hcl/hclsyntax: Don't eat errors in IndexExpr collection or key

Due to incorrect diagnostics discipline here, the result of the index
operation was overwriting any diagnostics from either the collection or
the key expressions.

hcl/hclsyntax/expression.go

@@ -604,8 +604,9 @@ func (e *IndexExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
 	diags = append(diags, collDiags...)
 	diags = append(diags, keyDiags...)
 
-	val, diags := hcl.Index(coll, key, &e.SrcRange)
-	setDiagEvalContext(diags, e, ctx)
+	val, indexDiags := hcl.Index(coll, key, &e.SrcRange)
+	setDiagEvalContext(indexDiags, e, ctx)
+	diags = append(diags, indexDiags...)
 	return val, diags
 }
 

hcl/hclsyntax/expression_test.go

@@ -981,6 +981,16 @@ upper(
 			cty.StringVal("hello"),
 			0,
 		},
+		{
+			`["boop"].foo[index]`, // index is a variable to force IndexExpr instead of traversal
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"index": cty.NumberIntVal(0),
+				},
+			},
+			cty.DynamicVal,
+			1, // expression ["boop"] does not have attributes
+		},
 
 		{
 			`foo`,