that aside, I get the impression you are trying to work around the fact that when an expression appears in a canonical type, presumably because that expression is dependent on an NTTP, we can't rely on uniquing anymore to compare if they are the same type, as we lack in Clang the equivalent concept of canonicalization for expressions.

But this however is a bit hard to implement. Are we sure the standard requires this, or can we simply consider these types always different?


================
Comment at: clang/lib/AST/ASTContext.cpp:5149-5151
+    Expr *E = new (*this)
+        DeclRefExpr(*this, NTTP, /*enclosing*/ false, T,
+                    Expr::getValueKindForType(NTTPType), NTTP->getLocation());
----------------



================
Comment at: clang/lib/AST/ASTContext.cpp:5154-5156
+      E = new (*this) PackExpansionExpr(
+          NTTPType->isUndeducedAutoType() ? NTTPType : DependentTy, E,
+          NTTP->getLocation(), None);
----------------
I don't know if this change is necessary for this patch, as this looks part of the workaround in `SemaConcept.h`,
but I think a better way to preserve the type here might be to always use `NTTPType`, but then add an additional `Dependent` parameter to `PackExpansionExpr` which can be used to force the expression to be dependent.


================
Comment at: clang/lib/Sema/SemaConcept.cpp:827
+    assert(FoldE->isRightFold() && FoldE->getOperator() == BO_LAnd);
+    E = FoldE->getPattern();
+  }
----------------
If you need to dig down into the pattern, then I think the pattern might also contain casts and parenthesis which you would need to keep ignoring for the rest of the code to work.

I would consider adding a test for that.


================
Comment at: clang/lib/Sema/SemaOverload.cpp:9675-9676
-  // when comparing template functions. 
-  if (Cand1.Function && Cand2.Function && Cand1.Function->hasPrototype() &&
-      Cand2.Function->hasPrototype()) {
     auto *PT1 = cast<FunctionProtoType>(Cand1.Function->getFunctionType());
----------------
Why are these `hasPrototype` checks not needed anymore?

I don't see other changes which would obliviate the need for it. Presumably the code below is assuming we have them when we perform that `FunctionProtoType` cast.

Maybe this was either not possible, or we simply never added tests for it.


================
Comment at: clang/lib/Sema/SemaTemplate.cpp:1267
       BuildDeclRefExpr(NTTP, NTTP->getType(), VK_PRValue, NTTP->getLocation());
-  if (!Ref)
-    return true;
+  assert(Ref);
   ExprResult ImmediatelyDeclaredConstraint = formImmediatelyDeclaredConstraint(
----------------
I don't think the `assert` is even necessary TBH, the function can't return nullptr.


================
Comment at: clang/lib/Sema/SemaTemplateDeduction.cpp:5247-5248
+  for (unsigned i = 0; i < NumParams1; ++i)
+    if (FD1->getParamDecl(i)->getType().getCanonicalType() !=
+        FD2->getParamDecl(i)->getType().getCanonicalType())
+      return nullptr;
----------------



================
Comment at: clang/lib/Sema/SemaTemplateDeduction.cpp:5256-5257
+  //   than the other.
+  if (TPOC == TPOC_Conversion && FD1->getReturnType().getCanonicalType() !=
+                                     FD2->getReturnType().getCanonicalType())
+    return nullptr;
----------------



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  https://reviews.llvm.org/D128750/new/

https://reviews.llvm.org/D128750