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

src/exprutil.c

@@ -25174,6 +25174,24 @@ C mode.
 }  /* still_an_lvalue */
 
 
+static void record_local_auto_object(
+                                   an_expr_or_stmt_traversal_block_ptr tblock)
+/*
+Called from examine_expr_for_auto_object when the addressing path leads to a
+local variable or parameter.  Normally that ends the traversal.  When
+tblock->look_for_temporaries is TRUE, keep going, so that a temporary on
+another part of the path (e.g., the other operand of a "?" operator) is
+still found and reported instead.
+*/
+{
+  if (!tblock->is_temp) {
+    tblock->result = TRUE;
+    tblock->is_temp = FALSE;
+    if (!tblock->look_for_temporaries) tblock->terminate = TRUE;
+  }  /* if */
+}  /* record_local_auto_object */
+
+
 static void examine_expr_for_auto_object(
                                     an_expr_node_ptr                    expr,
                                     an_expr_or_stmt_traversal_block_ptr tblock)
@@ -25217,25 +25235,28 @@ that identifies an object or a prvalue that is a pointer to an object.
            from a scope enclosing a lambda), do not consider it local in this
            context. */
         if (var->source_corresp.enclosing_routine == curr_routine_or_null()) {
-          tblock->result = TRUE;
-          tblock->is_temp = FALSE;
-          tblock->terminate = TRUE;
+          record_local_auto_object(tblock);
         }  /* if */
       }  /* if */
     } else if (node_is(expr, enk_param_ref)) {
       /* An lvalue for a parameter outside an associated function
          definition.  An enk_param_ref for "this" won't get here because
          it's not an lvalue, but the answer would be right even if it did. */
-      tblock->result = TRUE;
-      tblock->is_temp = FALSE;
-      tblock->terminate = TRUE;
+      record_local_auto_object(tblock);
     } else if (node_is(expr, enk_temp_init)) {
       if (!expr->variant.init.dynamic_init->static_temp) {
         /* An lvalue for a nonstatic temporary. */
         tblock->result = TRUE;
         tblock->is_temp = TRUE;
         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
+         materialized for it. */
+      tblock->result = TRUE;
+      tblock->is_temp = TRUE;
+      tblock->terminate = TRUE;
     }  /* if */
   } else {
     /* The expression passed in is a pointer to an object. */
@@ -25254,13 +25275,14 @@ that identifies an object or a prvalue that is a pointer to an object.
 }  /* examine_expr_for_auto_object */
 
 
-a_boolean is_glvalue_for_auto_object(an_expr_node_ptr expr,
-                                     a_boolean        *is_temp)
+static a_boolean glvalue_for_auto_object(an_expr_node_ptr expr,
+                                         a_boolean        follow_prvalues,
+                                         a_boolean        *is_temp)
 /*
-Return TRUE if expr is a glvalue whose underlying object is known to
-be an automatic (stack-based) entity.  The safe answer is FALSE.  If the
-underlying entity is a temporary, return *is_temp set to TRUE.  
-is_temp can be NULL if that information is not needed.
+Do the work for is_glvalue_for_auto_object and
+is_glvalue_for_auto_object_or_temp_subobject.  If follow_prvalues is TRUE,
+the traversal continues through class prvalues, so that a subobject of a
+temporary (e.g., "S().x") is found.
 */
 {
   a_boolean is_auto_object = FALSE;
@@ -25271,14 +25293,42 @@ is_temp can be NULL if that information is not needed.
     clear_expr_or_stmt_traversal_block(&tblock);
     tblock.process_expr = examine_expr_for_auto_object;
     tblock.follow_addressing_path = TRUE;
+    tblock.follow_class_rvalue_addressing_path = follow_prvalues;
+    tblock.look_for_temporaries = follow_prvalues;
     traverse_expr(expr, &tblock);
     is_auto_object = tblock.result;
     if (is_temp != NULL) *is_temp = tblock.is_temp;
   }  /* if */
   return is_auto_object;
+}  /* glvalue_for_auto_object */
+
+
+a_boolean is_glvalue_for_auto_object(an_expr_node_ptr expr,
+                                     a_boolean        *is_temp)
+/*
+Return TRUE if expr is a glvalue whose underlying object is known to
+be an automatic (stack-based) entity.  The safe answer is FALSE.  If the
+underlying entity is a temporary, return *is_temp set to TRUE.  
+is_temp can be NULL if that information is not needed.
+*/
+{
+  return glvalue_for_auto_object(expr, /*follow_prvalues=*/FALSE, is_temp);
 }  /* is_glvalue_for_auto_object */
 
