Constant evaluation: find overriders below non-overriding classes [GH #421]

A virtual call is now dispatched by finding the outermost enclosing
subobject from which it can be dispatched, and looking up the final
overrider in the override information of that class's base class entry for
the subobject of the callee's class (identified by its offset).  Previously,
only the entries for the direct base classes along the way were consulted,
so an overrider declared in a class derived from the callee's class through
a class that does not itself override the function was not found.

Author: daveedvdv-nvidia

src/interpret.c

@@ -22554,49 +22554,79 @@ if *p_this_arg is not initialized.)
        constructor has not completely run yet, or because the object has
        been partially destroyed). */
   } else {
-    /* The final overrider is a member of the class of the subobject from which
-       it is found, and that subobject is the object for the call. */
-    a_byte  *callee_subobj = subobj;
+    /* Find the outermost enclosing subobject from which the call is
+       dispatched (outer_subobj, of class outer_class). */
+    a_byte            *outer_subobj = subobj;
+    a_type_ptr        outer_class = NULL;
+    a_type_ptr        callee_class = skip_typerefs(parent_class_of(*p_callee));
+    a_base_class_ptr  bcp;
     do {
-      a_base_class_ptr  bcp = *(a_base_class_ptr*)subobj;
-      a_byte_count      offset, retval_offset_step = 0;
-      a_routine_ptr     overrider = NULL;
-      an_overriding_virtual_function_ptr
-                        ovfp;
+      a_byte_count  offset;
+      bcp = *(a_base_class_ptr*)outer_subobj;
       if (bcp == NULL) {
         /* A most derived subobject (i.e., a field or array element).  We are
            done. */
         break;
       }  /* if */
-      check_assertion(!bcp->is_pack_expansion);
-      ovfp = bcp->variant.overriding_virtual_functions;
-      for (; ovfp != NULL; ovfp = ovfp->next) {
-        if (ovfp->primary_function == *p_callee) {
+      get_mapped_byte_count(&persistent_map, bcp, offset);
+      outer_subobj -= offset;
+      outer_class = bcp->derived_class;
+      if (!subobject_is_initialized(outer_subobj, complete_obj)) {
+        /* The next-enclosing subobject is not constructed.  Do not dispatch
+           from anything enclosing it. */
+        break;
+      }  /* if */
+    } while (outer_subobj != complete_obj);
+    if (outer_class != NULL) {
+      /* The override information for the subobject of the callee's class
+         in outer_class gives the final overrider in outer_class (which may
+         be a member of an intermediate class).  The base class entry for
+         that subobject is identified by its offset, since the callee's
+         class can be a base class of outer_class more than once. */
+      an_overriding_virtual_function_ptr
+                        ovfp;
+      for (bcp = base_classes_of(outer_class); bcp != NULL; bcp = bcp->next) {
+        if (!bcp->is_pack_expansion &&
+            same_entities(skip_typerefs(bcp->type), callee_class) &&
+            outer_subobj + compute_interpreter_base_offset(
+                                         bcp, (a_base_class_ptr*)NULL) ==
+                                                                    subobj) {
+          break;
+        }  /* if */
+      }  /* for */
+      check_assertion(bcp != NULL);
+      do {
+        /* If the overrider found is a member of an intermediate class, it
+           is normally the final overrider, but a function that overrides
+           it without being recorded as overriding the original function
+           (e.g., because of an invalid covariant return type) is found
+           through the base class entry for that intermediate class. */
+        ovfp = bcp->variant.overriding_virtual_functions;
+        while (ovfp != NULL && ovfp->primary_function != *p_callee) {
+          ovfp = ovfp->next;
+        }  /* while */
+        if (ovfp != NULL) {
           a_base_class_ptr  ret_base = ovfp->return_adjustment_base_class;
+          *p_callee = ovfp->overriding_function;
+          bcp = ovfp->base_class;
+          if (bcp == NULL) {
+            /* The overrider is a member of outer_class. */
+            this_val->address = outer_subobj;
+          } else {
+            this_val->address =
+                       outer_subobj +
+                       compute_interpreter_base_offset(bcp,
+                                                       (a_base_class_ptr*)NULL);
+          }  /* if */
           if (ret_base != NULL) {
             /* A covariant return override.  The return value will need
                adjustment (possibly to an indirect base class). */
-            retval_offset_step = compute_interpreter_base_offset(
+            *p_retval_offset += compute_interpreter_base_offset(
                                            ret_base, (a_base_class_ptr*)NULL);
           }  /* if */
-          overrider = ovfp->overriding_function;
-          break;
         }  /* if */
-      }  /* for */
-      get_mapped_byte_count(&persistent_map, bcp, offset);
-      subobj -= offset;
-      if (overrider != NULL) {
-        *p_callee = overrider;
-        *p_retval_offset += retval_offset_step;
-        callee_subobj = subobj;
-      }  /* if */
-      if (!subobject_is_initialized(subobj, complete_obj)) {
-        /* The next-enclosing subobject is not constructed.  Do not dispatch
-           from it. */
-        break;
-      }  /* if */
-    } while (subobj != complete_obj);
-    this_val->address = callee_subobj;
+      } while (ovfp != NULL && bcp != NULL);
+    }  /* if */
   }  /* if */
   return result;
 }  /* adjust_virtual_callee */

