Lifetime extension through a member of a temporary in GNU and Clang modes [GH #90]

src/cmd_line.c

@@ -4317,7 +4317,9 @@ default mode (e.g., exception handling).
     if (!option_kind_used[(int)optk_digit_separators]) {
       digit_separators_enabled = TRUE;
     }  /* if */
-    if (rvalue_references_enabled && !gpp_mode && !clang_mode) {
+    if (rvalue_references_enabled &&
+        !gpp_version_is(<90000) && !clangcpp_version_is(<40000)) {
+      /* A field selection on a class prvalue is an xvalue (CWG 616). */
       selection_from_prvalue_is_xvalue = TRUE;
     }  /* if */
     relaxed_constexpr_enabled = TRUE;

src/exprutil.c

@@ -25271,6 +25271,10 @@ 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;
+    /* A prvalue.field xvalue (CWG 616) names a subobject of the class
+       prvalue, so the class object must be considered as well. */
+    tblock.follow_class_rvalue_addressing_path =
+                                         selection_from_prvalue_is_xvalue;
     traverse_expr(expr, &tblock);
     is_auto_object = tblock.result;
     if (is_temp != NULL) *is_temp = tblock.is_temp;
@@ -25321,6 +25325,10 @@ underlying entity is a temporary, return *is_temp set to TRUE.
     clear_expr_or_stmt_traversal_block(&tblock);
     tblock.process_expr = examine_expr_for_auto_object;
     tblock.follow_addressing_path = TRUE;
+    /* A pointer into a prvalue.field xvalue (CWG 616) addresses a
+       subobject of the class prvalue. */
+    tblock.follow_class_rvalue_addressing_path =
+                                         selection_from_prvalue_is_xvalue;
     traverse_expr(expr, &tblock);
     is_addr_of_auto = tblock.result;
     *is_temp = tblock.is_temp;  

src/il_walk.c

@@ -3266,10 +3266,10 @@ Tips for proper use of the follow_addressing_path mode:
       switch (op) {
         case eok_dot_field:
           /* x.y:  Follow x. */
-          /* We don't have to test for the prvalue.field case here because its
-             result is a prvalue and therefore wouldn't get here, unless
-             follow_class_rvalue_addressing_path is TRUE, in which case
-             we want to keep going anyway. */
+          /* We don't have to test for the prvalue.field case here: it gets
+             here only if follow_class_rvalue_addressing_path is TRUE or if
+             its result is an xvalue (selection_from_prvalue_is_xvalue,
+             CWG 616), and in both cases we want to keep going. */
           traverse_expr(operand1, tblock);
           break;
         case eok_points_to_field:
@@ -3278,10 +3278,10 @@ Tips for proper use of the follow_addressing_path mode:
           break;
         case eok_pm_field:
           /* x.*pm:  Follow x. */
-          /* We don't have to test for the prvalue.*pm case here because its
-             result is a prvalue and therefore wouldn't get here, unless
-             follow_class_rvalue_addressing_path is TRUE, in which case
-             we want to keep going anyway. */
+          /* We don't have to test for the prvalue.*pm case here: it gets
+             here only if follow_class_rvalue_addressing_path is TRUE or if
+             its result is an xvalue (selection_from_prvalue_is_xvalue,
+             CWG 616), and in both cases we want to keep going. */
           traverse_expr(operand1, tblock);
           break;
         case eok_pm_points_to_field:

