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keep C++11 constexpr templates with C++14-style statements usable as constants in GNU and Clang modes, fix for [GH #356] - #386
In GNU (12.0+) and Clang (3.3+) C++11 modes, a statement that C++14 allows in a constexpr constructor or function template (an if, for example) only gets a warning, but statement() still set constexpr_ruled_out, so it could not be used in constant expressions.
statement() now tracks whether a statement would be allowed under the C++14 rules. If it is, it no longer rules out a constexpr constructor (GNU and Clang modes) or constexpr function (Clang mode only). This matches GCC 14/15 and Clang 18 with -std=c++11: both evaluate such a constructor, and only Clang evaluates such a function. Statements C++14 does not allow either (goto, asm, try, ...) still rule it out. Declarations behave as before, since their C++14 checks only run when relaxed constexpr is enabled. Other modes are unchanged.
Summary by CodeRabbit
Bug Fixes
Fixed C++11-mode handling of C++14-style statements in constexpr constructors, allowing eligible constructors to be used in constant expressions.
Eligible constexpr function templates can now be used in constant expressions in Clang mode. GNU mode still does not allow this, and goto continues to prevent constant-expression use.
These statements may still produce compiler warnings.
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Walkthrough
The change tracks relaxed-constexpr eligibility for statements and uses it when determining constant-evaluation eligibility for template constructors and functions. New regression tests cover C++11 behavior in GNU and Clang modes.
statement tracks relaxed-constexpr eligibility separately and updates it for declarations and statements, including goto, asm, and try.
Apply eligibility and validate constexpr use src/statements.c, tests/tests/changes/GH356.sft.cpp, tests/tests/changes/.GH356.rto/*, src/pending_changes/gh-356-cpp11-constexpr-cpp14-body.txt
For template constructors and functions, the diagnostic path uses relaxed-constexpr eligibility to determine whether constant evaluation remains possible. Tests cover constructor and function templates, plus a goto case. Expected compiler outputs and a change description are added.
The change has no established merge-blocking risk.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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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 4 functions across 2 files. (3 skipped: 3 …
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
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Description Check
Check skipped - CodeRabbit’s high-level summary is enabled.
Title check
The title clearly identifies the main change: preserving C++11 constexpr usability for templates with C++14-style statements in GNU and Clang modes, and it references GH #356.
Linked Issues check
Issue #356 requires C++11 constexpr templates with C++14-allowed statements to remain usable in constant expressions when GNU or Clang downgrades the diagnostic to a warning. src/statements.c tracks…
Out of Scope Changes check
The changed compiler logic directly implements issue #356. The GH356 regression test and expected outputs verify the required GNU and Clang behavior. The pending-change note documents the same fix. …
Full details: Linked Issues check
Explanation
Issue #356 requires C++11 constexpr templates with C++14-allowed statements to remain usable in constant expressions when GNU or Clang downgrades the diagnostic to a warning. src/statements.c tracks allowed_by_relaxed_constexpr and preserves constexpr_ruled_out for the affected constructor case in GNU and Clang, and for the function-template case in Clang. It still rules out C++14-disallowed statements such as goto. tests/tests/changes/GH356.sft.cpp covers the reported constructor, a constructor with return, the Clang-only function behavior, and rejected goto behavior.
Full details: Out of Scope Changes check
Explanation
The changed compiler logic directly implements issue #356. The GH356 regression test and expected outputs verify the required GNU and Clang behavior. The pending-change note documents the same fix. No unrelated changes appear in the reviewed diff.
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 4 functions across 2 files. (3 skipped: 3 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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Actionable comments posted: 2
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Inline comments:
Review comments at @src/statements.c:
- Around line 8095-8103: Compute C++14 declaration eligibility separately from
the C++11 result produced by decl_statement, and use that result for
allowed_by_relaxed_constexpr while preserving can_appear_in_constexpr_body
unchanged. Apply the repository’s existing mode and version rules, including
pre-C++23 rejection of uninitialized, static, and thread-local locals.
- Around line 8167-8180: Update the tok_return handling for constexpr functions
so constexpr constructors set allowed_by_relaxed_constexpr to FALSE, preventing
bare returns from preserving constexpr eligibility. Keep the existing behavior
for non-constructor constexpr functions and the diagnostic handling for return
expressions.
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Reviewing files that changed from the base of the PR and between 35b7c47 and 68a3128.
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Fixes #356.
In GNU (12.0+) and Clang (3.3+) C++11 modes, a statement that C++14 allows in a constexpr constructor or function template (an
if, for example) only gets a warning, butstatement()still setconstexpr_ruled_out, so it could not be used in constant expressions.statement()now tracks whether a statement would be allowed under the C++14 rules. If it is, it no longer rules out a constexpr constructor (GNU and Clang modes) or constexpr function (Clang mode only). This matches GCC 14/15 and Clang 18 with-std=c++11: both evaluate such a constructor, and only Clang evaluates such a function. Statements C++14 does not allow either (goto,asm,try, ...) still rule it out. Declarations behave as before, since their C++14 checks only run when relaxed constexpr is enabled. Other modes are unchanged.Summary by CodeRabbit
constexprconstructors, allowing eligible constructors to be used in constant expressions.constexprfunction templates can now be used in constant expressions in Clang mode. GNU mode still does not allow this, andgotocontinues to prevent constant-expression use.