src/pending_changes/gh-421-virtual-call-indirect-overrider.txt

@@ -0,0 +1,16 @@
+[GH #421]
+Constant evaluation of virtual calls to an overrider in a derived class
+
+During constant evaluation, a virtual function call did not find the final
+overrider if the class of which it is a member is derived from the class of
+the called function through an intermediate class that does not itself
+override the function: The called function was not replaced.  For example,
+with --c++20:
+
+  struct A { constexpr virtual int f() const { return 1; } };
+  struct NA : A {};
+  struct B : NA { constexpr int f() const override { return 2; } };
+  constexpr int g() { B b; const A* p = &b; return p->f(); }
+  static_assert(g() == 2);  // Previously failed.
+
+This is now fixed.

tests/expectations/edg_x86_64/imported.log

@@ -7147,7 +7147,6 @@ gnu/cpp/cpp2a/paren-init30.sft.cpp:fn:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/paren-init33.sft.cpp:rp:-DTEST_NUMBER=1 --c++20:FAIL(COMPILE):
 gnu/cpp/cpp2a/paren-init35.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/paren-init4.sft.cpp:rp:-DTEST_NUMBER=1 --c++20:FAIL(COMPILE):
-gnu/cpp/cpp2a/pr117317-1.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/pr99833.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:ABORT:
 gnu/cpp/cpp2a/spaceship-constexpr2/test.mft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:
 gnu/cpp/cpp2a/spaceship-narrowing1.sft.cpp:fp:-DTEST_NUMBER=1 --c++20:FAIL:

tests/tests/changes/.gh-421.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++20 Test_name.c