src/interpret.c

@@ -23649,10 +23649,21 @@ after the constructor body has been interpreted.
       p_arg_ptr += 1;
       arg_size += 1;
     }  /* for */
-    if (dip->variant.constructor.value_initialization) {
-      /* If this is for value initialization, clear the storage first.
-         Do not, however, overwrite the first word (which may record the
-         inheritance hierarchy). */
+    if (!dip->variant.constructor.value_initialization) {
+      /* Nothing to do. */
+    } else if (!type_has_user_provided_default_constructor(class_type)) {
+      /* Value initialization of a class without a user-provided default
+         constructor zero-initializes the object before calling the
+         constructor, which need not initialize every subobject (e.g., a
+         member of a class whose implicit default constructor calls a
+         user-provided base class constructor).  Mark the subobjects as
+         initialized, so that reading them is valid. */
+      init_subobject_to_zero(ips, result_storage, class_type,
+                             complete_object);
+    } else {
+      /* Otherwise, the object is not zero-initialized, but clear the
+         storage anyway.  Do not, however, overwrite the first word (which
+         may record the inheritance hierarchy). */
       a_byte_count  n_class_bytes;
       a_boolean     dummy = TRUE;
       n_class_bytes = f_value_bytes_for_type(ips, class_type, &dummy);
@@ -31878,8 +31889,9 @@ the value representation of the integer value.
                 result_addr = *(a_constexpr_address*)opnd1_value;
               } else {
                 /* The first operand is a class rvalue: Create an address for
-                   it. */
-                mark_complete_object_initialized(opnd1_value);
+                   it.  Do not mark the object as fully initialized: since
+                   C++20, a constexpr constructor can leave members
+                   uninitialized, and reading those must be diagnosed. */
                 clear_address(&result_addr, opnd1_value);
                 result_addr.alloc_seq_number = ips->storage_stack.alloc_seq();
               }  /* if */
@@ -32003,8 +32015,9 @@ the value representation of the integer value.
                 result_addr = *(a_constexpr_address*)opnd1_value;
               } else {
                 /* The first operand is a class rvalue: Create an address for
-                   it. */
-                mark_complete_object_initialized(opnd1_value);
+                   it.  Do not mark the object as fully initialized: since
+                   C++20, a constexpr constructor can leave members
+                   uninitialized, and reading those must be diagnosed. */
                 clear_address(&result_addr, opnd1_value);
                 result_addr.alloc_seq_number = ips->storage_stack.alloc_seq();
               }  /* if */

src/pending_changes/gh-90-lifetime-extension-member-of-temporary.txt

@@ -0,0 +1,20 @@
+[GH #90]
+GNU and Clang C++ modes: lifetime extension through a member of a temporary
+
+When emulating GCC 9 or later or Clang 4 or later, selecting a member of a
+class prvalue now produces an xvalue (Core issue 616), as it already did in
+standard C++14 and later modes.  Binding a reference to the member now
+extends the lifetime of the whole temporary.  For example, with
+--g++ --gnu_version=150000:
+
+  struct A { int b; ~A(); };
+  const int &r = A().b;  // A() was previously destroyed here.
+
+Constant evaluation also no longer treats every member of a class prvalue
+as initialized when one is selected with "." or ".*".  Since C++20, a
+constexpr constructor can leave members uninitialized, and reading such a
+member is now diagnosed instead of producing an indeterminate value.  For
+example, with --c++20:
+
+  struct W { int n; constexpr W() {} };
+  static_assert(W().n == 0);  // Error: n is uninitialized.

src/pending_changes/gh-90-value-init-zeroed-members.txt

@@ -0,0 +1,12 @@
+[GH #90]
+Constexpr reads of zero-initialized members after value initialization
+
+Value initialization of a class without a user-provided default constructor
+zero-initializes the object before calling the constructor.  During
+constant evaluation, the zeroed members were not treated as initialized, so
+reading one that the constructor does not set was rejected.  For example,
+with --c++20:
+
+  struct A { constexpr A() {} int x; };
+  struct B : A { int y; };
+  static_assert(B().x == 0);  // Previously rejected.

tests/expectations/edg_x86_64/imported.log

@@ -7077,7 +7077,6 @@ gnu/cpp/cpp2a/consteval25.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/consteval32.sft.cpp:fn:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/constexpr-99287.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/constexpr-dynamic13.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
-gnu/cpp/cpp2a/constexpr-init18.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/constexpr-inst1.sft.cpp:fn:-DTEST_NUMBER=1 --c++14:FAIL:
 gnu/cpp/cpp2a/constexpr-inst2.sft.cpp:fp:-DTEST_NUMBER=1 --c++11:FAIL:
 gnu/cpp/cpp2a/constexpr-new11/test.mft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
@@ -7757,7 +7756,6 @@ gnu/cpp/init/new22.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 gnu/cpp/init/new39.sft.cpp:rp:-DTEST_NUMBER=1:FAIL:
 gnu/cpp/init/new40.sft.cpp:rp:-DTEST_NUMBER=1:FAIL:
 gnu/cpp/init/null1.sft.cpp:fp:-DTEST_NUMBER=1 --c++11:FAIL:
-gnu/cpp/init/ref19.sft.cpp:rp:-DTEST_NUMBER=1:FAIL:
 gnu/cpp/init/ref23.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 gnu/cpp/init/reference2.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 gnu/cpp/init/self1.sft.cpp:fn:-DTEST_NUMBER=1 --c++11:FAIL:

