[clang] [clang] CTAD: implement the missing IsDeducible constraint for alias templates (PR #89358)

[Matheus Izvekov via cfe-commits]

================
@@ -3237,6 +3237,40 @@ static TemplateDeductionResult FinishTemplateArgumentDeduction(
 
   return TemplateDeductionResult::Success;
 }
+/// Complete template argument deduction for DeduceTemplateArgumentsFromType.
+/// FIXME: this is mostly duplicated with the above two versions. Deduplicate
+/// the three implementations.
+static TemplateDeductionResult FinishTemplateArgumentDeduction(
+    Sema &S, TemplateDecl *TD,
+    SmallVectorImpl<DeducedTemplateArgument> &Deduced,
+    TemplateDeductionInfo &Info) {
+  // Unevaluated SFINAE context.
+  EnterExpressionEvaluationContext Unevaluated(
+      S, Sema::ExpressionEvaluationContext::Unevaluated);
+  Sema::SFINAETrap Trap(S);
+
+  Sema::ContextRAII SavedContext(S, getAsDeclContextOrEnclosing(TD));
+
+  // C++ [temp.deduct.type]p2:
+  //   [...] or if any template argument remains neither deduced nor
+  //   explicitly specified, template argument deduction fails.
+  SmallVector<TemplateArgument, 4> SugaredBuilder, CanonicalBuilder;
+  if (auto Result = ConvertDeducedTemplateArguments(
+          S, TD, /*IsPartialOrdering=*/false, Deduced, Info, SugaredBuilder,
+          CanonicalBuilder);
+      Result != TemplateDeductionResult::Success)
+    return Result;
+
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mizvekov wrote:

This is missing a consistency check, ie [temp.deduct.type]p1.

Otherwise we would incorrectly accept a match where there would be a non-deduced mismatch.

https://github.com/llvm/llvm-project/pull/89358