[clang] [clang] Implement CTAD for type alias template. (PR #77890)

[Erich Keane via cfe-commits]

================
@@ -2258,6 +2258,94 @@ class ExtractTypeForDeductionGuide
   }
 };
 
+// Build a deduction guide with the specified parameter types.
+FunctionTemplateDecl *
+buildDeductionGuide(Sema &SemaRef, TemplateDecl *OriginalTemplate,
+                    TemplateParameterList *TemplateParams,
+                    CXXConstructorDecl *Ctor, ExplicitSpecifier ES,
+                    TypeSourceInfo *TInfo, SourceLocation LocStart,
+                    SourceLocation Loc, SourceLocation LocEnd, bool IsImplicit,
+                    llvm::ArrayRef<TypedefNameDecl *> MaterializedTypedefs = {}) {
+  DeclContext *DC = OriginalTemplate->getDeclContext();
+  auto DeductionGuideName =
+      SemaRef.Context.DeclarationNames.getCXXDeductionGuideName(
+          OriginalTemplate);
+
+  DeclarationNameInfo Name(DeductionGuideName, Loc);
+  ArrayRef<ParmVarDecl *> Params =
+      TInfo->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams();
+
+  // Build the implicit deduction guide template.
+  auto *Guide =
+      CXXDeductionGuideDecl::Create(SemaRef.Context, DC, LocStart, ES, Name,
+                                    TInfo->getType(), TInfo, LocEnd, Ctor);
+  Guide->setImplicit(IsImplicit);
+  Guide->setParams(Params);
+
+  for (auto *Param : Params)
+    Param->setDeclContext(Guide);
+  for (auto *TD : MaterializedTypedefs)
+    TD->setDeclContext(Guide);
+
+  auto *GuideTemplate = FunctionTemplateDecl::Create(
+      SemaRef.Context, DC, Loc, DeductionGuideName, TemplateParams, Guide);
+  GuideTemplate->setImplicit(IsImplicit);
+  Guide->setDescribedFunctionTemplate(GuideTemplate);
+
+  if (isa<CXXRecordDecl>(DC)) {
+    Guide->setAccess(AS_public);
+    GuideTemplate->setAccess(AS_public);
+  }
+
+  DC->addDecl(GuideTemplate);
+  return GuideTemplate;
+}
+
+// Transform a given template type parameter `TTP`.
+TemplateTypeParmDecl *
+transformTemplateTypeParam(Sema &SemaRef, DeclContext *DC,
+                           TemplateTypeParmDecl *TTP,
+                           MultiLevelTemplateArgumentList &Args,
+                           unsigned NewDepth, unsigned NewIndex) {
+  // TemplateTypeParmDecl's index cannot be changed after creation, so
+  // substitute it directly.
+  auto *NewTTP = TemplateTypeParmDecl::Create(
+      SemaRef.Context, DC, TTP->getBeginLoc(), TTP->getLocation(), NewDepth,
+      NewIndex, TTP->getIdentifier(), TTP->wasDeclaredWithTypename(),
+      TTP->isParameterPack(), TTP->hasTypeConstraint(),
+      TTP->isExpandedParameterPack()
+          ? std::optional<unsigned>(TTP->getNumExpansionParameters())
+          : std::nullopt);
+  if (const auto *TC = TTP->getTypeConstraint())
+    SemaRef.SubstTypeConstraint(NewTTP, TC, Args,
+                                /*EvaluateConstraint*/ true);
----------------
erichkeane wrote:

For the most part, we WANT the 'old' constraint to attach.  All the work to instantiate the constraints end up going against the original template.

By doing this constraint evaluation, you're now making substitution failures observable way before they are allowed to be observed from the looks of it.

https://github.com/llvm/llvm-project/pull/77890