[dev.typeparams] cmd/compile/internal/types2: report constant overflow in binary ops

This is the go.types changes of https://golang.org/cl/271706
ported to types2.

Also: Fixed a bug in the go/types version (was using the
wrong position in the error message).

Change-Id: I798b80243a66f0be5b943a6951d7a1ff769abca2
Reviewed-on: https://go-review.googlesource.com/c/go/+/271806
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/expr.go

@@ -897,7 +897,7 @@ var binaryOpPredicates = opPredicates{
 }
 
 // The binary expression e may be nil. It's passed in for better error messages only.
-func (check *Checker) binary(x *operand, e *syntax.Operation, lhs, rhs syntax.Expr, op syntax.Operator) {
+func (check *Checker) binary(x *operand, e *syntax.Operation, lhs, rhs syntax.Expr, op syntax.Operator, opPos syntax.Pos) {
 	var y operand
 
 	check.expr(x, lhs)
@@ -977,6 +977,14 @@ func (check *Checker) binary(x *operand, e *syntax.Operation, lhs, rhs syntax.Ex
 			tok = token.QUO_ASSIGN
 		}
 		x.val = constant.BinaryOp(xval, tok, yval)
+		// report error if valid operands lead to an invalid result
+		if xval.Kind() != constant.Unknown && yval.Kind() != constant.Unknown && x.val.Kind() == constant.Unknown {
+			// TODO(gri) We should report exactly what went wrong. At the
+			//           moment we don't have the (go/constant) API for that.
+			//           See also TODO in go/constant/value.go.
+			check.errorf(opPos, "constant result is not representable")
+			// TODO(gri) Should we mark operands with unknown values as invalid?
+		}
 		// Typed constants must be representable in
 		// their type after each constant operation.
 		if isTyped(typ) {
@@ -1791,7 +1799,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		}
 
 		// binary expression
-		check.binary(x, e, e.X, e.Y, e.Op)
+		check.binary(x, e, e.X, e.Y, e.Op, e.Y.Pos()) // TODO(gri) should have OpPos here (like in go/types)
 		if x.mode == invalid {
 			goto Error
 		}

src/cmd/compile/internal/types2/fixedbugs/issue20583.src

@@ -0,0 +1,12 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue20583
+const (
+	_ = 6e886451608 /* ERROR malformed constant */ /2
+	_ = 6e886451608i /* ERROR malformed constant */ /2
+	_ = 0 * 1e+1000000000 // ERROR malformed constant
+	x = 1e100000000
+	_ = x*x*x*x*x*x*x /* ERROR not representable */ // TODO(gri) this error should be at the last *
+)

src/cmd/compile/internal/types2/stmt.go

@@ -396,7 +396,7 @@ func (check *Checker) stmt(ctxt stmtContext, s syntax.Stmt) {
 			}
 
 			var x operand
-			check.binary(&x, nil, lhs[0], rhs[0], s.Op)
+			check.binary(&x, nil, lhs[0], rhs[0], s.Op, rhs[0].Pos()) // TODO(gri) should have TokPos here (like in go/types)
 			if x.mode == invalid {
 				return
 			}

src/go/types/expr.go

@@ -890,7 +890,7 @@ func (check *Checker) binary(x *operand, e *ast.BinaryExpr, lhs, rhs ast.Expr, o
 			// TODO(gri) We should report exactly what went wrong. At the
 			//           moment we don't have the (go/constant) API for that.
 			//           See also TODO in go/constant/value.go.
-			check.errorf(atPos(e.OpPos), _InvalidConstVal, "constant result is not representable")
+			check.errorf(atPos(opPos), _InvalidConstVal, "constant result is not representable")
 			// TODO(gri) Should we mark operands with unknown values as invalid?
 		}
 		// Typed constants must be representable in