[SR-3254] Unhelpful diagnostics for compound name reference.

Description

func foo(x: Int) {}
let f = foo(_:)

test.swift:2:9: error: use of unresolved identifier 'foo'
let f = foo(_:)
        ^~~~~~~
test.swift:1:6: note: did you mean 'foo'?
func foo(x: Int) {}
     ^

test.swift:2:9: error: use of unresolved name 'foo(_:)'
let f = foo(_:)
        ^~~~~~~
test.swift:1:6: note: did you mean 'foo(x:)'?
func foo(x: Int) {}
     ^

Contributor guide

Tech stack

swift

Domain

developer experience

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-3 hours

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Swift language basicsUnderstanding of compiler error messages

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

45

Research direction

Investigate the Swift compiler's diagnostic emission for compound name references. The relevant code is likely in lib/Sema/TypeCheckExpr.cpp or similar files that handle unresolved identifier errors. The goal is to improve the error message from 'use of unresolved identifier' to something like 'use of unresolved name foo( :)' with a suggestion to use 'foo(x:)'. Look at how the compiler tokenizes and prints function signatures. Also review the related issue SR 4867 for additional context on similar diagnostic improvements.