[dev.typeparams] go/types: report better error for invalid untyped operation

This is port of CL 328053 for types2 to go/type.

The change is identical, but for some tweaks to the error positions in
tests.

Updates #46749

Change-Id: I8d34c5b1669e59e4ec7d91f81dcf655b2bfd89a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/328869
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/errorcodes.go

@@ -875,7 +875,7 @@ const (
 	// context in which it is used.
 	//
 	// Example:
-	//  var _ = 1 + ""
+	//  var _ = 1 + nil
 	_InvalidUntypedConversion
 
 	// _BadOffsetofSyntax occurs when unsafe.Offsetof is called with an argument

src/go/types/expr.go

@@ -927,6 +927,19 @@ func (check *Checker) binary(x *operand, e ast.Expr, lhs, rhs ast.Expr, op token
 		return
 	}
 
+	canMix := func(x, y *operand) bool {
+		if IsInterface(x.typ) || IsInterface(y.typ) {
+			return true
+		}
+		if isBoolean(x.typ) != isBoolean(y.typ) {
+			return false
+		}
+		if isString(x.typ) != isString(y.typ) {
+			return false
+		}
+		return true
+	}
+	if canMix(x, &y) {
 		check.convertUntyped(x, y.typ)
 		if x.mode == invalid {
 			return
@@ -936,6 +949,7 @@ func (check *Checker) binary(x *operand, e ast.Expr, lhs, rhs ast.Expr, op token
 			x.mode = invalid
 			return
 		}
+	}
 
 	if isComparison(op) {
 		check.comparison(x, &y, op)

src/go/types/testdata/check/const0.src

@@ -27,7 +27,7 @@ const (
 	ub1 = true
 	ub2 = 2 < 1
 	ub3 = ui1 == uf1
-	ub4 = true /* ERROR "cannot convert" */ == 0
+	ub4 = true /* ERROR "mismatched types untyped bool and untyped int" */ == 0
 
 	// integer values
 	ui0 = 0

src/go/types/testdata/check/decls1.src

@@ -83,7 +83,7 @@ var (
 
 // Constant expression initializations
 var (
-	v1 = 1 /* ERROR "cannot convert" */ + "foo"
+	v1 = 1 /* ERROR "mismatched types untyped int and untyped string" */ + "foo"
 	v2 = c + 255
 	v3 = c + 256 /* ERROR "overflows" */
 	v4 = r + 2147483647

src/go/types/testdata/check/expr1.src

@@ -111,10 +111,10 @@ type mystring string
 func _(x, y string, z mystring) {
 	x = x + "foo"
 	x = x /* ERROR not defined */ - "foo"
-	x = x + 1 // ERROR cannot convert
+	x = x /* ERROR mismatched types string and untyped int */ + 1
 	x = x + y
 	x = x /* ERROR not defined */ - y
-	x = x * 10 // ERROR cannot convert
+	x = x /* ERROR mismatched types string and untyped int */* 10
 }
 
 func f() (a, b int) { return }

src/go/types/testdata/check/expr2.src

@@ -10,7 +10,7 @@ func _bool() {
 	const t = true == true
 	const f = true == false
 	_ = t /* ERROR "cannot compare" */ < f
-	_ = 0 /* ERROR "cannot convert" */ == t
+	_ = 0 /* ERROR "mismatched types untyped int and untyped bool" */ == t
 	var b bool
 	var x, y float32
 	b = x < y

src/go/types/testdata/check/expr3.src

@@ -103,7 +103,7 @@ func indexes() {
 	var ok mybool
 	_, ok = m["bar"]
 	_ = ok
-	_ = m[0 /* ERROR "cannot use 0" */ ] + "foo" // ERROR "cannot convert"
+	_ = m/* ERROR "mismatched types int and untyped string" */[0 /* ERROR "cannot use 0" */ ] + "foo"
 
 	var t string
 	_ = t[- /* ERROR "negative" */ 1]

src/go/types/testdata/check/stmt0.src

@@ -49,18 +49,18 @@ func assignments1() {
 	b = true
 
 	i += 1
-	i += "foo" /* ERROR "cannot convert.*int" */
+	i /* ERROR "mismatched types int and untyped string" */+= "foo"
 
 	f -= 1
 	f /= 0
 	f = float32(0)/0 /* ERROR "division by zero" */
-	f -= "foo" /* ERROR "cannot convert.*float64" */
+	f /* ERROR "mismatched types float64 and untyped string" */-= "foo"
 
 	c *= 1
 	c /= 0
 
 	s += "bar"
-	s += 1 /* ERROR "cannot convert.*string" */
+	s /* ERROR "mismatched types string and untyped int" */+= 1
 
 	var u64 uint64
 	u64 += 1<<u64
@@ -937,13 +937,13 @@ func issue6766b() {
 // errors reported).
 func issue10148() {
 	for y /* ERROR declared but not used */ := range "" {
-		_ = "" /* ERROR cannot convert */ + 1
+		_ = "" /* ERROR mismatched types untyped string and untyped int */ + 1
 	}
 	for range 1 /* ERROR cannot range over 1 */ {
-		_ = "" /* ERROR cannot convert */ + 1
+		_ = "" /* ERROR mismatched types untyped string and untyped int */ + 1
 	}
 	for y := range 1 /* ERROR cannot range over 1 */ {
-		_ = "" /* ERROR cannot convert */ + 1
+		_ = "" /* ERROR mismatched types untyped string and untyped int */ + 1
 	}
 }