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make returning a reference bound to a temporary an error in C++26 (P2748), fix for [GH #106] - #388
A return statement that binds the returned reference to a temporary is ill-formed in C++26 (P2748). The front end already detected this in check_for_returning_reference_to_local_entity, but only issued the "returning reference to local temporary" warning.
In C++26 that diagnostic is now an error. When emulating GCC before 14 or Clang before 19 it stays a warning, as in those compilers. GCC 14 and 15 with -std=c++26 error on the same cases (prvalue, converted value, member of a temporary, rvalue reference) and still only warn about returning a reference to a local variable, which is unchanged here.
Known gap, not changed here: in standard (non-GNU, non-Clang) modes, return S().x; is not detected as binding to a temporary at all (no warning before this change either), while GNU and Clang modes do catch it. A broader local regression run is in progress; I will follow up if it finds anything.
Summary by CodeRabbit
Bug Fixes
In C++26, returning a reference bound to a temporary—including a converted value or a member of a temporary—produces an error with GCC 14 or later and Clang 19 or later, except when the initializer is a similar lvalue. With older compiler versions, these cases remain warnings.
Returning a reference to a local variable continues to produce a warning.
Tests
Added coverage for reference-return diagnostics across C++23 and C++26, including compiler-version differences and conversion checks.
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Files selected for processing: 9
src/exprutil.c
tests/tests/changes/.GH106.rto/default.1.1.txt
tests/tests/changes/.GH106.rto/default.2.1.txt
tests/tests/changes/.GH106.rto/default.3.1.txt
tests/tests/changes/.GH106_a.rto/default.1.1.txt
tests/tests/changes/.GH106_a.rto/default.2.1.txt
tests/tests/changes/.GH106_a.rto/default.3.1.txt
tests/tests/changes/GH106.sft.cpp
tests/tests/changes/GH106_a.sft.cpp
Files skipped from review as they are similar to previous changes: 9
tests/tests/changes/.GH106.rto/default.3.1.txt
tests/tests/changes/.GH106_a.rto/default.2.1.txt
tests/tests/changes/.GH106_a.rto/default.3.1.txt
tests/tests/changes/.GH106.rto/default.1.1.txt
tests/tests/changes/.GH106.rto/default.2.1.txt
tests/tests/changes/.GH106_a.rto/default.1.1.txt
tests/tests/changes/GH106_a.sft.cpp
src/exprutil.c
tests/tests/changes/GH106.sft.cpp
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Walkthrough
In C++26 mode, returning a reference to a temporary produces an error when the source is not a similar-type lvalue, subject to compatibility behavior for older GCC and Clang versions. Returning a reference to a local variable remains a warning. Tests cover these cases across language modes and compiler-version settings.
Expression traversal can follow class-prvalue addressing paths and report temporary subobjects. The existing automatic-object check uses the shared traversal helper.
The diagnostic uses source similarity and temporary status to select an error or warning. Tests and expected output cover C++23 and C++26 modes, compiler-version settings, temporary expressions, and local-variable references. The pending note describes the diagnostic behavior.
sequenceDiagram
participant ExprTraversal
participant ReferenceInitializer
participant ReturnReferenceCheck
ExprTraversal->>ReferenceInitializer: report temporary status
ReferenceInitializer->>ReturnReferenceCheck: pass temporary status and similar-lvalue flag
ReturnReferenceCheck->>ReturnReferenceCheck: select error or warning
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Possibly related issues
edgcpp/compiler#106: The issue requests P2748R5 implementation, which this change adds for returned references bound to temporaries.
Possibly related PRs
edgcpp/compiler#389: It changes class-prvalue member access and temporary-subobject traversal, which affect cases tested by this change.
Suggested labels:bug
Suggested reviewers:daveedvdv-nvidia
Merge Risk: Minimal · up to 6bbbc
No actionable issue is established for this change; it is ready for normal merge checks.
Security architecture risk: Minimal · up to bd64e
The change strengthens compile-time diagnostics for references bound to temporaries. The reviewed behavior remains within expression analysis and diagnostic emission; no material security risk introduced or worsened by this PR was identified.
Retained concerns
No architecture-level concerns identified.
Security review details
Security Blast Radius
inferred — Source supplied for compilation can influence the traversed expressions and resulting diagnostic severity. The demonstrated changed outcome is compilation acceptance or diagnostics; the inspected path does not add authority over credentials, tenants, services, or persistent configuration.
Trust Boundaries and Controls
observed — The helper installs a compiler-owned callback on a cleared local block. The existing walker invokes that callback on expression nodes, honors termination, and follows the selected addressing path. The added flag changes classification preference rather than introducing external callback registration.
