Michaelrfairhurst/package deadcode 9 rule 0-2-3

c/common/test/rules/unusedtypedeclarations/UnusedTypeDeclarations.expected

@@ -2,3 +2,4 @@
 | test.c:7:18:7:18 | D | Type declaration D is not used. |
 | test.c:28:11:28:11 | R | Type declaration R is not used. |
 | test.c:41:12:41:12 | (unnamed class/struct/union) | Type declaration (unnamed class/struct/union) is not used. |
+| test.c:56:3:56:4 | T2 | Type declaration T2 is not used. |

c/common/test/rules/unusedtypedeclarations/test.c

@@ -49,4 +49,13 @@ void test_nested_struct() {
   s.f1;
   s.f3;
   s.f5;
-}
+}
+
+typedef struct {
+  int m1;
+} T2; // NON_COMPLIANT
+typedef struct {
+  int m1;
+} T1; // COMPLIANT - used in function below
+
+void test_typedef() { T1 t1; }

change_notes/2026-02-18-unused-type-improvements.md

@@ -0,0 +1,7 @@
+ - `RULE-2-3`, `A0-1-6` - `UnusedTypeDeclarations.ql`:
+   - Type usage analysis has been improved to find more possible type usages, including:
+     - Previous behavior considered anonymous types in variable declarations to be considered used by the variable definition itself. This has been improved to require that a field of the anonymous type is accessed for the type to be considered used.
+     - Usages of a template type inside a specialization of that template are no longer considered usages of the template type.
+     - Hidden friend declarations are no longer considered usages of the class they are declaring friendship for.
+     - Improved exclusions generally, for cases such as nested types and functions within functions. These previously were a source of incorrectly identified type uses.
+     - Additional case added to detect `template <Enum = Enum::Value>` as a usage of `Enum`, without an explicit `tpl<Enum::Value>` usage.

cpp/common/src/codingstandards/cpp/ast/Class.qll

@@ -0,0 +1,28 @@
+import cpp
+
+/**
+ * The last declaration in a class by location order.
+ *
+ * This may fail to capture certain cases such as hidden friends (see HiddenFriend.qll).
+ */
+class LastClassDeclaration extends Declaration {
+  Class cls;
+
+  pragma[nomagic]
+  LastClassDeclaration() {
+    cls.getADeclaration() = this and
+    getLocation().getEndLine() = getLastLineOfClassDeclaration(cls)
+  }
+}
+
+/**
+ * Gets the line number of the last line of the declaration of `cls`.
+ *
+ * This is often more performant to use than `LastClassDeclaration.getLocation().getEndLine()`.
+ */
+int getLastLineOfClassDeclaration(Class cls) {
+  result =
+    max(int endLine |
+      endLine = pragma[only_bind_out](cls).getADeclaration().getLocation().getEndLine()
+    )
+}

