Add getAMatchingSuccessorType

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphImpl.qll

@@ -5,6 +5,7 @@
  */
 
 import go
+private import codeql.controlflow.SuccessorType
 
 /** A block statement that is not the body of a `switch` or `select` statement. */
 class PlainBlock extends BlockStmt {
@@ -542,6 +543,30 @@ module CFG {
     }
   }
 
+  /**
+   *  Gets a successor type that matches completion `c`.
+   *
+   * Note that there isn't a 1-to-1 correspondence between completions and
+   * successor types, so this is an approximation. For example sometimes the
+   * completion `Done()` is used when the successor type would be
+   * `ReturnSuccessor` or `ExitSuccessor`.
+   */
+  private SuccessorType getAMatchingSuccessorType(Completion c) {
+    c = Done() and result instanceof DirectSuccessor
+    or
+    exists(boolean b | c = Bool(b) | result.(BooleanSuccessor).getValue() = b)
+    or
+    c = Break(_) and result instanceof BreakSuccessor
+    or
+    c = Continue(_) and result instanceof ContinueSuccessor
+    or
+    c = Fallthrough() and result instanceof DirectSuccessor
+    or
+    c = Return() and result instanceof ReturnSuccessor
+    or
+    c = Panic() and result instanceof ExceptionSuccessor
+  }
+
   /**
    * Holds if `e` should have an evaluation node in the control-flow graph.
    *