Allow lambdas to name template parameters from function decl [GH #338] by mborland · Pull Request #339 · edgcpp/compiler

mborland · 2026-10-21T17:38:53Z

This PR enable lambda to name template parameters from template function declaration such in the case of:

template <class T>
  void f(char (*)[sizeof([](T y) { return y; }(T{}))]) {}
  void g() { char a[4]; f<int>(&a); }  // Was an error: identifier "T" is undefined

Closes: #338

Summary by CodeRabbit

Bug Fixes
- Lambdas in function-template declarations can now use function-template parameters during substitution and rescanning, including in enable_if conditions.
- Improved overload resolution for affected templates when lambda expressions reference template parameters.
- Generic lambdas, lambdas with their own template parameters, and references to template parameter packs remain unsupported in this context.
Tests
- Added coverage across C++20 and C++23 for supported cases and documented boundaries.

…pp#338]

edgcpp-bot · 2026-10-21T20:43:28Z

AcknowlEDG review results updated for 86215d0800f6:

Test - Changes (new)

edgcpp-bot · 2026-10-21T20:56:54Z

AcknowlEDG review results updated for 86215d0800f6:

Bench - C (new)
Bench - C++ (new)
Test - Changes (unchanged)

CI success.

coderabbitai · 2026-10-22T16:07:05Z

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📥 Commits

Reviewing files that changed from the base of the PR and between 8360528 and 4cb3ad5.

⛔ Files ignored due to path filters: 1

tests/expectations/edg_x86_64/imported.log is excluded by !**/*.log

📒 Files selected for processing: 51

src/Changes
src/il.c
src/overload.c
tests/tests/changes/.gh-338-lambda-template-param-enable-if.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-enable-if.rto/default.2.1.txt
tests/tests/changes/.gh-338-lambda-template-param-generic.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-generic.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-gnu.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-gnu.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-incomplete.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-incomplete.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.2.1.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.2.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.3.1.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.3.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.4.1.txt
tests/tests/changes/.gh-338-lambda-template-param-limits.rto/default.4.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-mismatch.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-mismatch.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-outer.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-outer.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-outer.rto/default.2.1.txt
tests/tests/changes/.gh-338-lambda-template-param-outer.rto/default.2.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-overload.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-overload.rto/default.2.1.txt
tests/tests/changes/.gh-338-lambda-template-param-qualified.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-qualified.rto/default.2.1.txt
tests/tests/changes/.gh-338-lambda-template-param-variations-negative.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-variations-negative.rto/default.1.matches.txt
tests/tests/changes/.gh-338-lambda-template-param-variations.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param-variations.rto/default.2.1.txt
tests/tests/changes/.gh-338-lambda-template-param.rto/default.1.1.txt
tests/tests/changes/.gh-338-lambda-template-param.rto/default.2.1.txt
tests/tests/changes/gh-338-lambda-template-param-enable-if.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-generic.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-gnu.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-incomplete.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-limits.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-mismatch.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-outer.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-overload.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-qualified.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-variations-negative.sft.cpp
tests/tests/changes/gh-338-lambda-template-param-variations.sft.cpp
tests/tests/changes/gh-338-lambda-template-param.sft.cpp
tests/tests/imported/clang/cpp/SemaCXX/.lambda-unevaluated.rto/default.1.1.txt
tests/tests/imported/clang/cpp/SemaCXX/.lambda-unevaluated.rto/default.2.1.txt
tests/tests/imported/gnu/cpp/cpp2a/.lambda-targ4.rto/default.1.1.txt
tests/tests/imported/gnu/cpp/cpp2a/.lambda-uneval7.rto/default.1.1.txt

💤 Files with no reviewable changes: 2

tests/tests/imported/gnu/cpp/cpp2a/.lambda-uneval7.rto/default.1.1.txt
tests/tests/imported/gnu/cpp/cpp2a/.lambda-targ4.rto/default.1.1.txt

