break in a statement expression in a switch condition [GH #434]

Author: daveedvdv-nvidia

src/interpret.c

@@ -9704,6 +9704,7 @@ transfer, or perform their cleanup and let it propagate further.
              (see the handling of ips->switch_entry above). */
           ips->switch_entry = case_stmt;
           push_stmt_work(ips, stmt->variant.switch_stmt.body_statement);
+          item->phase = iwp_2nd_resume;
           pop_when_done = FALSE;
           goto done;
         }  /* if */
@@ -9928,10 +9929,11 @@ transfer, or perform their cleanup and let it propagate further.
           do_constexpr_fail(result);
         }  /* if */
       }  /* if */
-      if (!result && !ips->failed &&
+      if (!result && !ips->failed && item->phase == iwp_2nd_resume &&
           ips->transfer.kind == ctk_switch_break) {
         /* Break out of the switch statement (which completes the execution of
-           the break statement). */
+           the break statement).  A break statement in the condition (in a GNU
+           statement expression) applies to an enclosing statement instead. */
         ips->transfer.kind = ctk_none;
         result = TRUE;
       }  /* if */

src/pending_changes/gh-434-break-in-switch-condition.txt

@@ -0,0 +1,28 @@
+[GH #434]
+GNU statement expressions and switch statements
+
+In GNU modes, a break statement inside a statement expression that appears as
+the value a switch statement switches on was bound to that switch statement
+instead of (as with GCC and Clang) to the enclosing loop or switch statement.
+Similarly, a case or default label in such a statement expression was
+associated with that switch statement.  For example, with --g++ --c++20:
+
+  int f(int y, bool c) {
+    switch (y) {
+    case 0:
+      switch (({ if (c) break; 1; })) {
+      case 1:
+        return 10;
+      }
+      return 1;
+    }
+    return 2;
+  }
+
+f(0, true) returned 1 instead of 2.  Constant evaluation had the same
+problem, and also let a switch statement complete a break statement executed
+while computing the value it switches on.  This is now fixed.  Such a break
+statement is now diagnosed if there is no enclosing loop or switch
+statement, and such a case or default label now applies to an enclosing
+switch statement (and is diagnosed as a transfer of control into a statement
+expression).

src/statements.c

@@ -2995,6 +2995,7 @@ statement is the top block of a GNU statement expression ({ ... }).
   }  /* if */
   sssep->switch_has_dependent_case
                                = FALSE;
+  sssep->in_switch_condition   = FALSE;
   sssep->contains_user_label   = FALSE;
   sssep->contains_active_switch_case = FALSE;
   sssep->record_declared_entities = FALSE;
@@ -3538,8 +3539,9 @@ static a_struct_stmt_stack_entry_ptr find_enclosing_struct_stmt(
                                                      a_boolean find_loop)
 /*
 Search the structured statement stack from the current entry outward,
-looking for a switch statement (if find_switch is TRUE) or a loop
-statement (while, do, or for; if find_loop is TRUE).  Return a pointer
+looking for a switch statement whose condition is not being scanned (if
+find_switch is TRUE) or a loop statement (while, do, or for; if find_loop is
+TRUE).  Return a pointer
 to the first structured statement stack entry found, or NULL if none 
 was found.
 */
@@ -3552,7 +3554,7 @@ was found.
      the compound statement that defines the function. */
   while (sssep != &struct_stmt_stack[0]) {
     kind = sssep->kind;
-    if (find_switch && kind == ssk_switch) {
+    if (find_switch && kind == ssk_switch && !sssep->in_switch_condition) {
       goto found;
     } else if (find_loop &&
                (kind == ssk_while || kind == ssk_do || kind == ssk_for ||
@@ -4589,7 +4591,9 @@ See also 3.6.4.2.
   (void)required_token(tok_lparen, ec_exp_lparen);
   add_stop_token(tok_rparen);
   /* Scan the "condition", which in C++ may be a condition declaration. */
+  struct_stmt_stack[depth_stmt_stack].in_switch_condition = TRUE;
   scan_condition(sp, &is_condition_decl);
+  struct_stmt_stack[depth_stmt_stack].in_switch_condition = FALSE;
   if (!is_error_node(sp->expr)) {
     /* Issue a remark if the selector is constant. */
     if (is_constant_node(skip_parens(sp->expr))) {

src/statements.h

@@ -331,6 +331,12 @@ typedef struct a_struct_stmt_stack_entry {
 			/* Set only in entries with kind == ssk_switch.
 			   Indicates that at least one case contains a
 			   template-dependent constant. */
+  a_bit_field	in_switch_condition:1;
+			/* Set only in entries with kind == ssk_switch.  TRUE
+			   while the condition of the switch statement is
+			   being scanned.  A break statement or a case label in
+			   the condition (in a GNU statement expression) does
+			   not apply to that switch statement. */
   a_bit_field	contains_user_label:1;
 			/* TRUE if the structured statement contains a user-
 			   declared label (used to avoid spurious reachability

tests/tests/changes/.gh-434.rto/default.1.1.txt

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

tests/tests/changes/.gh-434.rto/default.1.2.txt

@@ -0,0 +1 @@
+./a.out

tests/tests/changes/gh-434.sft.cpp

@@ -0,0 +1,62 @@
+//type:rp
+//remark:A break in a GNU statement expression in a switch condition
+//options_all:--g++ --c++20
+
+constexpr int f(int y, bool c) {
+  switch (y) {
+  case 0:
+    switch (({ if (c) break; 1; })) {
+    case 1:
+      return 10;
+    }
+    return 1;
+  }
+  return 2;
+}
+static_assert(f(0, false) == 10);
+static_assert(f(0, true) == 2);
+static_assert(f(5, true) == 2);
+
+constexpr int g(bool c) {
+  int r = 0;
+  for (int i = 0; i < 3; ++i) {
+    switch (({ if (c) break; i; })) {
+    default:
+      r += 1;
+    }
+    r += 100;
+  }
+  return r;
+}
+static_assert(g(false) == 303);
+static_assert(g(true) == 0);
+
+constexpr int h(int c) {
+  int r = 0;
+  do {
+    switch (({ int v = 0;
+               switch (c) {
+               case 1:
+                 v = 5;
+                 break;
+               }
+               v; })) {
+    case 5:
+      r = 50;
+      break;
+    default:
+      r = 1;
+    }
+    r += 2;
+  } while (false);
+  return r;
+}
+static_assert(h(1) == 52);
+static_assert(h(0) == 3);
+
+int main() {
+  volatile bool t = true, f0 = false;
+  volatile int zero = 0, one = 1, five = 5;
+  return !(f(zero, f0) == 10 && f(zero, t) == 2 && f(five, t) == 2 &&
+           g(f0) == 303 && g(t) == 0 && h(one) == 52 && h(zero) == 3);
+}

tests/tests/imported/clang/c/Parser/.bad-control.rto/default.1.1.txt

@@ -7,4 +7,8 @@ fe_only -DTEST_NUMBER=1 --clang -e 200 --no_wrap_diagnostics --clang_version 999
     continue; /* expected-error {{'continue' statement not in loop statement}} */
     ^
 
-2 errors detected in the compilation of "Test_name.c".
+"Test_name.c", line 14: error: a break statement may only be used within a loop or switch
+    switch(({ if (first) { first = 0; break; } 1; })) { // expected-error {{'break' statement not in loop or switch statement}}
+                                      ^
+
+3 errors detected in the compilation of "Test_name.c".