carbon-language/carbon-lang

Call to local function inside generic function fails

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#6793 opened on Feb 25, 2026

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good first issuetoolchain

Description

Description of the bug:

A call to a local function defined inside a generic function fails with a compiler error.

What did you do, or what's a simple way to reproduce the bug?

fn F(unused T:! type) {
  fn Local() {}
  Local();
}

What did you expect to happen?

This should compile successfully.

What actually happened?

The compiler cannot deduce the generic parameters of the enclosing function while deducing parameters of the local function.

  // CHECK:STDERR: demo.carbon:[[@LINE+7]]:3: error: cannot deduce value for generic parameter `T` [DeductionIncomplete]
  // CHECK:STDERR:   Local();
  // CHECK:STDERR:   ^~~~~~~
  // CHECK:STDERR: demo.carbon:[[@LINE-4]]:3: note: while deducing parameters of generic declared here [DeductionGenericHere]
  // CHECK:STDERR:   fn Local() {}
  // CHECK:STDERR:   ^~~~~~~~~~~~
  // CHECK:STDERR:

Any other information, logs, or outputs that you want to share?

No response

Contributor guide

Tech stack
cpp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
basic Carbon syntaxunderstanding of generics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the compiler's implementation of generic parameter deduction, specifically how local functions inside generic functions are handled. Look for how the enclosing function's generic parameters are made available during deduction of the local function's calls. The error message indicates a failure to deduce `T`, so focus on the deduction context propagation.