C++ Interop: Add Core.CppCompat.Long32 as a distinct type for Cpp.long when long is 32 bits (#6364)

For now, only support implicit conversions from and to `i32`.

See #6275 for rationale.

Part of #5263.

toolchain/check/cpp/import.cpp

@@ -1112,6 +1112,11 @@ static auto MapBuiltinIntegerType(Context& context, SemIR::LocId loc_id,
     return ExprAsType(context, Parse::NodeId::None,
                       MakeCharTypeLiteral(context, Parse::NodeId::None));
   }
+  if (ast_context.hasSameType(qual_type, ast_context.LongTy) && width == 32) {
+    return ExprAsType(context, Parse::NodeId::None,
+                      LookupNameInCore(context, Parse::NodeId::None,
+                                       {"CppCompat", "Long32"}));
+  }
   return TypeExpr::None;
 }
 

toolchain/check/cpp/type_mapping.cpp

@@ -104,6 +104,8 @@ static auto LookupCppType(
 // type is not supported.
 static auto TryMapClassType(Context& context, SemIR::ClassType class_type)
     -> clang::QualType {
+  clang::ASTContext& ast_context = context.ast_context();
+
   // If the class was imported from C++, return the original C++ type.
   auto clang_decl_id =
       context.name_scopes()
@@ -112,7 +114,7 @@ static auto TryMapClassType(Context& context, SemIR::ClassType class_type)
   if (clang_decl_id.has_value()) {
     clang::Decl* clang_decl = context.clang_decls().Get(clang_decl_id).key.decl;
     auto* tag_type_decl = clang::cast<clang::TagDecl>(clang_decl);
-    return context.ast_context().getCanonicalTagType(tag_type_decl);
+    return ast_context.getCanonicalTagType(tag_type_decl);
   }
 
   // If the class represents a Carbon type literal, map it to the corresponding
@@ -135,22 +137,28 @@ static auto TryMapClassType(Context& context, SemIR::ClassType class_type)
           CARBON_FATAL("Unexpected invalid numeric type literal");
         }
         case SemIR::NumericTypeLiteralInfo::Float: {
-          return context.ast_context().getRealTypeForBitwidth(
+          return ast_context.getRealTypeForBitwidth(
               bit_width, clang::FloatModeKind::NoFloat);
         }
         case SemIR::NumericTypeLiteralInfo::Int: {
-          return context.ast_context().getIntTypeForBitwidth(bit_width, true);
+          return ast_context.getIntTypeForBitwidth(bit_width, true);
         }
         case SemIR::NumericTypeLiteralInfo::UInt: {
-          return context.ast_context().getIntTypeForBitwidth(bit_width, false);
+          return ast_context.getIntTypeForBitwidth(bit_width, false);
         }
       }
     }
     case SemIR::TypeLiteralInfo::Char: {
-      return context.ast_context().CharTy;
+      return ast_context.CharTy;
+    }
+    case SemIR::TypeLiteralInfo::CppLong32: {
+      if (ast_context.getIntWidth(ast_context.LongTy) == 32) {
+        return ast_context.LongTy;
+      }
+      break;
     }
     case SemIR::TypeLiteralInfo::CppNullptrT: {
-      return context.ast_context().NullPtrTy;
+      return ast_context.NullPtrTy;
     }
     case SemIR::TypeLiteralInfo::Str: {
       return LookupCppType(context, {"std", "string_view"});