C#: Narrow the set of declarations where we make explicit interface implementations.

Author: michaelnebel

csharp/ql/src/Stubs/Stubs.qll

@@ -400,7 +400,7 @@ private string stubAccessibility(Member m) {
   if
     m.getDeclaringType() instanceof Interface
     or
-    exists(getSingleSpecificImplementedInterface(m))
+    exists(useExplicitImplementedInterface(m))
     or
     m instanceof Constructor and m.isStatic()
   then result = ""
@@ -713,13 +713,49 @@ private string stubEventAccessors(Event e) {
   else result = ";"
 }
 
-private Interface getSingleSpecificImplementedInterface(Member c) {
-  result = unique(Interface i | i = c.(Virtualizable).getExplicitlyImplementedInterface())
+/**
+ * Returns an interface that `c` explicitly implements, if either or the
+ * following holds.
+ * (1) `c` is not static.
+ * (2) `c` is static and an implementation of a generic with type constraints.
+ * (3) `c` is static and there is another member with the same name
+ * but different return types.
+ *
+ * We use these rules, as explicit interfaces are needed in some cases, eg.
+ * for compilation purposes (both to distinguish members but also to ensure
+ * type constraints are satisfied). We can't always use the explicit interface
+ * due to the generic math support, because then in some cases we will only be
+ * able to access a static via a type variable with type
+ * constraints (C# 11 language feature).
+ */
+private Interface useExplicitImplementedInterface(Virtualizable c) {
+  result = unique(Interface i | i = c.getExplicitlyImplementedInterface()) and
+  (
+    not c.isStatic()
+    or
+    c.isStatic() and
+    (
+      not c instanceof Method
+      or
+      c instanceof Method and
+      (
+        exists(TypeParameter t | t = c.getImplementee().(UnboundGeneric).getATypeParameter() |
+          exists(t.getConstraints().getATypeConstraint())
+        )
+        or
+        exists(Member m |
+          (not m.isStatic() or m.(Method).getReturnType() != c.(Method).getReturnType()) and
+          m.getName() = c.getName() and
+          m.getDeclaringType() = c.getDeclaringType()
+        )
+      )
+    )
+  )
 }
 
 private string stubExplicitImplementation(Member c) {
-  if exists(getSingleSpecificImplementedInterface(c))
-  then result = stubClassName(getSingleSpecificImplementedInterface(c)) + "."
+  if exists(useExplicitImplementedInterface(c))
+  then result = stubClassName(useExplicitImplementedInterface(c)) + "."
   else result = ""
 }