[cxx-interop] Support C++ function templates in Swift. (swiftlang#33053)

This patch adds rudimentary support for C++ template functions in swift.

Author: zoecarver

include/swift/AST/ASTContext.h

@@ -23,11 +23,12 @@
 #include "swift/AST/Import.h"
 #include "swift/AST/SearchPathOptions.h"
 #include "swift/AST/Type.h"
-#include "swift/AST/Types.h"
 #include "swift/AST/TypeAlignments.h"
+#include "swift/AST/Types.h"
 #include "swift/Basic/LangOptions.h"
 #include "swift/Basic/Located.h"
 #include "swift/Basic/Malloc.h"
+#include "clang/AST/DeclTemplate.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
@@ -628,6 +629,13 @@ class ASTContext final {
     ArrayRef<SILParameterInfo> params, Optional<SILResultInfo> result,
     SILFunctionType::Representation trueRep);
 
+  /// Instantiates "Impl.Converter" if needed, then calls
+  /// ClangTypeConverter::getClangTemplateArguments.
+  std::unique_ptr<TemplateInstantiationError> getClangTemplateArguments(
+      const clang::TemplateParameterList *templateParams,
+      ArrayRef<Type> genericArgs,
+      SmallVectorImpl<clang::TemplateArgument> &templateArgs);
+
   /// Get the Swift declaration that a Clang declaration was exported from,
   /// if applicable.
   const Decl *getSwiftDeclForExportedClangDecl(const clang::Decl *decl);

include/swift/AST/ClangModuleLoader.h

@@ -14,7 +14,9 @@
 #define SWIFT_AST_CLANG_MODULE_LOADER_H
 
 #include "swift/AST/ModuleLoader.h"
+#include "swift/AST/SubstitutionMap.h"
 #include "swift/Basic/TaggedUnion.h"
+#include "clang/AST/DeclTemplate.h"
 
 namespace clang {
 class ASTContext;
@@ -219,6 +221,18 @@ class ClangModuleLoader : public ModuleLoader {
   /// based on subtleties like the target module interface format.
   virtual bool isSerializable(const clang::Type *type,
                               bool checkCanonical) const = 0;
+
+  virtual clang::FunctionDecl *
+  instantiateCXXFunctionTemplate(ASTContext &ctx,
+                                 clang::FunctionTemplateDecl *func,
+                                 SubstitutionMap subst) = 0;
+};
+
+/// Used to describe a template instantiation error.
+struct TemplateInstantiationError {
+  /// Generic types that could not be converted to QualTypes using the
+  /// ClangTypeConverter.
+  SmallVector<Type, 4> failedTypes;
 };
 
 } // namespace swift

include/swift/AST/Decl.h

@@ -5960,9 +5960,10 @@ class FuncDecl : public AbstractFunctionDecl {
                                   DeclContext *Parent);
 
   static FuncDecl *createImported(ASTContext &Context, SourceLoc FuncLoc,
-                                  DeclName Name, SourceLoc NameLoc,
-                                  bool Async, bool Throws,
-                                  ParameterList *BodyParams, Type FnRetType,
+                                  DeclName Name, SourceLoc NameLoc, bool Async,
+                                  bool Throws, ParameterList *BodyParams,
+                                  Type FnRetType,
+                                  GenericParamList *GenericParams,
                                   DeclContext *Parent, ClangNode ClangN);
 
   bool isStatic() const;

include/swift/AST/DiagnosticsSema.def

@@ -1674,6 +1674,11 @@ ERROR(where_nongeneric_ctx,none,
 ERROR(where_nongeneric_toplevel,none,
       "'where' clause cannot be applied to a non-generic top-level "
       "declaration", ())
+ERROR(unable_to_convert_generic_swift_types,none,
+      "could not generate C++ types from the generic Swift types provided. "
+      "The following Swift type(s) provided to '%0' were unable to be "
+      "converted: %1.",
+      (StringRef, StringRef))
 
 // Type aliases
 ERROR(type_alias_underlying_type_access,none,

include/swift/ClangImporter/ClangImporter.h

@@ -472,6 +472,11 @@ class ClangImporter final : public ClangModuleLoader {
 
   bool isSerializable(const clang::Type *type,
                       bool checkCanonical) const override;
+
+  clang::FunctionDecl *
+  instantiateCXXFunctionTemplate(ASTContext &ctx,
+                                 clang::FunctionTemplateDecl *func,
+                                 SubstitutionMap subst) override;
 };
 