Pre-merge checks: 5
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Explanation
Description Check
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Title check
The title clearly identifies the main change: returning a reference bound to a temporary becomes an error in C++26 under P2748. It also includes the related issue number.
Linked Issues check
Issue #106 requires P2748R5. check_for_returning_reference_to_local_entity now reports references bound to temporaries as errors in C++26 for GCC 14+ and Clang 19+, while older compatibility modes r…
Out of Scope Changes check
The source changes support issue #106 by detecting temporary subobjects, preserving the local-variable diagnostic, and applying the C++26 severity rule. The expected-output updates and regression test…
Docstring Coverage
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
Full details: Linked Issues check
Explanation
Issue #106 requires P2748R5. check_for_returning_reference_to_local_entity now reports references bound to temporaries as errors in C++26 for GCC 14+ and Clang 19+, while older compatibility modes retain warnings. Local-variable references remain warnings. The shared traversal detects temporary subobjects, call-result members, conditional operands, and array temporaries. The GH106 and P2748R5 tests verify the diagnostics and is_convertible exception.
Full details: Out of Scope Changes check
Explanation
The source changes support issue #106 by detecting temporary subobjects, preserving the local-variable diagnostic, and applying the C++26 severity rule. The expected-output updates and regression tests verify the required cases. No unrelated change is demonstrated.
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@nv-cmeerw the return check is now skipped for the invented function of the is_convertible check, so __is_convertible_to(long, const int&) stays true in C++26, matching GCC 15 and Clang 20. It also now catches return S().x; in standard modes. changes/GH106 covers both. Could you please take another look?
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Actionable comments posted: 4
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/exprutil.c:
- Line 25306: Update traverse_addressing_subtree’s eok_question operand handling
so a local result found when follow_class_rvalue_addressing_path is enabled
records is_temp as FALSE without terminating traversal; allow a later temporary
match to set is_temp to TRUE and terminate, while preserving the existing early
termination behavior for is_glvalue_for_auto_object.
- Line 25275: In the return-expression path that uses
tblock.follow_class_rvalue_addressing_path, create or locate the top temporary
for the operand expression before checking for a reference to a local entity.
Use find_top_temporary with class-temporary creation enabled so class-prvalue
calls such as make().x are recognized by the return check.
Review comments at @src/overload.c:
- Around line 25443-25444: Keep is_glvalue_for_auto_object_or_temp_subobject’s
broader traversal for warnings, but gate the C++26 P2748 error using the lowered
expression form so only P2748 temporary-expression paths are rejected; preserve
valid indirect paths such as the field-and-array access through S{}.
- Around line 25734-25735: Update the `similar_lvalue_source` calculation to
exclude bit-field operands, using `source_operand` and the existing bit-field
detection helper. Preserve the current rvalue and similar-type checks for
non-bit-field operands.
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Actionable comments posted: 1
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Inline comments:
Review comments at @src/exprutil.c:
- Around line 25250-25259: Update traverse_expr_for_addressing_path to traverse
non-glvalue array operands for array-to-pointer decay only when
look_for_temporaries is enabled, and update examine_expr_for_auto_object to
admit array expressions under the same condition. Preserve existing behavior for
automatic-object queries when look_for_temporaries is disabled.
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Files selected for processing: 11
src/exprutil.c
src/il_walk.c
src/il_walk.h
tests/tests/changes/.GH106.rto/default.1.1.txt
tests/tests/changes/.GH106.rto/default.2.1.txt
tests/tests/changes/.GH106.rto/default.3.1.txt
tests/tests/changes/.GH106_a.rto/default.1.1.txt
tests/tests/changes/.GH106_a.rto/default.2.1.txt
tests/tests/changes/.GH106_a.rto/default.3.1.txt
tests/tests/changes/GH106.sft.cpp
tests/tests/changes/GH106_a.sft.cpp
Files skipped from review as they are similar to previous changes: 6
tests/tests/changes/.GH106.rto/default.2.1.txt
tests/tests/changes/.GH106_a.rto/default.2.1.txt
tests/tests/changes/.GH106_a.rto/default.3.1.txt
tests/tests/changes/.GH106_a.rto/default.1.1.txt
tests/tests/changes/.GH106.rto/default.3.1.txt
tests/tests/changes/.GH106.rto/default.1.1.txt
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@nv-cmeerw updated with a few more cases found in review: a reference bound to a member of a call result (return make().x;), to a temporary operand of ?:, and to an array temporary (return A{1, 2};) is now also an error in C++26, matching GCC and Clang. changes/GH106 covers them. Could you please take another look?
