Fix a nil deref, take two (#245)

The change in 0d2eda1 did not fix the panic condition, which occurs if the underlying *StringExpr is nil. Check for nil using reflection, and add a test to verify the fix.

ast/expr.go

@@ -17,6 +17,7 @@ package ast
 import (
 	"encoding/json"
 	"fmt"
+	"reflect"
 	"strings"
 
 	"github.com/hashicorp/hcl/v2"
@@ -59,7 +60,7 @@ type exprConstraint interface {
 
 func exprPosition[T exprConstraint](expr T) (*hcl.Range, string) {
 	var zero T
-	if expr != zero {
+	if expr != zero && !reflect.ValueOf(expr).IsNil() {
 		if syntax := expr.Syntax(); syntax != nil {
 			return syntax.Syntax().Range(), syntax.Syntax().Path()
 		}

ast/expr_test.go

@@ -0,0 +1,45 @@
+// Copyright 2024, Pulumi Corporation.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ast
+
+import (
+	"testing"
+
+	"github.com/pulumi/esc/syntax"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestExprError(t *testing.T) {
+	type args struct {
+		expr    Expr
+		summary string
+	}
+	var ss *StringExpr
+	tests := []struct {
+		name string
+		args args
+		want *syntax.Diagnostic
+	}{
+		{"nil *StringExpr",
+			args{
+				ss, "",
+			}, syntax.Error(nil, "", "")},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equalf(t, tt.want, ExprError(tt.args.expr, tt.args.summary), "ExprError(%v, %v)", tt.args.expr, tt.args.summary)
+		})
+	}
+}