 ImportDecl *createImportDecl(ASTContext &Ctx, DeclContext *DC, ClangNode ClangN,

lib/AST/ASTContext.cpp

@@ -18,13 +18,13 @@
 #include "ClangTypeConverter.h"
 #include "ForeignRepresentationInfo.h"
 #include "SubstitutionMapStorage.h"
-#include "swift/AST/ForeignAsyncConvention.h"
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/ConcreteDeclRef.h"
 #include "swift/AST/DiagnosticEngine.h"
 #include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/FileUnit.h"
+#include "swift/AST/ForeignAsyncConvention.h"
 #include "swift/AST/ForeignErrorConvention.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/GenericSignature.h"
@@ -41,19 +41,19 @@
 #include "swift/AST/PropertyWrappers.h"
 #include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/RawComment.h"
+#include "swift/AST/SILLayout.h"
 #include "swift/AST/SemanticAttrs.h"
 #include "swift/AST/SourceFile.h"
 #include "swift/AST/SubstitutionMap.h"
-#include "swift/AST/SILLayout.h"
 #include "swift/AST/TypeCheckRequests.h"
 #include "swift/Basic/Compiler.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
 #include "swift/Basic/StringExtras.h"
-#include "swift/Syntax/References.h"
-#include "swift/Syntax/SyntaxArena.h"
 #include "swift/Strings.h"
 #include "swift/Subsystems.h"
+#include "swift/Syntax/References.h"
+#include "swift/Syntax/SyntaxArena.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringMap.h"
@@ -4563,6 +4563,21 @@ ASTContext::getCanonicalClangFunctionType(
   return ty ? ty->getCanonicalTypeInternal().getTypePtr() : nullptr;
 }
 
+std::unique_ptr<TemplateInstantiationError>
+ASTContext::getClangTemplateArguments(
+    const clang::TemplateParameterList *templateParams,
+    ArrayRef<Type> genericArgs,
+    SmallVectorImpl<clang::TemplateArgument> &templateArgs) {
+  auto &impl = getImpl();
+  if (!impl.Converter) {
+    auto *cml = getClangModuleLoader();
+    impl.Converter.emplace(*this, cml->getClangASTContext(), LangOpts.Target);
+  }
+
+  return impl.Converter->getClangTemplateArguments(templateParams, genericArgs,
+                                                   templateArgs);
+}
+
 const Decl *
 ASTContext::getSwiftDeclForExportedClangDecl(const clang::Decl *decl) {
   auto &impl = getImpl();

lib/AST/ClangTypeConverter.cpp

@@ -807,3 +807,33 @@ Decl *ClangTypeConverter::getSwiftDeclForExportedClangDecl(
   auto it = ReversedExportMap.find(decl);
   return (it != ReversedExportMap.end() ? it->second : nullptr);
 }
+
+std::unique_ptr<TemplateInstantiationError>
+ClangTypeConverter::getClangTemplateArguments(
+    const clang::TemplateParameterList *templateParams,
+    ArrayRef<Type> genericArgs,
+    SmallVectorImpl<clang::TemplateArgument> &templateArgs) {
+  // Keep track of the types we failed to convert so we can return a useful
+  // error.
+  SmallVector<Type, 2> failedTypes;
+  for (clang::NamedDecl *param : *templateParams) {
+    // Note: all template parameters must be template type parameters. This is
+    // verified when we import the clang decl.
+    auto templateParam = cast<clang::TemplateTypeParmDecl>(param);
+    auto replacement = genericArgs[templateParam->getIndex()];
+    auto qualType = convert(replacement);
+    if (qualType.isNull()) {
+      failedTypes.push_back(replacement);
+      // Find all the types we can't convert.
+      continue;
+    }
+    templateArgs.push_back(clang::TemplateArgument(qualType));
+  }
+  if (failedTypes.empty())
+    return nullptr;
+  auto errorInfo = std::make_unique<TemplateInstantiationError>();
+  llvm::for_each(failedTypes, [&errorInfo](auto type) {
+    errorInfo->failedTypes.push_back(type);
+  });
+  return errorInfo;
+}

lib/AST/ClangTypeConverter.h

@@ -84,8 +84,18 @@ class ClangTypeConverter :
   /// Swift declaration.
   Decl *getSwiftDeclForExportedClangDecl(const clang::Decl *decl) const;
 
