go/types: in string(x) conversions, x must be of integer type

Port of https://go-review.googlesource.com/11365

Fixes #11357.

Change-Id: Icd20fa038696a8853d1d14477e1c1132938b3e2e
Reviewed-on: https://go-review.googlesource.com/11368Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/conversions.go

@@ -20,7 +20,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 		switch t := T.Underlying().(*Basic); {
 		case representableConst(x.val, check.conf, t.kind, &x.val):
 			ok = true
-		case x.isInteger() && isString(t):
+		case isInteger(x.typ) && isString(t):
 			codepoint := int64(-1)
 			if i, ok := exact.Int64Val(x.val); ok {
 				codepoint = i

src/go/types/testdata/conversions.src

@@ -32,6 +32,11 @@ func string_conversions() {
 	const _ = string(true /* ERROR "cannot convert" */ )
 	const _ = string(1.2 /* ERROR "cannot convert" */ )
 	const _ = string(nil /* ERROR "cannot convert" */ )
+
+	// issues 11357, 11353: argument must be of integer type
+	_ = string(0.0 /* ERROR "cannot convert" */ )
+	_ = string(0i /* ERROR "cannot convert" */ )
+	_ = string(1 /* ERROR "cannot convert" */ + 2i)
 }
 
 func interface_conversions() {