go/types: print assignment operation for invalid operation errors

This is port of CL 357229 for types2 to go/types.

Change-Id: I35ed6b784969210a00ea5b36238df7d6b7fa18bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/357230
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: cuonglm

src/go/types/expr.go

@@ -1000,7 +1000,11 @@ func (check *Checker) binary(x *operand, e ast.Expr, lhs, rhs ast.Expr, op token
 			if e != nil {
 				posn = e
 			}
-			check.invalidOp(posn, _MismatchedTypes, "%s (mismatched types %s and %s)", e, x.typ, y.typ)
+			if e != nil {
+				check.invalidOp(posn, _MismatchedTypes, "%s (mismatched types %s and %s)", e, x.typ, y.typ)
+			} else {
+				check.invalidOp(posn, _MismatchedTypes, "%s %s= %s (mismatched types %s and %s)", lhs, op, rhs, x.typ, y.typ)
+			}
 		}
 		x.mode = invalid
 		return

src/go/types/testdata/fixedbugs/issue48472.go2

@@ -9,3 +9,8 @@ func g() {
 	var i int
 	_ = s /* ERROR invalid operation: s \+ i \(mismatched types string and int\) */ + i
 }
+
+func f(i int) int {
+        i /* ERROR invalid operation: i \+= "1" \(mismatched types int and untyped string\) */ += "1"
+        return i
+}