I checked 6bbbc0a against GCC 15 and Clang 21 (-std=c++26), with using A = int[2];. Binding the returned reference to the whole array temporary (return A{1, 2}; for A&& or const A&, and return *A{1, 2};) is now diagnosed, but returning a reference to an element still is not, in any mode:
GCC and Clang reject all three in C++26. These go through eok_subscript and then the eok_array_to_pointer case of traverse_expr_for_addressing_path, which follows only a glvalue operand, so the array temporary is never seen. CodeRabbit's original two-line suggestion handles this: when look_for_temporaries is set, follow the array prvalue operand of eok_array_to_pointer, and accept array prvalues in examine_expr_for_auto_object.
The patch below does that on top of 6bbbc0a. It keeps your eok_indirect check, which is still needed for the whole-array case. It also adds a const A& case and an element case to changes/GH106 and changes/GH106_a (with their recordings) and mentions arrays in the pending Changes entry. Since look_for_temporaries is set only by the return check, nothing else should be affected. I ran the day posture (edg_x86_64 and edg_x86_64_cp) with it. The only differences are in cwg/cwg1835 and cwg/cwg2881, and those mismatch the same way with 6bbbc0a alone.
An alternative to the eok_indirect pattern match: the whole-array case stops in the walker at an eok_lvalue_adjust from the element lvalue to the array type. So traverse_expr_for_addressing_path could also follow eok_lvalue_adjust and eok_ref_cast with a non-trivial type change when look_for_temporaries is set. That handles the whole-array case without special-casing its representation. As a side effect, the return check would also warn for return (const intptr_t&)a; with a local int a[1], as GCC's test imported/gnu/cpp/warn/Wreturn-local-addr-6 expects; that changes the test's recording.
Separately, int&& f() { return A{1, 2}[0]; } is rejected outright ("an rvalue reference cannot be bound to an lvalue"), because the front end does not implement Core issue 1213 (subscripting an array prvalue or xvalue gives an xvalue). That is why the element cases above use const int&. It is a separate problem; see #440.
diff --git a/src/exprutil.c b/src/exprutil.c
index 7a3c1faa3e..abc3c3fac6 100644
--- a/src/exprutil.c+++ b/src/exprutil.c@@ -25204,9 +25204,11 @@ that identifies an object or a prvalue that is a pointer to an object.
{
if (is_glvalue_node(expr) ||
(tblock->follow_class_rvalue_addressing_path &&
- is_class_struct_union_type(expr->type))) {+ is_class_struct_union_type(expr->type)) ||+ (tblock->look_for_temporaries && is_array_type(expr->type))) {
/* The expression passed in is a glvalue for an object (or a class
- prvalue object). */+ prvalue object, or, when looking for temporaries, an array+ prvalue). */
/* Skip some operation nodes (like value casts) that do not affect the
underlying object. */
while (is_operation_node(expr)) {
@@ -25251,8 +25253,8 @@ that identifies an object or a prvalue that is a pointer to an object.
tblock->terminate = TRUE;
} /* if */
} else if (tblock->look_for_temporaries && !is_glvalue_node(expr)) {
- /* A class prvalue (e.g., a function call) on the addressing path, for- example the object expression of "f().x": a temporary is+ /* A class or array prvalue (e.g., a function call) on the addressing+ path, for example the object expression of "f().x": a temporary is
materialized for it. */
tblock->result = TRUE;
tblock->is_temp = TRUE;
diff --git a/src/il_walk.c b/src/il_walk.c
index 744982edad..4a0d7cbc30 100644
--- a/src/il_walk.c+++ b/src/il_walk.c@@ -3409,8 +3409,10 @@ Tips for proper use of the follow_addressing_path mode:
traverse_expr(operand1, tblock);
break;
case eok_array_to_pointer:
- /* Array-to-pointer decay. Watch out for the array prvalue case. */- if (is_glvalue_node(operand1)) {+ /* Array-to-pointer decay. Watch out for the array prvalue case,+ which is followed only when looking for temporaries (e.g., for+ the "A{1, 2}" in "A{1, 2}[0]"). */+ if (is_glvalue_node(operand1) || tblock->look_for_temporaries) {
traverse_expr(operand1, tblock);
} /* if */
break;
diff --git a/src/il_walk.h b/src/il_walk.h
index ca285dab7f..734c3b7515 100644
--- a/src/il_walk.h+++ b/src/il_walk.h@@ -393,9 +393,9 @@ typedef struct an_expr_or_stmt_traversal_block {
a_boolean look_for_temporaries;
/* TRUE if a temporary anywhere on the addressing
path (e.g., in either operand of a "?" operator,
- or a class prvalue whose member is selected)- should be reported in preference to a local- variable. */+ a class prvalue whose member is selected, or an+ array prvalue) should be reported in preference+ to a local variable. */
#if HANDLE_VIRTUAL_BASES_IN_COMPLETE_CTOR_DTORS
a_variable_ptr
orig_params;
diff --git a/src/pending_changes/gh-106-return-ref-to-temporary.txt b/src/pending_changes/gh-106-return-ref-to-temporary.txt
index e50677572a..c3b88ff492 100644
--- a/src/pending_changes/gh-106-return-ref-to-temporary.txt+++ b/src/pending_changes/gh-106-return-ref-to-temporary.txt@@ -12,8 +12,10 @@ Clang versions before 19, which do the same). For example, with --c++26:
Returning a reference to a member of a temporary, as in g, was previously not
diagnosed at all in the standard (non-GNU, non-Clang) modes; before C++26 it
-now gets the same warning as the other cases. Returning a reference to a-local variable is still only a warning.+now gets the same warning as the other cases. The same is true of returning+a reference to an array temporary or to one of its elements (e.g.,+"return A{1, 2};" or "return A{1, 2}[0];" with "using A = int[2];") in all+modes. Returning a reference to a local variable is still only a warning.