cpp/common/src/codingstandards/cpp/ast/HiddenFriend.qll

@@ -0,0 +1,165 @@
+/**
+ * The C++ extractor does not support hidden friends, which are friend functions defined within a
+ * class, rather than declared:
+ *
+ * ```cpp
+ * struct A {
+ *   friend void hidden_friend(A) {} // Definition: this is a hidden friend
+ *   friend void not_hidden_friend(A); // Declaration: this is not a hidden friend
+ * };
+ * ```
+ *
+ * In the database, a `FriendDecl` is not created for the hidden friend. The hidden friend function
+ * is created as a `TopLevel` function with no enclosing element. However, we can identify it as a
+ * hidden friend by its location.
+ */
+
+import cpp
+import codingstandards.cpp.ast.Class
+
+/**
+ * A class that, by our best logic, appears to possibly be a hidden friend.
+ *
+ * Hidden friends are not directly represented in the database. Instances of this class have been
+ * found to have a location "within" the "body" of a class, and to satisfy other criteria that
+ * indicates it may be a hidden friend.
+ */
+class PossibleHiddenFriend extends HiddenFriendCandidate {
+  ClassCandidate cls;
+
+  PossibleHiddenFriend() { hidesFriend(cls, this) }
+
+  Class getFriendClass() { result = cls }
+}
+
+/**
+ * Begin by limiting the number of candidate functions to consider.
+ *
+ * Only inline top level functions can be hidden friends.
+ */
+private class HiddenFriendCandidate extends TopLevelFunction {
+  HiddenFriendCandidate() { this.isInline() }
+}
+
+/**
+ * Only consider files which contain hidden friend candidates.
+ */
+private class FileCandidate extends File {
+  FileCandidate() { exists(HiddenFriendCandidate c | c.getFile() = this) }
+}
+
+/**
+ * Only consider classes in candidate files, that include hidden friend candidates.
+ */
+private class ClassCandidate extends Class {
+  ClassCandidate() { getFile() instanceof FileCandidate }
+
+  /**
+   * Find the next declaration after this class that shares an enclosing element.
+   *
+   * This may be the next declaration after this class, or `getNextOrphanedDeclaration` may find the
+   * true next declaration after this class. These are split for performance reasons.
+   */
+  Declaration getNextSiblingDeclaration() {
+    result =
+      min(Declaration decl |
+        decl.getEnclosingElement() = this.getEnclosingElement() and
+        pragma[only_bind_out](decl.getFile()) = pragma[only_bind_out](this.getFile()) and
+        decl.getLocation().getStartLine() > getLastLineOfClassDeclaration(this)
+      |
+        decl order by decl.getLocation().getStartLine(), decl.getLocation().getStartColumn()
+      )
+  }
+
+  /**
+   * Get the next declaration after this class that does not have an enclosing element.
+   *
+   * This may be the next declaration after this class, or `getNextSiblingDeclaration` may find the
+   * true next declaration after this class. These are split for performance reasons.
+   *
+   * Note that `OrphanedDeclaration` excludes hidden friend candidates, so this will find the next
+   * orphan that is definitely not a hidden friend.
+   */
+  Declaration getNextOrphanedDeclaration() {
+    result =
+      min(OrphanedDeclaration decl, int startLine, int startColumn |
+        orphanHasLocation(decl, this.getFile(), startLine, startColumn) and
+        startLine > getLastLineOfClassDeclaration(this)
+      |
+        decl order by startLine, startColumn
+      )
+  }
+
+  /**
+   * Get the first declaration definitely after this class, and not a hidden friend declaration, to
+   * determine the "end" location of this class.
+   */
+  Declaration getFirstNonClassDeclaration() {
+    result =
+      min(Declaration decl |
+        decl = getNextSiblingDeclaration() or decl = getNextOrphanedDeclaration()
+      |
+        decl order by decl.getLocation().getStartLine(), decl.getLocation().getStartColumn()
+      )
+  }
+}
+
+/**
+ * Helper predicate to improve join performance.
+ */
+pragma[nomagic]
+private predicate orphanHasLocation(
+  OrphanedDeclaration orphan, FileCandidate file, int startLine, int startColumn
+) {
+  orphan.getFile() = file and
+  orphan.getLocation().getEndLine() = startLine and
+  orphan.getLocation().getEndColumn() = startColumn
+}
+
+/**
+ * Orphaned declarations to be found by `getNextOrphanedDeclaration`.
+ *
+ * These are declarations with no enclosing element. Note that we exclude hidden friend candidates,
+ * as this class is used to find the declarations that are definitely not part of some class. This
+ * is done so we can detect if hidden friends may be within that class definition. Therefore we must
+ * exclude hidden friend candidates, even though those are also orphaned.
+ */
+private class OrphanedDeclaration extends Declaration {
+  OrphanedDeclaration() {
+    not exists(getEnclosingElement()) and
+    not this instanceof HiddenFriendCandidate and
+    getFile() instanceof FileCandidate and
+    not isFromTemplateInstantiation(_)
+  }
+}
+
+/**
+ * Helper predicate to improve join performance.
+ */
+pragma[nomagic]
+private predicate classCandidateHasFile(ClassCandidate c, FileCandidate f) { c.getFile() = f }
+
+/**
+ * Helper predicate to improve join performance.
+ */
+pragma[nomagic]
+private predicate hiddenFriendCandidateHasFile(HiddenFriendCandidate h, FileCandidate f) {
+  h.getFile() = f
+}
+
+/**
+ * Find the class locations that have declarations that could be hidden friend declarations, by
+ * comparing the locations of the candidate hidden friend functions to the location of the first
+ * declaration that clearly is outside that class.
+ */
+pragma[nomagic]
+private predicate hidesFriend(ClassCandidate c, HiddenFriendCandidate f) {
+  exists(FileCandidate file, Location cloc, Location floc |
+    classCandidateHasFile(c, file) and
+    hiddenFriendCandidateHasFile(f, file) and
+    cloc = c.getLocation() and
+    floc = f.getLocation() and
+    cloc.getEndLine() < floc.getStartLine() and
+    floc.getEndLine() < c.getFirstNonClassDeclaration().getLocation().getStartLine()
+  )
+}