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📝 Walkthrough

Changes

Lambda template-parameter substitution

Layer / File(s)	Summary
Bind parameters during lambda rescanning `src/il.c`	Lambda rescanning identifies unqualified template parameters, applies available arguments, and restores symbol state afterward.
Scope enable_if substitution `src/overload.c`, `tests/tests/changes/gh-338-lambda-template-param-enable-if.sft.cpp`, `tests/tests/changes/.gh-338-lambda-template-param-enable-if.rto/*`	`enable_if` substitution uses the associated template instantiation scope. Tests cover C++20 and C++23.
Validate supported lambda references `tests/tests/changes/gh-338-lambda-template-param.sft.cpp`, `tests/tests/changes/.gh-338-lambda-template-param.rto/*`	Regression tests cover declaration contexts, qualified names, overloads, nested templates, member templates, partial specializations, and GNU-specific cases.
Validate unsupported cases and diagnostics `tests/tests/imported/*`, `tests/tests/changes/gh-338-lambda-template-param-generic.sft.cpp`, `tests/tests/changes/gh-338-lambda-template-param-limits.sft.cpp`, `tests/tests/changes/gh-338-lambda-template-param-variations-negative.sft.cpp`	Tests preserve undefined-name diagnostics for generic lambdas, template parameter packs, and other unsupported cases. Affected expected diagnostics are updated. `src/Changes` documents the supported boundaries.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant FunctionTemplateSubstitution
  participant copy_template_param_lambda_expr
  participant TemplateInstantiationScope
  participant LambdaRescan
  FunctionTemplateSubstitution->>copy_template_param_lambda_expr: substitute lambda expression
  copy_template_param_lambda_expr->>TemplateInstantiationScope: locate matching template parameter list
  TemplateInstantiationScope->>copy_template_param_lambda_expr: provide available arguments
  copy_template_param_lambda_expr->>LambdaRescan: rescan cached lambda tokens
  LambdaRescan-->>FunctionTemplateSubstitution: return substituted lambda

Suggested reviewers: nv-cmeerw

Merge Risk: ⚪ Minimal · up to 4cb3a

The change appears mergeable after normal checks. The documented unsupported lambda cases remain outside its intended scope.

Pre-merge checks: 4 · 1

Failed checks: 1 inconclusive

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 71 functions across 12 files. (37 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly summarizes the main change: allowing lambdas in function-template declarations to name function template parameters. It is concise and related to the pull request objectives.
Linked Issues check		The PR satisfies the coding requirements in [`#338`]. `src/il.c` binds available template arguments while rescanning cached non-generic lambda expressions and restores the symbol state. `src/overload.c`…
Out of Scope Changes check		The changes remain within [`#338`]. The release note, regression tests, expected diagnostics, and imported-test updates support the lambda substitution behavior or record its changed diagnostics. No unr…

Full details: Linked Issues check

Explanation

The PR satisfies the coding requirements in [#338]. src/il.c binds available template arguments while rescanning cached non-generic lambda expressions and restores the symbol state. src/overload.c adds the required instantiation scope for enable_if substitution. The new tests cover the reported parameter-type case and additional declaration contexts, including default template arguments, constraints, exception specifications, and nested uses. The tests also record the documented limits for generic lambdas and template parameter packs.

Full details: Out of Scope Changes check

Explanation

The changes remain within [#338]. The release note, regression tests, expected diagnostics, and imported-test updates support the lambda substitution behavior or record its changed diagnostics. No unrelated public API or unrelated feature change appears in the supplied whole-PR summary.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 71 functions across 12 files. (37 skipped: 35 unsupported, 2 too large.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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mborland · 2026-10-22T16:12:02Z

A follow up issue has been added for generics and packs: #346

edgcpp-bot · 2026-10-22T17:29:46Z

AcknowlEDG review results updated for 4cb3ad5656b4:

Bench - C (updated)
Bench - C++ (updated)
Test - Changes (updated)

CI success.

nv-cmeerw · 2026-10-28T09:44:26Z

 }  /* substituted_cast_is_valid */


+static a_boolean lambda_tokens_name_template_param(


What's the reason to do this name matching at all? Shouldn't the scopes always be set up correctly (if nothing refers to those template parameters, then setting up the scope wasn't needed, but it shouldn't do any harm either)?

Allow lambdas to name template parameters from function decl [GH edgc…

a56953f

…pp#338]

mborland self-assigned this Oct 21, 2026

mborland added the bug Something isn't working label Oct 21, 2026

mborland removed their assignment Oct 21, 2026

Fix 32 bit testing failures []

86215d0

nv-cmeerw requested changes Oct 22, 2026

View reviewed changes

Comment thread src/Changes

Comment thread src/il.c Outdated

Comment thread src/il.c Outdated

Add additional tests from review, refactor, and state limitations []

4cb3ad5

mborland requested a review from nv-cmeerw October 22, 2026 16:06

nv-cmeerw reviewed Oct 28, 2026

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Allow lambdas to name template parameters from function decl [GH #338] - #339

Allow lambdas to name template parameters from function decl [GH #338]#339
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		} /* substituted_cast_is_valid */


		static a_boolean lambda_tokens_name_template_param(

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