SA4023: generate better message when we know that the related information will be dropped

Updates gh-972

Author: dominikh

analysis/report/report.go

@@ -130,6 +130,17 @@ func shortRange(node ast.Node) (pos, end token.Pos) {
 	}
 }
 
+func HasRange(node Positioner) bool {
+	// we don't know if getRange will be called with shortRange set to
+	// true, so make sure that both work.
+	_, _, ok := getRange(node, false)
+	if !ok {
+		return false
+	}
+	_, _, ok = getRange(node, true)
+	return ok
+}
+
 func getRange(node Positioner, short bool) (pos, end token.Pos, ok bool) {
 	switch n := node.(type) {
 	case sourcer:

staticcheck/lint.go

@@ -4306,8 +4306,13 @@ func CheckTypedNilInterface(pass *analysis.Pass) (interface{}, error) {
 					default:
 						panic("unreachable")
 					}
-					report.Report(pass, binop, fmt.Sprintf("this comparison is %s true", qualifier),
-						report.Related(x.X, "the lhs of the comparison gets its value from here and has a concrete type"))
+					if report.HasRange(x.X) {
+						report.Report(pass, binop, fmt.Sprintf("this comparison is %s true", qualifier),
+							report.Related(x.X, "the lhs of the comparison gets its value from here and has a concrete type"))
+					} else {
+						// we can't generate related information for this, so make the diagnostic itself slightly more useful
+						report.Report(pass, binop, fmt.Sprintf("this comparison is %s true; the lhs of the comparison has been assigned a concretely typed value", qualifier))
+					}
 					continue
 				}
 				if obj == nil {

staticcheck/testdata/src/CheckTypedNilInterface/CheckTypedNilInterface.go

@@ -204,4 +204,7 @@ func test() {
 	_ = gen20() == nil
 	_ = gen21() == nil
 	_ = gen22() == nil // want `never true`
+
+	var v1 interface{} = 0
+	_ = v1 == nil // want `never true; the lhs`
 }