go/types: don't panic in complex division

Make sure that in complex division we reject divisors that would
underflow to zero when using the textbook complex-division method we
currently use.

This change does for go/types what golang.org/cl/42650 did for gc.

Fixes #20227

Change-Id: Iaa784ac5e60141f51c501eb0e3ce0e9c1c2993d0
Reviewed-on: https://go-review.googlesource.com/44590
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/expr.go

@@ -800,10 +800,24 @@ func (check *Checker) binary(x *operand, e *ast.BinaryExpr, lhs, rhs ast.Expr, o
 		return
 	}
 
-	if (op == token.QUO || op == token.REM) && (x.mode == constant_ || isInteger(x.typ)) && y.mode == constant_ && constant.Sign(y.val) == 0 {
-		check.invalidOp(y.pos(), "division by zero")
-		x.mode = invalid
-		return
+	if op == token.QUO || op == token.REM {
+		// check for zero divisor
+		if (x.mode == constant_ || isInteger(x.typ)) && y.mode == constant_ && constant.Sign(y.val) == 0 {
+			check.invalidOp(y.pos(), "division by zero")
+			x.mode = invalid
+			return
+		}
+
+		// check for divisor underflow in complex division (see issue 20227)
+		if x.mode == constant_ && y.mode == constant_ && isComplex(x.typ) {
+			re, im := constant.Real(y.val), constant.Imag(y.val)
+			re2, im2 := constant.BinaryOp(re, token.MUL, re), constant.BinaryOp(im, token.MUL, im)
+			if constant.Sign(re2) == 0 && constant.Sign(im2) == 0 {
+				check.invalidOp(y.pos(), "division by zero")
+				x.mode = invalid
+				return
+			}
+		}
 	}
 
 	if x.mode == constant_ && y.mode == constant_ {

src/go/types/stdlib_test.go

@@ -171,7 +171,6 @@ func TestStdFixed(t *testing.T) {
 		"issue18459.go", // go/types doesn't check validity of //go:xxx directives
 		"issue18882.go", // go/types doesn't check validity of //go:xxx directives
 		"issue20232.go", // go/types handles larger constants than gc
-		"issue20227.go", // go/types does not handle this yet
 		"issue20529.go", // go/types does not have constraints on stack size
 	)
 }