 
+a_boolean is_glvalue_for_auto_object_or_temp_subobject(
+                                                   an_expr_node_ptr expr,
+                                                   a_boolean        *is_temp)
+/*
+Like is_glvalue_for_auto_object, but also return TRUE (with *is_temp set to
+TRUE) if expr is a glvalue for a subobject of a class prvalue temporary,
+e.g., "S().x".
+*/
+{
+  return glvalue_for_auto_object(expr, /*follow_prvalues=*/TRUE, is_temp);
+}  /* is_glvalue_for_auto_object_or_temp_subobject */
+
+
 a_boolean is_prvalue_for_auto_object(an_expr_node_ptr expr,
                                      a_boolean        *is_temp)
 /*

src/exprutil.h

@@ -3149,6 +3149,10 @@ extern a_boolean still_an_lvalue(a_type_ptr type_before_cast,
 extern a_boolean is_glvalue_for_auto_object(an_expr_node_ptr expr,
                                              a_boolean        *is_temp);
 
+extern a_boolean is_glvalue_for_auto_object_or_temp_subobject(
+                                                   an_expr_node_ptr expr,
+                                                   a_boolean        *is_temp);
+
 extern a_boolean is_prvalue_for_auto_object(an_expr_node_ptr expr,
                                             a_boolean        *is_temp);
 

src/il_walk.c

@@ -2878,6 +2878,7 @@ default values.
   tblock->complete_object_type = NULL;
   tblock->call_case = FALSE;
   tblock->is_temp = FALSE;
+  tblock->look_for_temporaries = FALSE;
 #if HANDLE_VIRTUAL_BASES_IN_COMPLETE_CTOR_DTORS
   tblock->orig_params = NULL;
   tblock->new_params = NULL;

src/il_walk.h

@@ -390,6 +390,12 @@ typedef struct an_expr_or_stmt_traversal_block {
   /* Fields used by is_glvalue_for_auto_object: */
   a_boolean	is_temp;
 			/* TRUE if the underlying object is a temporary. */
+  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. */
 #if HANDLE_VIRTUAL_BASES_IN_COMPLETE_CTOR_DTORS
   a_variable_ptr
 		orig_params;

src/overload.c

@@ -25425,23 +25425,39 @@ 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;
 
   if (is_expression_operand(operand)) {
-    if (is_glvalue_for_auto_object(operand->variant.expression, &is_temp) ||
+    if (is_glvalue_for_auto_object_or_temp_subobject(
+                                    operand->variant.expression, &is_temp) ||
         is_prvalue_for_auto_object(operand->variant.expression, &is_temp)) {
       /* 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);
+         entity (possibly a subobject of a temporary).  Use a different
+         message for temporaries and local variables. */
+      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 +25630,7 @@ 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 +25728,11 @@ 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) {
@@ -26314,9 +26336,14 @@ the conversion.
                                     source_operand,
                                     (conv_context & CCO_STATIC_LIFETIME) != 0);
   } else if ((conv_context & CCO_INITIALIZING_RETURN_VALUE) &&
+             (conv_context & CCO_TYPE_TRAITS_CHECK) == 0 &&
              !template_case) {
-    /* Check for returning a reference to a local entity. */
-    check_for_returning_reference_to_local_entity(source_operand);
+    /* Check for returning a reference to a local entity.  This is not done
+       for the invented function of the is_convertible check: P2748, which
+       makes binding a returned reference to a temporary ill-formed in C++26,
+       does not apply to it. */
+    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 +1,20 @@
+[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:
+
+  struct S { int x; };
+  const int& f() { return 1; }      // Now an error.
+  const int& g() { return S().x; }  // Now an error.
+
+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.
+
+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
+true).

tests/expectations/edg_x86_64/imported.log

@@ -856,7 +856,6 @@ 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 +1,30 @@
+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 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 17: error: returning reference to local temporary
+  const int& f7() { return make().x; }      // Member of a call's result.
+                           ^
+
+"Test_name.c", line 19: error: returning reference to local temporary
+    return b ? static_cast<int&&>(i) : S().x;
+           ^
+
+6 errors detected in the compilation of "Test_name.c".

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

@@ -0,0 +1,34 @@
+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 17: error: returning reference to local temporary
+  const int& f7() { return make().x; }      // Member of a call's result.
+                           ^
+
+"Test_name.c", line 19: error: returning reference to local temporary
+    return b ? static_cast<int&&>(i) : S().x;
+           ^
+
+"Test_name.c", line 25: warning: returning reference to local temporary
+  const int *const *const &f6() { return pp; }
+                                         ^
+
+6 errors detected in the compilation of "Test_name.c".

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

@@ -0,0 +1,34 @@
+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 17: error: returning reference to local temporary
+  const int& f7() { return make().x; }      // Member of a call's result.
+                           ^
+
+"Test_name.c", line 19: error: returning reference to local temporary
+    return b ? static_cast<int&&>(i) : S().x;
+           ^
+
+"Test_name.c", line 25: warning: returning reference to local temporary
+  const int *const *const &f6() { return pp; }
+                                         ^
+
+6 errors detected in the compilation of "Test_name.c".