As P2748 specifies, this does not apply to the conversion checked by
std::is_convertible (e.g., __is_convertible_to(long, const int&) is still
diff --git a/tests/tests/changes/.GH106.rto/default.1.1.txt b/tests/tests/changes/.GH106.rto/default.1.1.txt
index 235999934e..7b675054b4 100644
--- a/tests/tests/changes/.GH106.rto/default.1.1.txt+++ b/tests/tests/changes/.GH106.rto/default.1.1.txt@@ -31,4 +31,12 @@ fe_only -DTEST_NUMBER=1 --c++26 Test_name.c
A&& f9() { return A{1, 2}; } // Array temporary.
^
-7 errors detected in the compilation of "Test_name.c".+"Test_name.c", line 23: error: returning reference to local temporary+ const A& f10() { return A{1, 2}; } // Array temporary.+ ^++"Test_name.c", line 24: error: returning reference to local temporary+ const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.+ ^++9 errors detected in the compilation of "Test_name.c".diff --git a/tests/tests/changes/.GH106.rto/default.2.1.txt b/tests/tests/changes/.GH106.rto/default.2.1.txt
index 79cfccf0fd..0bdfdb3dd8 100644
--- a/tests/tests/changes/.GH106.rto/default.2.1.txt+++ b/tests/tests/changes/.GH106.rto/default.2.1.txt@@ -31,8 +31,16 @@ fe_only -DTEST_NUMBER=2 --c++26 --gnu_version 140000 Test_name.c
A&& f9() { return A{1, 2}; } // Array temporary.
^
-"Test_name.c", line 27: warning: returning reference to local temporary+"Test_name.c", line 23: error: returning reference to local temporary+ const A& f10() { return A{1, 2}; } // Array temporary.+ ^++"Test_name.c", line 24: error: returning reference to local temporary+ const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.+ ^++"Test_name.c", line 29: warning: returning reference to local temporary
const int *const *const &f6() { return pp; }
^
-7 errors detected in the compilation of "Test_name.c".+9 errors detected in the compilation of "Test_name.c".diff --git a/tests/tests/changes/.GH106.rto/default.3.1.txt b/tests/tests/changes/.GH106.rto/default.3.1.txt
index 04dc789411..7a9c7a756c 100644
--- a/tests/tests/changes/.GH106.rto/default.3.1.txt+++ b/tests/tests/changes/.GH106.rto/default.3.1.txt@@ -31,8 +31,16 @@ fe_only -DTEST_NUMBER=3 --c++26 --clang_version 190000 Test_name.c
A&& f9() { return A{1, 2}; } // Array temporary.
^
-"Test_name.c", line 27: warning: returning reference to local temporary+"Test_name.c", line 23: error: returning reference to local temporary+ const A& f10() { return A{1, 2}; } // Array temporary.+ ^++"Test_name.c", line 24: error: returning reference to local temporary+ const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.+ ^++"Test_name.c", line 29: warning: returning reference to local temporary
const int *const *const &f6() { return pp; }
^
-7 errors detected in the compilation of "Test_name.c".+9 errors detected in the compilation of "Test_name.c".diff --git a/tests/tests/changes/.GH106_a.rto/default.1.1.txt b/tests/tests/changes/.GH106_a.rto/default.1.1.txt
index 11bd613034..4cd7b58862 100644
--- a/tests/tests/changes/.GH106_a.rto/default.1.1.txt+++ b/tests/tests/changes/.GH106_a.rto/default.1.1.txt@@ -31,3 +31,11 @@ fe_only -DTEST_NUMBER=1 --c++23 Test_name.c
A&& f9() { return A{1, 2}; } // Array temporary.