+  /// Translate Swift generic arguments to template arguments.
+  ///
+  /// \returns nullptr if successful. If an error occors, returns a unique_ptr
+  /// to a `TemplateInstantiationError` with a list of the failed types.
+  std::unique_ptr<TemplateInstantiationError> getClangTemplateArguments(
+      const clang::TemplateParameterList *templateParams,
+      ArrayRef<Type> genericArgs,
+      SmallVectorImpl<clang::TemplateArgument> &templateArgs);
+
 private:
   clang::QualType convert(Type type);
+
   clang::QualType convertMemberType(NominalTypeDecl *DC,
                                     StringRef memberName);
 

lib/AST/Decl.cpp

@@ -7227,16 +7227,15 @@ FuncDecl *FuncDecl::createImplicit(ASTContext &Context,
 }
 
 FuncDecl *FuncDecl::createImported(ASTContext &Context, SourceLoc FuncLoc,
-                                   DeclName Name, SourceLoc NameLoc,
-                                   bool Async, bool Throws,
-                                   ParameterList *BodyParams,
-                                   Type FnRetType, DeclContext *Parent,
-                                   ClangNode ClangN) {
+                                   DeclName Name, SourceLoc NameLoc, bool Async,
+                                   bool Throws, ParameterList *BodyParams,
+                                   Type FnRetType,
+                                   GenericParamList *GenericParams,
+                                   DeclContext *Parent, ClangNode ClangN) {
   assert(ClangN && FnRetType);
   auto *const FD = FuncDecl::createImpl(
       Context, SourceLoc(), StaticSpellingKind::None, FuncLoc, Name, NameLoc,
-      Async, SourceLoc(), Throws, SourceLoc(),
-      /*GenericParams=*/nullptr, Parent, ClangN);
+      Async, SourceLoc(), Throws, SourceLoc(), GenericParams, Parent, ClangN);
   FD->setParameters(BodyParams);
   FD->setResultInterfaceType(FnRetType);
   return FD;

lib/ClangImporter/ClangImporter.cpp

@@ -21,8 +21,9 @@
 #include "swift/AST/ClangModuleLoader.h"
 #include "swift/AST/DiagnosticEngine.h"
 #include "swift/AST/DiagnosticsClangImporter.h"
-#include "swift/AST/ImportCache.h"
+#include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/IRGenOptions.h"
+#include "swift/AST/ImportCache.h"
 #include "swift/AST/LinkLibrary.h"
 #include "swift/AST/Module.h"
 #include "swift/AST/NameLookup.h"
@@ -35,8 +36,6 @@
 #include "swift/ClangImporter/ClangModule.h"
 #include "swift/Config.h"
 #include "swift/Demangling/Demangle.h"
-#include "swift/ClangImporter/ClangModule.h"
-#include "swift/Config.h"
 #include "swift/Parse/Lexer.h"
 #include "swift/Parse/Parser.h"
 #include "swift/Strings.h"
@@ -4082,3 +4081,32 @@ swift::getModuleCachePathFromClang(const clang::CompilerInstance &Clang) {
   return llvm::sys::path::parent_path(SpecificModuleCachePath).str();
 }
 
+clang::FunctionDecl *ClangImporter::instantiateCXXFunctionTemplate(
+    ASTContext &ctx, clang::FunctionTemplateDecl *func, SubstitutionMap subst) {
+  SmallVector<clang::TemplateArgument, 4> templateSubst;
+  std::unique_ptr<TemplateInstantiationError> error =
+      ctx.getClangTemplateArguments(func->getTemplateParameters(),
+                                    subst.getReplacementTypes(), templateSubst);
+  if (error) {
+    std::string failedTypesStr;
+    llvm::raw_string_ostream failedTypesStrStream(failedTypesStr);
+    llvm::interleaveComma(error->failedTypes, failedTypesStrStream);
+    // TODO: Use the location of the apply here.
+    // TODO: This error message should not reference implementation details.
+    // See: https://github.com/apple/swift/pull/33053#discussion_r477003350
+    ctx.Diags.diagnose(SourceLoc(),
+                       diag::unable_to_convert_generic_swift_types.ID,
+                       {func->getName(), StringRef(failedTypesStr)});
+    // Return a valid FunctionDecl but, we'll never use it.
+    return func->getAsFunction();
+  }
+
+  // Instanciate a specialization of this template using the substitution map.
+  auto *templateArgList = clang::TemplateArgumentList::CreateCopy(
+      func->getASTContext(), templateSubst);
+  auto &sema = getClangInstance().getSema();
+  auto *spec = sema.InstantiateFunctionDeclaration(func, templateArgList,
+                                                   clang::SourceLocation());
+  sema.InstantiateFunctionDefinition(clang::SourceLocation(), spec);
+  return spec;
+}