tests/tests/changes/gh-421.sft.cpp

@@ -0,0 +1,94 @@
+//type:fp
+//options:--c++20
+//remark:[GH #421] Constant evaluation of virtual calls whose final overrider
+//remark:is declared below a class that does not override the function
+
+struct A {
+  constexpr virtual int f() const { return 1; }
+  int a = 0;
+};
+struct NA : A {};
+struct B : NA {
+  constexpr int f() const override { return 2; }
+};
+constexpr int g1() { B b; const A* p = &b; return p->f(); }
+static_assert(g1() == 2);  // Previously failed.
+constexpr int g1n() { B b; const NA* p = &b; return p->f(); }
+static_assert(g1n() == 2);  // Previously failed.
+
+// Two non-overriding levels.
+struct NNA : NA {};
+struct C : NNA { constexpr int f() const override { return 3; } };
+constexpr int g2() { C c; const A* p = &c; return p->f(); }
+static_assert(g2() == 3);  // Previously failed.
+
+// The overrider is in a middle class, above a non-overriding one.
+struct M : NA { constexpr int f() const override { return 4 + a; } };
+struct NM : M {};
+struct E { int e = 7; };
+struct T : E, NM {};
+constexpr int g3() { T t; t.a = 10; const A* p = &t; return p->f(); }
+static_assert(g3() == 14);  // Previously failed.
+
+// Repeated base class: only one path overrides.
+struct L : A { constexpr int f() const override { return 5 + a; } };
+struct R : A {};
+struct D : L, R {};
+constexpr int g4() {
+  D d;
+  static_cast<L&>(d).a = 1;
+  static_cast<R&>(d).a = 2;
+  const A* pl = static_cast<const L*>(&d);
+  const A* pr = static_cast<const R*>(&d);
+  return pl->f() * 10 + pr->f();
+}
+static_assert(g4() == 61);
+struct DD : D {
+  constexpr int f() const override { return 9; }
+};
+constexpr int g5() {
+  DD d;
+  const A* pl = static_cast<const L*>(&d);
+  const A* pr = static_cast<const R*>(&d);
+  return pl->f() * 10 + pr->f();
+}
+static_assert(g5() == 99);  // Previously failed.
+
+// Covariant return through a non-overriding intermediate class.
+struct X { int x = 'X'; };
+struct Y : X { int y = 'Y'; };
+struct Z0 { int z = 0; };
+struct Q : Z0, Y { int q = 'Q'; };
+struct CA { constexpr virtual const X* h() const { return &xx; } X xx; };
+struct CN : CA {};
+struct CB : CN {
+  constexpr const Q* h() const override { return &qq; }
+  Q qq;
+};
+constexpr bool g6() {
+  CB b;
+  const CA* p = &b;
+  return p->h() == static_cast<const X*>(&b.qq) && p->h()->x == 'X';
+}
+static_assert(g6());  // Previously failed.
+
+// Virtual call during construction of an intermediate class.
+struct K : NA {
+  int r;
+  constexpr K() : r(static_cast<const A*>(this)->f()) {}
+  constexpr int f() const override { return 6; }
+};
+struct NK : K {};
+struct KK : NK { constexpr int f() const override { return 8; } };
+constexpr int g7() { KK k; const A* p = &k; return k.r * 10 + p->f(); }
+static_assert(g7() == 68);  // Previously failed.
+struct W : NA {
+  int r;
+  constexpr W() : r(static_cast<const A*>(this)->f()) {}
+};
+struct WW : W { constexpr int f() const override { return 8; } };
+constexpr int g8() { WW w; const A* p = &w; return w.r * 10 + p->f(); }
+static_assert(g8() == 18);  // Previously failed.
+
+constexpr B gb;
+static_assert(static_cast<const A&>(gb).f() == 2);

tests/tests/imported/clang/cpp/SemaCXX/.constant-expression-cxx2a.rto/default.1.1.txt

@@ -13,24 +13,10 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
 "SemaCXX/constant-expression-cxx2a.cpp", line 176: error: static assertion failed
     static_assert(d.ba == 'B');
     ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 176: note: the final comparison was 65 == 66
+"SemaCXX/constant-expression-cxx2a.cpp", line 176: note: the final comparison was 68 == 66
     static_assert(d.ba == 'B');
                        ^
 
-"SemaCXX/constant-expression-cxx2a.cpp", line 181: error: static assertion failed
-    static_assert(a.f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 181: note: the final comparison was 65 == 68
-    static_assert(a.f() == 'D');
-                        ^
-
-"SemaCXX/constant-expression-cxx2a.cpp", line 182: error: static assertion failed
-    static_assert(b.f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 182: note: the final comparison was 66 == 68
-    static_assert(b.f() == 'D');
-                        ^
-
 "SemaCXX/constant-expression-cxx2a.cpp", line 199: error: return type is not identical to nor covariant with return type "const Virtual::A *" of overridden virtual function "Virtual::Covariant1::f"
       constexpr virtual const D *f() const { return &this->d; }
                                  ^
@@ -41,20 +27,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
                                  ^
           detected during instantiation of class "Virtual::Covariant3<T> [with T=Virtual::C]" at line 203
 
-"SemaCXX/constant-expression-cxx2a.cpp", line 209: error: static assertion failed
-    static_assert(cb1->f()->f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 209: note: the final comparison was 65 == 68
-    static_assert(cb1->f()->f() == 'D');
-                                ^
-
-"SemaCXX/constant-expression-cxx2a.cpp", line 212: error: static assertion failed
-    static_assert(cb2->f()->f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 212: note: the final comparison was 66 == 68
-    static_assert(cb2->f()->f() == 'D');
-                                ^
-
 "SemaCXX/constant-expression-cxx2a.cpp", line 232: error: initialization is not constant
     constexpr PureVirtualCall pure_virtual_call;
                               ^

tests/tests/imported/clang/cpp/SemaCXX/.constant-expression-cxx2a.rto/edg_x86_32.1.1.txt

