make returning a reference bound to a temporary an error in C++26 (P2748), fix for [GH #106]

src/overload.c

@@ -25425,10 +25425,16 @@ non-const to the indicated (rvalue) operand.
 }  /* microsoft_can_bind_ref_to_rvalue */
 
 
-static void check_for_returning_reference_to_local_entity(an_operand *operand)
+static void check_for_returning_reference_to_local_entity(
+                                          an_operand  *operand,
+                                          a_boolean   similar_lvalue_source)
 /*
 operand is the entity being bound to a reference in a return statement.
-Issue a warning if it is a local entity.
+Issue a warning if it is a local entity.  Binding the returned reference to
+a temporary is an error in C++26 (P2748).  similar_lvalue_source is TRUE if
+the original initializer was an lvalue of a type similar to the referenced
+type; the reference should then bind to it directly (Core issue 2352), so
+a temporary found here only gets the warning.
 */
 {
   a_boolean is_temp;
@@ -25439,9 +25445,18 @@ Issue a warning if it is a local entity.
       /* The expression is a glvalue or class prvalue (object) for a local
          entity.  Use a different message for temporaries and local
          variables. */
-      expr_pos_warning(is_temp ? ec_return_ref_init_requires_temp :
-                                 ec_returning_ref_to_local_variable,
-                       &operand->position);
+      if (!is_temp) {
+        expr_pos_warning(ec_returning_ref_to_local_variable,
+                         &operand->position);
+      } else if (cpp26_mode && !similar_lvalue_source &&
+                 !gpp_version_is(<140000) && !clangcpp_version_is(<190000)) {
+        /* GCC 14 and Clang 19 implement P2748.  Earlier versions only issue
+           a warning. */
+        expr_pos_error(ec_return_ref_init_requires_temp, &operand->position);
+      } else {
+        expr_pos_warning(ec_return_ref_init_requires_temp,
+                         &operand->position);
+      }  /* if */
     }  /* if */
   }  /* if */
 }  /* check_for_returning_reference_to_local_entity */
@@ -25614,6 +25629,7 @@ the conversion.
   a_boolean    ref_to_const = FALSE;
   a_boolean    ref_to_const_volatile = FALSE;
   a_boolean    operand_was_rvalue, operand_is_function_lvalue;
+  a_boolean    similar_lvalue_source;
   a_boolean    warn = FALSE, template_case = FALSE;
   a_boolean    initializing_variable =
                        (conv_context & CCO_INITIALIZING_VARIABLE) != 0;
@@ -25711,6 +25727,11 @@ the conversion.
   }  /* if */
   base_dest_type = type_pointed_to(dest_type);
   operand_was_rvalue = is_an_rvalue(source_operand);
+  /* An lvalue whose type is similar to the referenced type, e.g., an "int **"
+     lvalue for a "const int *const *const &".  The reference binds directly
+     to it (Core issue 2352). */
+  similar_lvalue_source = !operand_was_rvalue &&
+                          types_are_similar(orig_source_type, base_dest_type);
   operand_is_function_lvalue = is_a_function_designator(source_operand);
   adj_base_dest_type = base_dest_type;
   if (bitwise_assignment_param) {
@@ -26316,7 +26337,8 @@ the conversion.
   } else if ((conv_context & CCO_INITIALIZING_RETURN_VALUE) &&
              !template_case) {
     /* Check for returning a reference to a local entity. */
-    check_for_returning_reference_to_local_entity(source_operand);
+    check_for_returning_reference_to_local_entity(source_operand,
+                                                  similar_lvalue_source);
   }  /* if */
   if (!leave_as_object) {
     /* Final step: add the reference-to to turn the glvalue or class prvalue

src/pending_changes/gh-106-return-ref-to-temporary.txt

@@ -0,0 +1,11 @@
+[GH #106]
+C++26: returning a reference bound to a temporary is not allowed
+
+A return statement that binds the returned reference to a temporary is not
+allowed in C++26 (P2748).  The front end only issued a warning.  It is now an
+error in C++26 (still a warning when emulating GCC versions before 14 or
+Clang versions before 19, which do the same).  For example, with --c++26:
+
+  const int& f() { return 1; }  // Now an error.
+
+Returning a reference to a local variable is still only a warning.