cpp/common/src/codingstandards/cpp/exclusions/cpp/DeadCode9.qll

@@ -0,0 +1,26 @@
+//** THIS FILE IS AUTOGENERATED, DO NOT MODIFY DIRECTLY.  **/
+import cpp
+import RuleMetadata
+import codingstandards.cpp.exclusions.RuleMetadata
+
+newtype DeadCode9Query = TUnusedTypeWithLimitedVisibilityQuery()
+
+predicate isDeadCode9QueryMetadata(Query query, string queryId, string ruleId, string category) {
+  query =
+    // `Query` instance for the `unusedTypeWithLimitedVisibility` query
+    DeadCode9Package::unusedTypeWithLimitedVisibilityQuery() and
+  queryId =
+    // `@id` for the `unusedTypeWithLimitedVisibility` query
+    "cpp/misra/unused-type-with-limited-visibility" and
+  ruleId = "RULE-0-2-3" and
+  category = "advisory"
+}
+
+module DeadCode9Package {
+  Query unusedTypeWithLimitedVisibilityQuery() {
+    //autogenerate `Query` type
+    result =
+      // `Query` type for `unusedTypeWithLimitedVisibility` query
+      TQueryCPP(TDeadCode9PackageQuery(TUnusedTypeWithLimitedVisibilityQuery()))
+  }
+}

cpp/common/src/codingstandards/cpp/exclusions/cpp/RuleMetadata.qll

@@ -23,6 +23,7 @@ import DeadCode4
 import DeadCode5
 import DeadCode6
 import DeadCode7
+import DeadCode9
 import Declarations
 import ExceptionSafety
 import Exceptions1
@@ -101,6 +102,7 @@ newtype TCPPQuery =
   TDeadCode5PackageQuery(DeadCode5Query q) or
   TDeadCode6PackageQuery(DeadCode6Query q) or
   TDeadCode7PackageQuery(DeadCode7Query q) or
+  TDeadCode9PackageQuery(DeadCode9Query q) or
   TDeclarationsPackageQuery(DeclarationsQuery q) or
   TExceptionSafetyPackageQuery(ExceptionSafetyQuery q) or
   TExceptions1PackageQuery(Exceptions1Query q) or
@@ -179,6 +181,7 @@ predicate isQueryMetadata(Query query, string queryId, string ruleId, string cat
   isDeadCode5QueryMetadata(query, queryId, ruleId, category) or
   isDeadCode6QueryMetadata(query, queryId, ruleId, category) or
   isDeadCode7QueryMetadata(query, queryId, ruleId, category) or
+  isDeadCode9QueryMetadata(query, queryId, ruleId, category) or
   isDeclarationsQueryMetadata(query, queryId, ruleId, category) or
   isExceptionSafetyQueryMetadata(query, queryId, ruleId, category) or
   isExceptions1QueryMetadata(query, queryId, ruleId, category) or

cpp/common/src/codingstandards/cpp/types/Uses.qll

@@ -12,6 +12,7 @@
  */
 
 import cpp
+import codingstandards.cpp.ast.HiddenFriend
 
 /**
  * Gets a typedef with the same qualified name and declared at the same location.
@@ -35,7 +36,8 @@ private TypedefType getAnEquivalentTypeDef(TypedefType type) {
  * is from within the function signature or field declaration of the type itself.
  */
 Locatable getATypeUse(Type type) {
-  result = getATypeUse_i(type, _)
+  result = getATypeUse_i(type, _) and
+  not isWithinTypeDefinition(result, type)
   or
   // Identify `TypeMention`s of typedef types, where the underlying type is used.
   //
@@ -65,6 +67,30 @@ Locatable getATypeUse(Type type) {
   )
 }
 
