[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:

Hmm... but as-written it seems like it would cause problems where the original function-template isn't instantiate-able.  It seems to me that instead of substituting it, perhaps we should be 'converting' it in some way to update the references?  I'll have to think on it, though I'm @zygoloid  can come around and answer for us.

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