Make MethodCallNode char pred more accurate

When a function is assigned to a variable and called through that
variable then we previously didn't realise it was a function. With
this change we try use local flow to determine if the function being
called is a method.

Author: owen-mc

go/ql/lib/change-notes/2023-07-19-method-call-node.md

@@ -0,0 +1,4 @@
+---
+category: fix
+---
+* The definition of `DataFlow::MethodCallNode` has been expanded to include `DataFlow::CallNode`s where what is being called is a variable that has a method assigned to it within the calling function. This means we can now follow data flow into the receiver of such a method call.

go/ql/lib/semmle/go/dataflow/internal/DataFlowNodes.qll

@@ -631,9 +631,9 @@ module Public {
 
   /** A data flow node that represents a call to a method. */
   class MethodCallNode extends CallNode {
-    MethodCallNode() { expr.getTarget() instanceof Method }
+    MethodCallNode() { getACalleeSource(this) instanceof MethodReadNode }
 
-    override Method getTarget() { result = expr.getTarget() }
+    override Method getTarget() { result = getACalleeSource(this).(MethodReadNode).getMethod() }
 
     override MethodDecl getACallee() { result = super.getACallee() }
   }

go/ql/test/library-tests/semmle/go/dataflow/GenericFunctionsAndTypes/generictypesandmethods.go

@@ -51,7 +51,7 @@ func main() {
 		gs1 := GenericStruct1[string]{""}
 		gs1.Field = source()
 		f := gs1.Getter
-		sink(f()) // $ MISSING: hasValueFlow="call to f"
+		sink(f()) // $ hasValueFlow="call to f"
 	}
 	{
 		gs1 := GenericStruct1[string]{""}
@@ -62,7 +62,7 @@ func main() {
 		gs1 := GenericStruct1[string]{""}
 		f := gs1.Setter
 		f(source())
-		sink(gs1.Field) // $ MISSING: hasValueFlow="selection of Field"
+		sink(gs1.Field) // $ hasValueFlow="selection of Field"
 	}
 
 	{
@@ -87,7 +87,7 @@ func main() {
 		gs2 := GenericStruct2[string, string]{"", ""}
 		gs2.Field1 = source()
 		f := gs2.Getter1
-		sink(f()) // $ MISSING: hasValueFlow="call to f"
+		sink(f()) // $ hasValueFlow="call to f"
 	}
 	{
 		gs2 := GenericStruct2[string, string]{"", ""}
@@ -98,7 +98,7 @@ func main() {
 		gs2 := GenericStruct2[string, string]{"", ""}
 		f := gs2.Setter1
 		f(source())
-		sink(gs2.Field1) // $ MISSING: hasValueFlow="selection of Field1"
+		sink(gs2.Field1) // $ hasValueFlow="selection of Field1"
 	}
 
 	{
@@ -123,7 +123,7 @@ func main() {
 		gs2 := GenericStruct2[string, string]{"", ""}
 		gs2.Field2 = source()
 		f := gs2.Getter2
-		sink(f()) // $ MISSING: hasValueFlow="call to f"
+		sink(f()) // $ hasValueFlow="call to f"
 	}
 	{
 		gs2 := GenericStruct2[string, string]{"", ""}
@@ -134,6 +134,6 @@ func main() {
 		gs2 := GenericStruct2[string, string]{"", ""}
 		f := gs2.Setter2
 		f(source())
-		sink(gs2.Field2) // $ MISSING: hasValueFlow="selection of Field2"
+		sink(gs2.Field2) // $ hasValueFlow="selection of Field2"
 	}
 }