[clang] [clang-tools-extra] [clang] check deduction consistency when partial ordering function templates (PR #100692)

[Matheus Izvekov via cfe-commits]

================
@@ -5399,11 +5434,85 @@ static QualType GetImplicitObjectParameterType(ASTContext &Context,
   return Context.getLValueReferenceType(RawType);
 }
 
+static TemplateDeductionResult FinishTemplateArgumentDeduction(
+    Sema &S, FunctionTemplateDecl *FTD, int ArgIdx, QualType P, QualType A,
+    SmallVectorImpl<DeducedTemplateArgument> &Deduced,
+    TemplateDeductionInfo &Info) {
+  // Unevaluated SFINAE context.
+  EnterExpressionEvaluationContext Unevaluated(
+      S, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::SFINAETrap Trap(S);
+
+  Sema::ContextRAII SavedContext(S, getAsDeclContextOrEnclosing(FTD));
+
+  // C++ [temp.deduct.type]p2:
+  //   [...] or if any template argument remains neither deduced nor
+  //   explicitly specified, template argument deduction fails.
+  bool IsIncomplete = false;
+  SmallVector<TemplateArgument, 4> SugaredBuilder, CanonicalBuilder;
+  if (auto Result = ConvertDeducedTemplateArguments(
+          S, FTD, /*IsDeduced=*/true, Deduced, Info, SugaredBuilder,
+          CanonicalBuilder, /*CurrentInstantiationScope=*/nullptr,
+          /*NumAlreadyConverted=*/0, &IsIncomplete);
+      Result != TemplateDeductionResult::Success)
+    return Result;
+
+  // Form the template argument list from the deduced template arguments.
+  TemplateArgumentList *SugaredDeducedArgumentList =
+      TemplateArgumentList::CreateCopy(S.Context, SugaredBuilder);
+  TemplateArgumentList *CanonicalDeducedArgumentList =
+      TemplateArgumentList::CreateCopy(S.Context, CanonicalBuilder);
+
+  Info.reset(SugaredDeducedArgumentList, CanonicalDeducedArgumentList);
+
+  // Substitute the deduced template arguments into the argument
+  // and verify that the instantiated argument is both valid
+  // and equivalent to the parameter.
+  LocalInstantiationScope InstScope(S);
+
+  QualType InstP;
+  {
+    MultiLevelTemplateArgumentList MLTAL(FTD, SugaredBuilder,
+                                         /*Final=*/true);
+    if (ArgIdx != -1)
+      if (auto *MD = dyn_cast<CXXMethodDecl>(FTD->getTemplatedDecl());
+          MD && MD->isImplicitObjectMemberFunction())
+        ArgIdx -= 1;
+    Sema::ArgumentPackSubstitutionIndexRAII PackIndex(
+        S, ArgIdx != -1 ? ::getPackIndexForParam(S, FTD, MLTAL, ArgIdx) : -1);
+    InstP = S.SubstType(P, MLTAL, FTD->getLocation(), FTD->getDeclName());
+    if (InstP.isNull())
+      return TemplateDeductionResult::SubstitutionFailure;
+  }
+
+  if (auto *PA = dyn_cast<PackExpansionType>(A);
+      PA && !isa<PackExpansionType>(InstP))
+    A = PA->getPattern();
+  if (!S.Context.hasSameType(
+          S.Context.getUnqualifiedArrayType(InstP.getNonReferenceType()),
+          S.Context.getUnqualifiedArrayType(A.getNonReferenceType())))
+    return TemplateDeductionResult::NonDeducedMismatch;
----------------
mizvekov wrote:

We could consider both candidates for that example.
```C++
void g() {
  B b;
  f(0, b);
}
```

But in this case, we order the function template specializations, instead of the function templates, which this patch does not change.

I am not sure what is the rule for that, if we are even being compliant here. I will try to dig this up later.


https://github.com/llvm/llvm-project/pull/100692