Summary

Empty collection expressions [] immediately after ? in conditional (ternary) expressions are misparsed. The parser interprets cond? as a nullable type and [] as an array rank specifier, instead of recognizing the ternary operator.

Reproduction

var x = condition ? [] : list;

Expected: Parsed as conditional expression with empty collection expression as consequence

Actual: Parsed as nullable array type declaration condition?[] followed by errors

Analysis

The grammar has an ambiguity: when it sees identifier?[], it can mean either:

1. identifier ? [] : ... - ternary with empty collection expression
2. identifier?[] - nullable array type (like int?[])

The grammar currently prefers interpretation #2, causing parse errors when : follows.

What Works

Notably, non-empty collection expressions in ternaries parse correctly:

// These all work:
var a = [];                    // standalone empty collection
var b = [1, 2, 3];             // collection with elements
var c = [..list];              // spread operator
var d = cond ? [1] : list;     // non-empty in ternary ✓

// This fails:
var e = cond ? [] : list;      // empty in ternary ✗

The issue is specifically the token sequence ? [] where [] is empty.

Real-World Impact

This pattern appears in production code:

var items = shouldClear ? [] : existingItems;

var result = new Response {
    Tags = includeAll 
        ? []  // Parse error here
        : FilterTags(input)
};

Possible Fix

The grammar may need disambiguation rules to prefer conditional_expression interpretation when ? is followed by [ ] : sequence.

Related

This is a specific edge case of the broader C# 12 collection expressions feature. See #401 for the general feature request, though most collection expression syntax already works.