+predicate isWithinTypeDefinition(Locatable loc, Type type) {
+  loc = type
+  or
+  loc.getEnclosingElement*() = type
+  or
+  isWithinTypeDefinition(loc.(Function).getDeclaringType(), type)
+  or
+  isWithinTypeDefinition(loc.(MemberVariable).getDeclaringType(), type)
+  or
+  isWithinTypeDefinition(loc.(PossibleHiddenFriend).getFriendClass(), type)
+  or
+  isWithinTypeDefinition(loc.(Parameter).getFunction(), type)
+  or
+  isWithinTypeDefinition(loc.(Expr).getEnclosingFunction(), type)
+  or
+  isWithinTypeDefinition(loc.(LocalVariable).getFunction(), type)
+  or
+  exists(TemplateClass tpl, ClassTemplateSpecialization spec |
+    tpl = type and
+    tpl = spec.getPrimaryTemplate() and
+    isWithinTypeDefinition(loc, spec)
+  )
+}
+
 private Locatable getATypeUse_i(Type type, string reason) {
   (
     // Restrict to uses within the source checkout root
@@ -81,6 +107,7 @@ private Locatable getATypeUse_i(Type type, string reason) {
       type = v.getType() and
       // Ignore self referential variables and parameters
       not v.getDeclaringType().refersTo(type) and
+      not type.(UserType).isAnonymous() and
       not type = v.(Parameter).getFunction().getDeclaringType()
     ) and
     reason = "used as a variable type"
@@ -145,6 +172,10 @@ private Locatable getATypeUse_i(Type type, string reason) {
     // Temporary object creation of type `type`
     exists(TemporaryObjectExpr toe | result = toe | type = toe.getType()) and
     reason = "used in temporary object expr"
+    or
+    // template<Type t> ...
+    exists(Declaration decl | result = decl | type = decl.getATemplateArgumentKind()) and
+    reason = "used as a non-type template parameter"
   )
   or
   // Recursive case - used by a used type

cpp/common/test/rules/unusedtypedeclarations/UnusedTypeDeclarations.expected

@@ -4,3 +4,7 @@
 | test.cpp:77:11:77:11 | R | Type declaration R is not used. |
 | test.cpp:90:12:90:12 | (unnamed class/struct/union) | Type declaration (unnamed class/struct/union) is not used. |
 | test.cpp:111:29:111:30 | AA | Type declaration AA is not used. |
+| test.cpp:126:7:126:12 | Nested | Type declaration Nested is not used. |
+| test.cpp:135:9:135:20 | UnusedNested | Type declaration UnusedNested is not used. |
+| test.cpp:138:7:138:22 | NestedBlockScope | Type declaration NestedBlockScope is not used. |
+| test.cpp:149:11:149:16 | Unused | Type declaration Unused is not used. |

cpp/common/test/rules/unusedtypedeclarations/test.cpp

@@ -121,4 +121,31 @@ void test_temporary_object_creation() {
 
     return C1{p1};
   };
-}
+}
+
+class Nested { // NON_COMPLIANT - only used within itself
+public:
+  class NestedNested { // COMPLIANT - used by class `Nested`
+  public:
+    Nested f();
+  };
+
+  NestedNested g();
+
+  class UnusedNested {}; // NON_COMPLIANT - never used by class `Nested`
+};
+
+class NestedBlockScope { // NON_COMPLIANT - only used within itself
+public:
+  void f() {
+    class NestedFunction { // COMPLIANT - used in function below
+      void g() {
+        NestedBlockScope l1; // Doesn't count as use of `NestedBlockScope`.
+      }
+    };
+
+    NestedFunction l1;
+
+    class Unused {}; // NON_COMPLIANT - never used by function `f`
+  }
+};