[cxx-interop] Move incomplete template specialization check

lib/ClangImporter/ImportDecl.cpp

@@ -1501,23 +1501,6 @@ namespace {
         // or the original C type.
         clang::QualType ClangType = Decl->getUnderlyingType();
 
-        // Prevent import of typedefs to forward-declared explicit template
-        // specializations, which would trigger assertion in Clang.
-        if (auto *templateSpec = dyn_cast<clang::TemplateSpecializationType>(
-                importer::desugarIfElaborated(ClangType).getTypePtr())) {
-          if (auto *recordType =
-                  templateSpec->desugar()->getAs<clang::RecordType>()) {
-            if (auto *spec = dyn_cast<clang::ClassTemplateSpecializationDecl>(
-                    recordType->getDecl())) {
-              if (spec->getSpecializationKind() ==
-                      clang::TSK_ExplicitSpecialization &&
-                  !spec->isCompleteDefinition()) {
-                return nullptr;
-              }
-            }
-          }
-        }
-
         SwiftType = Impl.importTypeIgnoreIUO(
             ClangType, ImportTypeKind::Typedef,
             ImportDiagnosticAdder(Impl, Decl, Decl->getLocation()),
@@ -3346,6 +3329,15 @@ namespace {
             decl->getName() == "_Expr" || decl->getName() == "__val_expr"))
         return nullptr;
 
+      // Don't even try to specialize/import this template if it's
+      // a forward-declared specialization like this:
+      //
+      //    template <> struct MyTemplate<int>;
+      //
+      if (decl->getSpecializationKind() == clang::TSK_ExplicitSpecialization &&
+          !decl->isCompleteDefinition())
+        return nullptr;
+
       // `decl->getDefinition()` can return nullptr before the call to sema and
       // return its definition afterwards.
       clang::Sema &clangSema = Impl.getClangSema();

test/Interop/Cxx/templates/Inputs/ForwardDeclaredSpecialization.h

@@ -60,4 +60,11 @@ struct PartialTemplate<T*, double> {
 };
 typedef PartialTemplate<int*, double> CompletePartial;
 
+// Some functions that use forward-declared specializations
+void TakesIncompleteSpecialization(BasicTemplate<int>);
+BasicTemplate<int> ReturnsIncompleteSpecialization();
+
+void TakesPtrToIncompleteSpecialization(BasicTemplate<int> *);
+BasicTemplate<int> *ReturnsPtrToIncompleteSpecialization();
+
 #endif

test/Interop/Cxx/templates/forward-declared-specialization.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=default
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=default -suppress-remarks -suppress-notes
 
 import ForwardDeclaredSpecialization
 
@@ -30,3 +30,17 @@ func testCompletePartial(_ param: CompletePartial) {
   let _ = param.ptr
   let _ = param.value
 }
+
+func testFunctionsUsingIncompleteSpec() {
+  let inc = ReturnsIncompleteSpecialization()
+  // expected-error@-1 {{return type is unavailable in Swift}}
+  // expected-warning@-2 {{constant 'inc' inferred to have type 'Never', which is an enum with no cases}}
+  TakesIncompleteSpecialization(inc)
+  // expected-error@-1 {{cannot find 'TakesIncompleteSpecialization' in scope}}
+}
+
+func testFunctionsUsingPtrToIncompleteSpec(_ ptr: OpaquePointer) {
+  let incPtr = ReturnsPtrToIncompleteSpecialization()
+  TakesPtrToIncompleteSpecialization(incPtr)
+  TakesPtrToIncompleteSpecialization(ptr)
+}