proc: fix index access to already-loaded string values (#3184)

Fixes #3176

_fixtures/fncall.go

@@ -231,6 +231,9 @@ func main() {
 
 	d := &Derived{3, Base{4}}
 
+	var ref strings.Builder
+	fmt.Fprintf(&ref, "blah")
+
 	runtime.Breakpoint() // breakpoint here
 	call1(one, two)
 	fn2clos(2)
@@ -238,5 +241,5 @@ func main() {
 	d.Method()
 	d.Base.Method()
 	x.CallMe()
-	fmt.Println(one, two, zero, call, call0, call2, callexit, callpanic, callbreak, callstacktrace, stringsJoin, intslice, stringslice, comma, a.VRcvr, a.PRcvr, pa, vable_a, vable_pa, pable_pa, fn2clos, fn2glob, fn2valmeth, fn2ptrmeth, fn2nil, ga, escapeArg, a2, square, intcallpanic, onetwothree, curriedAdd, getAStruct, getAStructPtr, getVRcvrableFromAStruct, getPRcvrableFromAStructPtr, getVRcvrableFromAStructPtr, pa2, noreturncall, str, d, x, x2.CallMe(5), longstrs, regabistacktest, regabistacktest2, issue2698.String(), regabistacktest3, rast3, floatsum)
+	fmt.Println(one, two, zero, call, call0, call2, callexit, callpanic, callbreak, callstacktrace, stringsJoin, intslice, stringslice, comma, a.VRcvr, a.PRcvr, pa, vable_a, vable_pa, pable_pa, fn2clos, fn2glob, fn2valmeth, fn2ptrmeth, fn2nil, ga, escapeArg, a2, square, intcallpanic, onetwothree, curriedAdd, getAStruct, getAStructPtr, getVRcvrableFromAStruct, getPRcvrableFromAStructPtr, getVRcvrableFromAStructPtr, pa2, noreturncall, str, d, x, x2.CallMe(5), longstrs, regabistacktest, regabistacktest2, issue2698.String(), regabistacktest3, rast3, floatsum, ref)
 }

pkg/proc/eval.go

@@ -2084,7 +2084,21 @@ func (v *Variable) sliceAccess(idx int) (*Variable, error) {
 		return nil, fmt.Errorf("index out of bounds")
 	}
 	if v.loaded {
-		return &v.Children[idx], nil
+		if v.Kind == reflect.String {
+			s := constant.StringVal(v.Value)
+			if idx >= len(s) {
+				return nil, fmt.Errorf("index out of bounds")
+			}
+			r := v.newVariable("", v.Base+uint64(int64(idx)*v.stride), v.fieldType, v.mem)
+			r.loaded = true
+			r.Value = constant.MakeInt64(int64(s[idx]))
+			return r, nil
+		} else {
+			if idx >= len(v.Children) {
+				return nil, fmt.Errorf("index out of bounds")
+			}
+			return &v.Children[idx], nil
+		}
 	}
 	mem := v.mem
 	if v.Kind != reflect.Array {

pkg/proc/variables_test.go

@@ -1206,6 +1206,10 @@ func TestCallFunction(t *testing.T) {
 		// Issue 1577
 		{"1+2", []string{`::3`}, nil},
 		{`"de"+"mo"`, []string{`::"demo"`}, nil},
+
+		// Issue 3176
+		{`ref.String()[0]`, []string{`:byte:98`}, nil},
+		{`ref.String()[20]`, nil, errors.New("index out of bounds")},
 	}
 
 	var testcases112 = []testCaseCallFunction{