tests/expectations/edg_x86_64/imported.log

@@ -858,7 +858,6 @@ clang/cpp/CXX/special/class.ctor/p1.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 clang/cpp/CXX/special/class.temporary/p1.sft.cpp:fn:-DTEST_NUMBER=1 --c++11:FAIL:
 clang/cpp/CXX/special/class.temporary/p6.sft.cpp:fp:-DTEST_NUMBER=1 --c++17:FAIL:
 clang/cpp/CXX/special/class.temporary/p6.sft.cpp:fp:-DTEST_NUMBER=2 --c++23:FAIL:
-clang/cpp/CXX/stmt.stmt/stmt.return/p6.sft.cpp:fn:-DTEST_NUMBER=1 --c++26:FAIL:
 clang/cpp/CXX/stmt.stmt/stmt.select/stmt.if/p2.sft.cpp:fn:-DTEST_NUMBER=2 --c++17 -DUNDEFINED:FAIL:
 clang/cpp/CXX/stmt.stmt/stmt.select/stmt.switch/p4.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 clang/cpp/CXX/temp/temp.constr/temp.constr.order/non-template-functions.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:

tests/tests/changes/.GH106.rto/default.1.1.txt

@@ -0,0 +1,18 @@
+fe_only -DTEST_NUMBER=1 --c++26 Test_name.c
+"Test_name.c", line 11: error: returning reference to local temporary
+  const int& f1() { return 1; }             // Temporary: diagnosed.
+                           ^
+
+"Test_name.c", line 12: error: returning reference to local temporary
+  const int& f2(long l) { return l; }       // Temporary from a conversion.
+                                 ^
+
+"Test_name.c", line 14: error: returning reference to local temporary
+  int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+                      ^
+
+"Test_name.c", line 15: warning: returning reference to local variable
+  const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+                                      ^
+
+3 errors detected in the compilation of "Test_name.c".

tests/tests/changes/.GH106.rto/default.2.1.txt

@@ -0,0 +1,26 @@
+fe_only -DTEST_NUMBER=2 --c++26 --gnu_version 140000 Test_name.c
+"Test_name.c", line 11: error: returning reference to local temporary
+  const int& f1() { return 1; }             // Temporary: diagnosed.
+                           ^
+
+"Test_name.c", line 12: error: returning reference to local temporary
+  const int& f2(long l) { return l; }       // Temporary from a conversion.
+                                 ^
+
+"Test_name.c", line 13: error: returning reference to local temporary
+  const int& f3() { return S().x; }         // Member of a temporary.
+                           ^
+
+"Test_name.c", line 14: error: returning reference to local temporary
+  int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+                      ^
+
+"Test_name.c", line 15: warning: returning reference to local variable
+  const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+                                      ^
+
+"Test_name.c", line 20: warning: returning reference to local temporary
+  const int *const *const &f6() { return pp; }
+                                         ^
+
+4 errors detected in the compilation of "Test_name.c".

tests/tests/changes/.GH106.rto/default.3.1.txt

@@ -0,0 +1,26 @@
+fe_only -DTEST_NUMBER=3 --c++26 --clang_version 190000 Test_name.c
+"Test_name.c", line 11: error: returning reference to local temporary
+  const int& f1() { return 1; }             // Temporary: diagnosed.
+                           ^
+
+"Test_name.c", line 12: error: returning reference to local temporary
+  const int& f2(long l) { return l; }       // Temporary from a conversion.
+                                 ^
+
+"Test_name.c", line 13: error: returning reference to local temporary
+  const int& f3() { return S().x; }         // Member of a temporary.
+                           ^
+
+"Test_name.c", line 14: error: returning reference to local temporary
+  int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+                      ^
+
+"Test_name.c", line 15: warning: returning reference to local variable
+  const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+                                      ^
+
+"Test_name.c", line 20: warning: returning reference to local temporary
+  const int *const *const &f6() { return pp; }
+                                         ^
+
+4 errors detected in the compilation of "Test_name.c".

tests/tests/changes/.GH106_a.rto/default.1.1.txt

@@ -0,0 +1,17 @@
+fe_only -DTEST_NUMBER=1 --c++23 Test_name.c
+"Test_name.c", line 10: warning: returning reference to local temporary
+  const int& f1() { return 1; }             // Temporary: diagnosed.
+                           ^
+
+"Test_name.c", line 11: warning: returning reference to local temporary
+  const int& f2(long l) { return l; }       // Temporary from a conversion.
+                                 ^
+
+"Test_name.c", line 13: warning: returning reference to local temporary
+  int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+                      ^
+
+"Test_name.c", line 14: warning: returning reference to local variable
+  const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+                                      ^
+