@@ -13,24 +13,10 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
 "SemaCXX/constant-expression-cxx2a.cpp", line 176: error: static assertion failed
     static_assert(d.ba == 'B');
     ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 176: note: the final comparison was 65 == 66
+"SemaCXX/constant-expression-cxx2a.cpp", line 176: note: the final comparison was 68 == 66
     static_assert(d.ba == 'B');
                        ^
 
-"SemaCXX/constant-expression-cxx2a.cpp", line 181: error: static assertion failed
-    static_assert(a.f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 181: note: the final comparison was 65 == 68
-    static_assert(a.f() == 'D');
-                        ^
-
-"SemaCXX/constant-expression-cxx2a.cpp", line 182: error: static assertion failed
-    static_assert(b.f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 182: note: the final comparison was 66 == 68
-    static_assert(b.f() == 'D');
-                        ^
-
 "SemaCXX/constant-expression-cxx2a.cpp", line 199: error: return type is not identical to nor covariant with return type "const Virtual::A *" of overridden virtual function "Virtual::Covariant1::f"
       constexpr virtual const D *f() const { return &this->d; }
                                  ^
@@ -41,20 +27,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
                                  ^
           detected during instantiation of class "Virtual::Covariant3<T> [with T=Virtual::C]" at line 203
 
-"SemaCXX/constant-expression-cxx2a.cpp", line 209: error: static assertion failed
-    static_assert(cb1->f()->f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 209: note: the final comparison was 65 == 68
-    static_assert(cb1->f()->f() == 'D');
-                                ^
-
-"SemaCXX/constant-expression-cxx2a.cpp", line 212: error: static assertion failed
-    static_assert(cb2->f()->f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 212: note: the final comparison was 66 == 68
-    static_assert(cb2->f()->f() == 'D');
-                                ^
-
 "SemaCXX/constant-expression-cxx2a.cpp", line 232: error: initialization is not constant
     constexpr PureVirtualCall pure_virtual_call;
                               ^

tests/tests/imported/clang/cpp/SemaCXX/.constant-expression-cxx2a.rto/edg_x86_32_cp.1.1.txt

@@ -13,24 +13,10 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
 "SemaCXX/constant-expression-cxx2a.cpp", line 176: error: static assertion failed
     static_assert(d.ba == 'B');
     ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 176: note: the final comparison was 65 == 66
+"SemaCXX/constant-expression-cxx2a.cpp", line 176: note: the final comparison was 68 == 66
     static_assert(d.ba == 'B');
                        ^
 
-"SemaCXX/constant-expression-cxx2a.cpp", line 181: error: static assertion failed
-    static_assert(a.f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 181: note: the final comparison was 65 == 68
-    static_assert(a.f() == 'D');
-                        ^
-
-"SemaCXX/constant-expression-cxx2a.cpp", line 182: error: static assertion failed
-    static_assert(b.f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 182: note: the final comparison was 66 == 68
-    static_assert(b.f() == 'D');
-                        ^
-
 "SemaCXX/constant-expression-cxx2a.cpp", line 186: error: expression must have a constant value
     static_assert(d_not_constexpr.f() == 'D');
                   ^
@@ -48,20 +34,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
                                  ^
           detected during instantiation of class "Virtual::Covariant3<T> [with T=Virtual::C]" at line 203
 
-"SemaCXX/constant-expression-cxx2a.cpp", line 209: error: static assertion failed
-    static_assert(cb1->f()->f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 209: note: the final comparison was 65 == 68
-    static_assert(cb1->f()->f() == 'D');
-                                ^
-
-"SemaCXX/constant-expression-cxx2a.cpp", line 212: error: static assertion failed
-    static_assert(cb2->f()->f() == 'D');
-    ^
-"SemaCXX/constant-expression-cxx2a.cpp", line 212: note: the final comparison was 66 == 68
-    static_assert(cb2->f()->f() == 'D');
-                                ^
-
 "SemaCXX/constant-expression-cxx2a.cpp", line 232: error: initialization is not constant
     constexpr PureVirtualCall pure_virtual_call;
                               ^