Fix completion crash after closing bracket in contextually-typed array literals (#2297)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: DanielRosenwasser <[email protected]>

Author: Copilot

internal/fourslash/tests/manual/completionsInArrayLiteralWithContextualType_test.go

@@ -66,4 +66,19 @@ func TestCompletionsInArrayLiteralWithContextualType(t *testing.T) {
 			},
 		},
 	})
+
+	// Test 4: Completions after `]` in a tuple should not crash (issue #2296)
+	// When completing after the closing bracket, we're outside the array literal
+	// so we shouldn't be getting contextual types for array elements
+	const content4 = `let x: [number] = [123]/*d*/;`
+	f4, done4 := fourslash.NewFourslash(t, nil /*capabilities*/, content4)
+	defer done4()
+	// Just verify that completions don't crash - accept any completion list
+	f4.VerifyCompletions(t, "d", &fourslash.CompletionsExpectedList{
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{},
+	})
 }

internal/ls/completions.go

@@ -2998,6 +2998,13 @@ func getContextualType(previousToken *ast.Node, position int, file *ast.SourceFi
 			}
 		}
 		return nil
+	case ast.KindCloseBracketToken:
+		// When completing after `]` (e.g., `[x]/*here*/`), we should not provide a contextual type
+		// for the closing bracket token itself. Without this case, CloseBracketToken would fall through
+		// to the default case, and if the parent is an array literal, GetContextualType would try to
+		// find the token's index in the array elements (returning -1), leading to an out-of-bounds panic
+		// in getContextualTypeForElementExpression.
+		return nil
 	case ast.KindQuestionToken:
 		// When completing after `?` in a ternary conditional (e.g., `foo(a ? /*here*/)`),
 		// we need to look at the parent conditional expression to find the contextual type.