^
+"Test_name.c", line 22: warning: returning reference to local temporary+ const A& f10() { return A{1, 2}; } // Array temporary.+ ^++"Test_name.c", line 23: warning: returning reference to local temporary+ const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.+ ^+diff --git a/tests/tests/changes/.GH106_a.rto/default.2.1.txt b/tests/tests/changes/.GH106_a.rto/default.2.1.txt
index aa65ec6463..7efff46c95 100644
--- a/tests/tests/changes/.GH106_a.rto/default.2.1.txt+++ b/tests/tests/changes/.GH106_a.rto/default.2.1.txt@@ -31,7 +31,15 @@ fe_only -DTEST_NUMBER=2 --c++26 --gnu_version 130000 Test_name.c
A&& f9() { return A{1, 2}; } // Array temporary.
^
-"Test_name.c", line 26: warning: returning reference to local temporary+"Test_name.c", line 22: warning: returning reference to local temporary+ const A& f10() { return A{1, 2}; } // Array temporary.+ ^++"Test_name.c", line 23: warning: returning reference to local temporary+ const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.+ ^++"Test_name.c", line 28: warning: returning reference to local temporary
const int *const *const &f6() { return pp; }
^
diff --git a/tests/tests/changes/.GH106_a.rto/default.3.1.txt b/tests/tests/changes/.GH106_a.rto/default.3.1.txt
index bbed1554ab..07473ff9a1 100644
--- a/tests/tests/changes/.GH106_a.rto/default.3.1.txt+++ b/tests/tests/changes/.GH106_a.rto/default.3.1.txt@@ -31,7 +31,15 @@ fe_only -DTEST_NUMBER=3 --c++26 --clang_version 180000 Test_name.c
A&& f9() { return A{1, 2}; } // Array temporary.
^
-"Test_name.c", line 26: warning: returning reference to local temporary+"Test_name.c", line 22: warning: returning reference to local temporary+ const A& f10() { return A{1, 2}; } // Array temporary.+ ^++"Test_name.c", line 23: warning: returning reference to local temporary+ const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.+ ^++"Test_name.c", line 28: warning: returning reference to local temporary
const int *const *const &f6() { return pp; }
^
diff --git a/tests/tests/changes/GH106.sft.cpp b/tests/tests/changes/GH106.sft.cpp
index 2662dc4f77..b0fa09ddf4 100644
--- a/tests/tests/changes/GH106.sft.cpp+++ b/tests/tests/changes/GH106.sft.cpp@@ -20,6 +20,8 @@ int&& f8(bool b, int i) { // A "?" operand is a temporary.
}
using A = int[2];
A&& f9() { return A{1, 2}; } // Array temporary.
+const A& f10() { return A{1, 2}; } // Array temporary.+const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.
// Core issue 2352: the reference binds directly to pp, so this is not
// diagnosed as an error.
diff --git a/tests/tests/changes/GH106_a.sft.cpp b/tests/tests/changes/GH106_a.sft.cpp
index e85401c4ad..6ec8533790 100644
--- a/tests/tests/changes/GH106_a.sft.cpp+++ b/tests/tests/changes/GH106_a.sft.cpp@@ -19,6 +19,8 @@ int&& f8(bool b, int i) { // A "?" operand is a temporary.
}
using A = int[2];
A&& f9() { return A{1, 2}; } // Array temporary.
+const A& f10() { return A{1, 2}; } // Array temporary.+const int& f11() { return A{1, 2}[0]; } // Element of an array temporary.
// Core issue 2352: the reference binds directly to pp, so this is not
// diagnosed as an error.
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Fixes #106.
A return statement that binds the returned reference to a temporary is ill-formed in C++26 (P2748). The front end already detected this in
check_for_returning_reference_to_local_entity, but only issued the "returning reference to local temporary" warning.In C++26 that diagnostic is now an error. When emulating GCC before 14 or Clang before 19 it stays a warning, as in those compilers. GCC 14 and 15 with
-std=c++26error on the same cases (prvalue, converted value, member of a temporary, rvalue reference) and still only warn about returning a reference to a local variable, which is unchanged here.Known gap, not changed here: in standard (non-GNU, non-Clang) modes,
return S().x;is not detected as binding to a temporary at all (no warning before this change either), while GNU and Clang modes do catch it. A broader local regression run is in progress; I will follow up if it finds anything.Summary by CodeRabbit