[cxx-interop] Support generic types with isValueType trait

In some circumstances the missing trait resulted in picking the wrong
branch of some compile time conditionals resulting in code that would
not compile.

rdar://126709253

Author: Gabor Horvath

lib/PrintAsClang/PrintClangValueType.cpp

@@ -675,14 +675,14 @@ void ClangValueTypePrinter::printTypeGenericTraits(
     os << "> = true;\n";
   }
 
-  if (!isa<ClassDecl>(typeDecl) && !typeDecl->hasClangNode() &&
-      typeMetadataFuncRequirements.empty()) {
-    // FIXME: generic support.
-    os << "template<>\n";
+  if (!isa<ClassDecl>(typeDecl) && !typeDecl->hasClangNode()) {
+    assert(NTD && "not a nominal type?");
+    if (printer.printNominalTypeOutsideMemberDeclTemplateSpecifiers(NTD))
+      os << "template<>\n";
     os << "static inline const constexpr bool isValueType<";
     printer.printBaseName(typeDecl->getModuleContext());
     os << "::";
-    printer.printBaseName(typeDecl);
+    printer.printNominalTypeReference(NTD, moduleContext);
     os << "> = true;\n";
   }
   if (isOpaqueLayout) {

test/Interop/SwiftToCxx/generics/generic-struct-as-generic-func-arg-in-cpp.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -typecheck %t/generics.swift -typecheck -module-name Generics -enable-experimental-cxx-interop -emit-clang-header-path %t/Generics-Swift.h
+
+// RUN: %target-interop-build-clangxx -fno-exceptions -std=gnu++20 -c %t/generics.cpp -I %t -o %t/generics.o
+// RUN: %target-build-swift %t/generics.swift -o %t/generics -Xlinker %t/generics.o -module-name Generics -Xfrontend -entry-point-function-name -Xfrontend swiftMain
+
+//--- generics.swift
+
+public struct MyClass<T> {
+    public var a: T
+    public init(_ p: T) { self.a = p }
+}
+
+public func genericFunc<T>(_ p: T) -> T {
+    return p
+}
+
+//--- generics.cpp
+
+#include "Generics-Swift.h"
+using namespace Generics;
+
+int main() {
+  auto c = MyClass<int>::init(10);
+  auto result = genericFunc<MyClass<int>>(c);
+}

test/Interop/SwiftToCxx/generics/generic-struct-in-cxx.swift

@@ -284,6 +284,11 @@ public func inoutConcretePair(_ x: UInt16, _ y: inout GenericPair<UInt16, UInt16
 // CHECK-NEXT: #ifdef __cpp_concepts
 // CHECK-NEXT: requires swift::isUsableInGenericContext<T_0_0> && swift::isUsableInGenericContext<T_0_1>
 // CHECK-NEXT: #endif // __cpp_concepts
+// CHECK-NEXT: static inline const constexpr bool isValueType<Generics::GenericPair<T_0_0, T_0_1>> = true;
+// CHECK-NEXT: template<class T_0_0, class T_0_1>
+// CHECK-NEXT: #ifdef __cpp_concepts
+// CHECK-NEXT: requires swift::isUsableInGenericContext<T_0_0> && swift::isUsableInGenericContext<T_0_1>
+// CHECK-NEXT: #endif // __cpp_concepts
 // CHECK-NEXT: static inline const constexpr bool isOpaqueLayout<Generics::GenericPair<T_0_0, T_0_1>> = true;
 // CHECK-NEXT: template<class T_0_0, class T_0_1>
 // CHECK-NEXT: #ifdef __cpp_concepts

test/Interop/SwiftToCxx/stdlib/swift-stdlib-in-cxx.swift

@@ -49,6 +49,7 @@
 // CHECK: template<class T_0_0>
 // CHECK: template<class T_0_0>
 // CHECK: template<class T_0_0>
+// CHECK: template<class T_0_0>
 // CHECK-NEXT: #ifdef __cpp_concepts
 // CHECK-NEXT: requires swift::isUsableInGenericContext<T_0_0>
 // CHECK-NEXT: #endif