tests/tests/cwg/.cwg616.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++14 Test_name.c

tests/tests/cwg/.cwg616.rto/default.2.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=2 --c++14 --g++ --gnu_version=150000 Test_name.c

tests/tests/cwg/.cwg616.rto/default.3.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=3 --c++14 --clang --clang_version=190100 Test_name.c

tests/tests/cwg/cwg616.sft.cpp

@@ -0,0 +1,14 @@
+//type:fp
+//options_all:--c++14
+//options::--g++ --gnu_version=150000:--clang --clang_version=190100
+template <class T> struct is_xvalue { static const bool value = false; };
+template <class T> struct is_xvalue<T &&> { static const bool value = true; };
+struct A { int b; };
+// Selecting a member of a class prvalue produces an xvalue.
+static_assert(is_xvalue<decltype((A().b))>::value, "");
+static_assert(is_xvalue<decltype((A().*&A::b))>::value, "");
+
+//cwg: 616
+//title: Definition of "indeterminate value"
+//meeting: Bristol 4/13
+//edg_status: Passes

tests/tests/imported/clang/cpp/CXX/dcl.decl/dcl.init/dcl.init.general/.p16-cxx20.rto/default.1.1.txt

@@ -1,11 +1,4 @@
 fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_version 999999 --c++20 Test_name.c
-"Test_name.c", line 16: error: expression must have a constant value
-      static_assert(B().x == 0);
-                    ^
-"Test_name.c", line 16: note: access to uninitialized object
-      static_assert(B().x == 0);
-                       ^
-
 "Test_name.c", line 27: error: class "P0960R3::A" has no suitable copy constructor
       A a1(va);
            ^
@@ -144,4 +137,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       D d8 = {vdd}; // expected-error {{no matching constructor for initialization of 'D'}}
               ^
 
-31 errors detected in the compilation of "Test_name.c".
+30 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/default.3.1.txt

@@ -43,18 +43,6 @@ fe_only -DTEST_NUMBER=3 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -635,4 +623,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-107 errors detected in the compilation of "Test_name.c".
+104 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/default.4.1.txt

@@ -43,18 +43,6 @@ fe_only -DTEST_NUMBER=4 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -628,4 +616,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-107 errors detected in the compilation of "Test_name.c".
+104 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/default.5.1.txt

@@ -43,18 +43,6 @@ fe_only -DTEST_NUMBER=5 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -612,4 +600,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-103 errors detected in the compilation of "Test_name.c".
+100 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/default.6.1.txt

@@ -43,18 +43,6 @@ fe_only -DTEST_NUMBER=6 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -600,4 +588,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-100 errors detected in the compilation of "Test_name.c".
+97 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/default.7.1.txt

@@ -43,18 +43,6 @@ fe_only -DTEST_NUMBER=7 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -600,4 +588,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-100 errors detected in the compilation of "Test_name.c".
+97 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/edg_x86_32.3.1.txt

@@ -47,18 +47,6 @@ fe_only -DTEST_NUMBER=3 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -639,4 +627,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-107 errors detected in the compilation of "Test_name.c".
+104 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/CXX/drs/.cwg6xx.rto/edg_x86_32.4.1.txt

@@ -47,18 +47,6 @@ fe_only -DTEST_NUMBER=4 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       const std::type_info &an2 = typeid(n + 1);
                             ^
 
-"Test_name.c", line 213: error: invalid redeclaration of type name "cwg616::T" (declared at line 212)
-    using T = decltype((static_cast<S&&>(s).n));
-          ^
-
-"Test_name.c", line 214: error: invalid redeclaration of type name "cwg616::T" (declared at line 213)
-    using T = decltype((f().n));
-          ^
-
-"Test_name.c", line 215: error: invalid redeclaration of type name "cwg616::T" (declared at line 214)
-    using T = decltype(S().*&S::n);
-          ^
-
 "Test_name.c", line 246: error: incomplete type "int []" is not allowed
       sizeof(x);
             ^
@@ -632,4 +620,4 @@ At end of source: error: expected a "}"
     constexpr int c = true || f();
                   ^
 
-107 errors detected in the compilation of "Test_name.c".
+104 errors detected in the compilation of "Test_name.c".