go/types: fix unsymmetric test when typechecking comparisons

The existing code assumed that comparability and orderedness
was implied for the 2nd operand if the 1st operand satisfied
these predicates.

Fixes #28164.

Change-Id: I61d4e5eedb3297731a20a14acb3645d11b36fcc5
Reviewed-on: https://go-review.googlesource.com/c/143277
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/expr.go

@@ -590,10 +590,10 @@ func (check *Checker) comparison(x, y *operand, op token.Token) {
 		switch op {
 		case token.EQL, token.NEQ:
 			// spec: "The equality operators == and != apply to operands that are comparable."
-			defined = Comparable(x.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
+			defined = Comparable(x.typ) && Comparable(y.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
 		case token.LSS, token.LEQ, token.GTR, token.GEQ:
 			// spec: The ordering operators <, <=, >, and >= apply to operands that are ordered."
-			defined = isOrdered(x.typ)
+			defined = isOrdered(x.typ) && isOrdered(y.typ)
 		default:
 			unreachable()
 		}

src/go/types/testdata/expr2.src

@@ -208,6 +208,19 @@ func interfaces() {
 
 	_ = i /* ERROR mismatched types */ == s2
 	_ = i /* ERROR mismatched types */ == &s2
+
+	// issue #28164
+	// testcase from issue
+	_ = interface /* ERROR cannot compare */ {}(nil) == []int(nil)
+
+	// related cases
+	var e interface{}
+	var s []int
+	var x int
+	_ = e /* ERROR cannot compare */ == s
+	_ = s /* ERROR cannot compare */ == e
+	_ = e /* ERROR cannot compare */ < x
+	_ = x /* ERROR cannot compare */ < e
 }
 
 func slices() {