[6.2][cxx-interop] Only swiftify template instantiations behind type aliases

Explanation: C++ template instantiations that are not behind type
aliases don't have corresponding Swift names that are both syntactically
and semantically valid types. This PR prevents generating swiftified
overloads for those types.
Issue: rdar://151422108
Risk: Low, we swiftify functions less often.
Testing: Regression test added.
Original PR: #81973
Reviewer: @hnrklssn

Author: Xazax-hun

lib/ClangImporter/ImportDecl.cpp

@@ -62,8 +62,9 @@
 #include "clang/AST/DeclObjCCommon.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/PrettyPrinter.h"
-#include "clang/AST/StmtVisitor.h"
 #include "clang/AST/RecordLayout.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/AST/StmtVisitor.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Basic/TargetInfo.h"
@@ -9119,6 +9120,24 @@ namespace {
   };
 } // namespace
 
+namespace {
+// Searches for template instantiations that are not behind type aliases.
+// FIXME: make sure the generated code compiles for template
+// instantiations that are not behind type aliases.
+struct UnaliasedInstantiationVisitor
+    : clang::RecursiveASTVisitor<UnaliasedInstantiationVisitor> {
+  bool hasUnaliasedInstantiation = false;
+
+  bool TraverseTypedefType(const clang::TypedefType *) { return true; }
+
+  bool
+  VisitTemplateSpecializationType(const clang::TemplateSpecializationType *) {
+    hasUnaliasedInstantiation = true;
+    return false;
+  }
+};
+} // namespace
+
 // Don't try to transform any Swift types that _SwiftifyImport doesn't know how
 // to handle.
 static bool SwiftifiableCountedByPointerType(Type swiftType) {
@@ -9173,6 +9192,13 @@ void ClangImporter::Implementation::swiftify(AbstractFunctionDecl *MappedDecl) {
   if (ClangDecl->getNumParams() != MappedDecl->getParameters()->size())
     return;
 
+  {
+    UnaliasedInstantiationVisitor visitor;
+    visitor.TraverseType(ClangDecl->getType());
+    if (visitor.hasUnaliasedInstantiation)
+      return;
+  }
+
   llvm::SmallString<128> MacroString;
   // We only attach the macro if it will produce an overload. Any __counted_by
   // will produce an overload, since UnsafeBufferPointer is still an improvement

test/Interop/Cxx/stdlib/Inputs/std-span.h

@@ -165,12 +165,20 @@ inline SpanOfInt MixedFuncWithMutableSafeWrapper7(int * __counted_by(len) p, int
   return SpanOfInt(p, len);
 }
 
+template <typename X>
+struct S {};
+
 struct SpanWithoutTypeAlias {
   std::span<const int> bar() [[clang::lifetimebound]];
   void foo(std::span<const int> s [[clang::noescape]]);
+  void otherTemplatedType(ConstSpanOfInt copy [[clang::noescape]], S<int>);
+  void otherTemplatedType2(ConstSpanOfInt copy [[clang::noescape]], S<int> *);
 };
 
 inline void func(ConstSpanOfInt copy [[clang::noescape]]) {}
 inline void mutableKeyword(SpanOfInt copy [[clang::noescape]]) {}
 
+inline void spanWithoutTypeAlias(std::span<const int> s [[clang::noescape]]) {}
+inline void mutableSpanWithoutTypeAlias(std::span<int> s [[clang::noescape]]) {}
+
 #endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_SPAN_H

test/Interop/Cxx/stdlib/std-span-interface.swift

@@ -3,7 +3,7 @@
 // RUN: %FileCheck %s < %t/interface.swift
 
 // Make sure we trigger typechecking and SIL diagnostics
-// RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -I %S/Inputs -enable-experimental-feature SafeInteropWrappers -enable-experimental-feature Lifetimes -cxx-interoperability-mode=default -strict-memory-safety -warnings-as-errors -Xcc -std=c++20 %s
+// RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -I %S/Inputs -enable-experimental-feature SafeInteropWrappers -enable-experimental-feature Lifetimes -cxx-interoperability-mode=default -strict-memory-safety -warnings-as-errors -verify -Xcc -std=c++20 %s
 
 // REQUIRES: swift_feature_SafeInteropWrappers
 // REQUIRES: swift_feature_Lifetimes
@@ -34,15 +34,10 @@ import CxxStdlib
 // CHECK-NEXT: }
 // CHECK: struct SpanWithoutTypeAlias {
 // CHECK-NEXT:   init()
-// CHECK-NEXT:   /// This is an auto-generated wrapper for safer interop
-// CHECK-NEXT:   @available(visionOS 1.1, tvOS 12.2, watchOS 5.2, iOS 12.2, macOS 10.14.4, *)
-// CHECK-NEXT:   @_lifetime(borrow self)
-// CHECK-NEXT:   @_alwaysEmitIntoClient @_disfavoredOverload public mutating func bar() -> Span<CInt>
 // CHECK-NEXT:   mutating func bar() -> std.{{.*}}span<__cxxConst<CInt>, _C{{.*}}_{{.*}}>
-// CHECK-NEXT:   /// This is an auto-generated wrapper for safer interop
-// CHECK-NEXT:   @available(visionOS 1.1, tvOS 12.2, watchOS 5.2, iOS 12.2, macOS 10.14.4, *)
-// CHECK-NEXT:   @_alwaysEmitIntoClient @_disfavoredOverload public mutating func foo(_ s: Span<CInt>)
 // CHECK-NEXT:   mutating func foo(_ s: std.{{.*}}span<__cxxConst<CInt>, _C{{.*}}_{{.*}}>)
+// CHECK-NEXT:   mutating func otherTemplatedType(_ copy: ConstSpanOfInt, _: S<CInt>)
+// CHECK-NEXT:   mutating func otherTemplatedType2(_ copy: ConstSpanOfInt, _: UnsafeMutablePointer<S<CInt>>!)
 // CHECK-NEXT: }
 
 // CHECK:      /// This is an auto-generated wrapper for safer interop
@@ -150,7 +145,7 @@ func callMethodWithSafeWrapper(_ x: inout X, s: Span<CInt>) {
 }
 
 func callFooBar(_ x: inout SpanWithoutTypeAlias, _ s: ConstSpanOfInt) {
-    let _: Span<CInt> = x.bar()
+    let _: Span<CInt> = x.bar() // expected-error {{cannot convert value of type}}
     unsafe x.foo(s)
 }
 
@@ -242,3 +237,11 @@ func callMixedFuncWithSafeWrapper7(_ p: UnsafeBufferPointer<CInt>) {
 func callMutableKeyword(_ span: inout MutableSpan<CInt>) {
     mutableKeyword(&span)
 }
+
+func callSpanWithoutTypeAlias(_ span: Span<CInt>) {
+  spanWithoutTypeAlias(span) // expected-error {{cannot convert value of type}}
+}
+
+func callMutableSpanWithoutTypeAlias(_ span: consuming MutableSpan<CInt>) {
+  mutableSpanWithoutTypeAlias(&span) // expected-error {{cannot convert value of type}}
+}