Factor ErrorType out of two duplicate tests

Author: smowton

ql/src/InconsistentCode/MissingErrorCheck.ql

@@ -90,7 +90,7 @@ predicate returnUncheckedAtNode(
     err.getAPredecessor() = call.getResult(1) and
     call.asInstruction() = node and
     isDereferenceableType(ptr.getType()) and
-    err.getType().implements(Builtin::error().getType().getUnderlyingType()) and
+    err.getType() instanceof ErrorType and
     calleeMayReturnNilWithError(call)
     or
     // Recursive case: check that some predecessor is missing a check, and `node` does not itself

ql/src/Security/CWE-327/InsecureTLS.ql

@@ -59,7 +59,7 @@ predicate isReturnedWithError(DataFlow::Node node) {
   exists(ReturnStmt ret |
     ret.getExpr(0) = node.asExpr() and
     ret.getNumExpr() = 2 and
-    ret.getExpr(1).getType().implements(Builtin::error().getType().getUnderlyingType())
+    ret.getExpr(1).getType() instanceof ErrorType
     // That last condition implies ret.getExpr(1) is non-nil, since nil doesn't implement `error`
   )
 }

ql/src/semmle/go/Types.qll

@@ -630,6 +630,13 @@ class NamedType extends @namedtype, CompositeType {
   override Type getUnderlyingType() { result = getBaseType().getUnderlyingType() }
 }
 
+/**
+ * A type that implements the builtin interface `error`.
+ */
+class ErrorType extends Type {
+  ErrorType() { this.implements(Builtin::error().getType().getUnderlyingType()) }
+}
+
 /**
  * Holds if `i` is the empty interface type, which is implemented by every type with a method set.
  */