diff --git a/ql/ql/src/queries/bugs/IncompleteToString.ql b/ql/ql/src/queries/bugs/IncompleteToString.ql
new file mode 100644
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+++ b/ql/ql/src/queries/bugs/IncompleteToString.ql
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+/**
+ * @name Incomplete tostring
+ * @description A toString on a IPA type does not cover all branches.
+ * @kind problem
+ * @problem.severity warning
+ * @id ql/incomplete-tostring
+ * @tags correctness
+ *       maintainability
+ * @precision very-high
+ */
+
+import ql
+
+// TODO:
+// - Some classes are restircted in their charpred to some branch types.
+//   We should take that into account when computing coverage.
+// - Some classes rely on onther classes for cover. Example is
+//   `ReturnCompletion`:
+//   ```codeql
+// /**
+//  * A completion that represents evaluation of a statement or an
+//  * expression resulting in a return from a callable.
+//  */
+// class ReturnCompletion extends Completion {
+//   ReturnCompletion() {
+//     this = TReturnCompletion() or
+//     this = TNestedCompletion(_, TReturnCompletion(), _)
+//   }
+//   override ReturnSuccessor getAMatchingSuccessorType() { any() }
+//   override string toString() {
+//     // `NestedCompletion` defines `toString()` for the other case
+//     this = TReturnCompletion() and result = "return"
+//   }
+// }
+// ```
+//
+// `getUnionMember` and `Branches` help to recursively unfold
+// definitions of the form
+// `class TMethodBase = TMethod or TSingletonMethod;`
+TypeDeclaration getUnionMember(Class c) {
+  result = c.getUnionMember().getResolvedType().getDeclaration() or
+  result = getUnionMember(c.getUnionMember().getResolvedType().getDeclaration())
+}
+
+class Branches extends TypeDeclaration {
+  NewTypeBranch branch;
+
+  Branches() {
+    branch = this
+    or
+    branch = getUnionMember(this)
+  }
+
+  NewTypeBranch getBranch() { result = branch }
+  // NewType getNewType() { result = branch.getNewType() }
+}
+
+/**
+ * A toString member predicate
+ *
+ * Supports looking through the disjucts of its definition in order to
+ * find restrictions on the class type (`this`).
+ *
+ * - `getRestrictions` gets the branch types that the class type is restricted to.
+ * - `hasUnrestrictedDisjunct` holds if one of the disjucts does not restrict
+ *   the class type.
+ *
+ * Together, these two predicates can answer whether all branches are covered.
+ */
+class ToString extends ClassPredicate {
+  ToString() { this.getName() = "toString" }
+
+  private Disjunction getADisJunction() {
+    result = this.getBody()
+    or
+    result = this.getADisJunction().getAChild()
+  }
+
+  Formula getADisjunct() {
+    result = this.getADisJunction().getAChild() and
+    not result instanceof Disjunction
+    or
+    not exists(this.getADisJunction()) and
+    result = this.getBody()
+  }
+
+  private Branches getRestrictions(Formula disjunct) {
+    // Treat each branch on its own
+    disjunct = this.getADisjunct() and
+    (
+      // this = (BranchType)...
+      // TODO: consider `not`
+      exists(ThisAccess ta, ComparisonFormula eq |
+        eq = disjunct.getAChild*() and
+        eq.getOperator() = "=" and
+        eq.getAnOperand() = ta and
+        result = eq.getAnOperand().(PredicateCall).getTarget()
+      )
+      or
+      // this.(BranchType)
+      // we currently do not do this anywhere
+      exists(ThisAccess ta, InlineCast ic |
+        ic = disjunct.getAChild*() and
+        ic.getBase() = ta and
+        result = ic.getTypeExpr().getResolvedType().getDeclaration()
+      )
+      or
+      // this instanceof BranchType
+      // TODO: consider `not`
+      exists(ThisAccess ta, InstanceOf io |
+        io = disjunct.getAChild*() and
+        ta = io.getExpr() and
+        result = io.getType().getResolvedType().getDeclaration()
+      )
+    )
+  }
+
+  NewTypeBranch getRestrictions() { result = this.getRestrictions(_).getBranch() }
+
+  predicate hasUnrestrictedDisjunct() {
+    exists(Formula disjunct | disjunct = this.getADisjunct() |
+      not exists(this.getRestrictions(disjunct))
+    )
+  }
+}
+
+ToString getToStringsOnClassExtendingNewtype(NewType nt) {
+  nt = result.getParent().getType().getASuperType+().getDeclaration() and
+  not result.hasAnnotation("abstract") and
+  // if the class of the toString also inherits directly
+  // from a branch type, it is not interesting.
+  not exists(NewTypeBranch branch | branch.getNewType() = nt |
+    branch = result.getParent().getType().getASuperType+().getDeclaration()
+  )
+}
+
+from ToString ts, NewType nt, NewTypeBranch branch
+where
+  ts = getToStringsOnClassExtendingNewtype(nt) and
+  branch.getNewType() = nt and
+  not branch = ts.getRestrictions() and
+  not ts.hasUnrestrictedDisjunct()
+select ts, "$@ does not define a result for $@ of $@.", ts, "This toString predicate", branch,
+  "this branch", nt, "this newtype"