tests/tests/changes/.GH106_a.rto/default.2.1.txt

@@ -0,0 +1,25 @@
+fe_only -DTEST_NUMBER=2 --c++26 --gnu_version 130000 Test_name.c
+"Test_name.c", line 10: warning: returning reference to local temporary
+  const int& f1() { return 1; }             // Temporary: diagnosed.
+                           ^
+
+"Test_name.c", line 11: warning: returning reference to local temporary
+  const int& f2(long l) { return l; }       // Temporary from a conversion.
+                                 ^
+
+"Test_name.c", line 12: warning: returning reference to local temporary
+  const int& f3() { return S().x; }         // Member of a temporary.
+                           ^
+
+"Test_name.c", line 13: warning: returning reference to local temporary
+  int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+                      ^
+
+"Test_name.c", line 14: warning: returning reference to local variable
+  const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+                                      ^
+
+"Test_name.c", line 19: warning: returning reference to local temporary
+  const int *const *const &f6() { return pp; }
+                                         ^
+

tests/tests/changes/.GH106_a.rto/default.3.1.txt

@@ -0,0 +1,25 @@
+fe_only -DTEST_NUMBER=3 --c++26 --clang_version 180000 Test_name.c
+"Test_name.c", line 10: warning: returning reference to local temporary
+  const int& f1() { return 1; }             // Temporary: diagnosed.
+                           ^
+
+"Test_name.c", line 11: warning: returning reference to local temporary
+  const int& f2(long l) { return l; }       // Temporary from a conversion.
+                                 ^
+
+"Test_name.c", line 12: warning: returning reference to local temporary
+  const int& f3() { return S().x; }         // Member of a temporary.
+                           ^
+
+"Test_name.c", line 13: warning: returning reference to local temporary
+  int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+                      ^
+
+"Test_name.c", line 14: warning: returning reference to local variable
+  const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+                                      ^
+
+"Test_name.c", line 19: warning: returning reference to local temporary
+  const int *const *const &f6() { return pp; }
+                                         ^
+

tests/tests/changes/GH106.sft.cpp

@@ -0,0 +1,20 @@
+//type:fn
+//options:--c++26:--c++26 --gnu_version 140000:--c++26 --clang_version 190000
+//remark:[GH #106] C++26: returning a reference bound to a temporary
+//
+// P2748 makes a return statement that binds the returned reference to a
+// temporary ill-formed in C++26.  Returning a reference to a local variable
+// is still only a warning.
+
+struct S { int x; };
+
+const int& f1() { return 1; }             // Temporary: diagnosed.
+const int& f2(long l) { return l; }       // Temporary from a conversion.
+const int& f3() { return S().x; }         // Member of a temporary.
+int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+
+// Core issue 2352: the reference binds directly to pp, so this is not
+// diagnosed as an error.
+int **pp;
+const int *const *const &f6() { return pp; }

tests/tests/changes/GH106_a.sft.cpp

@@ -0,0 +1,19 @@
+//type:fp
+//options:--c++23:--c++26 --gnu_version 130000:--c++26 --clang_version 180000
+//remark:[GH #106] Returning a reference bound to a temporary before C++26
+//
+// Before C++26, and in C++26 when emulating GCC before 14 or Clang before
+// 19, returning a reference bound to a temporary is only a warning.
+
+struct S { int x; };
+
+const int& f1() { return 1; }             // Temporary: diagnosed.
+const int& f2(long l) { return l; }       // Temporary from a conversion.
+const int& f3() { return S().x; }         // Member of a temporary.
+int&& f4() { return 1; }                  // Rvalue reference to a temporary.
+const int& f5() { int i = 0; return i; }  // Local variable: only a warning.
+
+// Core issue 2352: the reference binds directly to pp, so this is not
+// diagnosed as an error.
+int **pp;
+const int *const *const &f6() { return pp; }

tests/tests/cwg/.p2748R5.rto/default.1.1.txt

@@ -1,9 +1,9 @@
 fe_only -DTEST_NUMBER=1 --c++26 Test_name.c
-"Test_name.c", line 4: warning: returning reference to local temporary
+"Test_name.c", line 4: error: returning reference to local temporary
       return 42;  // ill-formed
              ^
 
-"Test_name.c", line 8: warning: returning reference to local temporary
+"Test_name.c", line 8: error: returning reference to local temporary
       return x;   // ill-formed
              ^
 
@@ -12,4 +12,4 @@ At end of source: error: expected a "}"
   auto&& f3() {
               ^
 
-1 error detected in the compilation of "Test_name.c".
+3 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/stmt.stmt/stmt.return/.p6.rto/default.1.1.txt

@@ -1,9 +1,10 @@
 fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_version 999999 --c++26 Test_name.c
-"Test_name.c", line 6: warning: returning reference to local temporary
+"Test_name.c", line 6: error: returning reference to local temporary
     return 42; // expected-error{{returning reference to local temporary object}}
            ^
 
-"Test_name.c", line 10: warning: returning reference to local temporary
+"Test_name.c", line 10: error: returning reference to local temporary
     return x; // expected-error{{returning reference to local temporary object}}
            ^
 
+2 errors detected in the compilation of "Test_name.c".