Hi!
As the following testcase shows, we mishandle break statements in expansion
statements during constant evaluation.
finish_expansion_stmt changes the BREAK_STMT/CONTINUE_STMTs to GOTO_EXPRs
to corresponding labels and marks those labels with
LABEL_DECL_BREAK/LABEL_DECL_CONTINUE so that constexpr.cc is happy about
those. The continue label (if any is needed) is right after each
iteration's instantiated body, the break label (if any is needed) is after
the last body.
Now, continue seems to work properly, when we encounter it, we set
*jump_target to it and continues predicate is true on it, but
cxx_eval_statement_list has
/* We've found a continue, so skip everything until we reach
the label its jumping to. */
if (continues (jump_target))
{
if (label_matches (ctx, jump_target, stmt))
/* Found it. */
*jump_target = NULL_TREE;
else
continue;
}
...
if (returns (jump_target)
|| breaks (jump_target)
|| throws (jump_target))
break;
and so it properly iterates through statement lists until it finds
the label decl.
But unfortunately it doesn't work for break, we set *jump_target on
the GOTO_EXPR, breaks predicate is true, but then break out of any
STATEMENT_LISTs and the only way to resume processing of statements
in that case is when cxx_eval_loop_expr does
if (breaks (jump_target))
{
*jump_target = NULL_TREE;
break;
}
or similarly switch handling. But for expansion stmt there is
nothing like that in the IL, so either we break some outer loop
(foo in the testcase) instead, or fail because we think there was no return
in the function.
The following patch fixes this by wrapping the series of instantiated
expansion stmt bodies (for all iterations) in an artificial
do ... while (0); statement, but does that only if break; was actually
needed (i.e. when we are emitting a break_label).
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2026-06-05 Jakub Jelinek <[email protected]>
PR c++/125601
* pt.cc (finish_expansion_stmt): If break; was seen in any of the
expansion stmt bodies, wrap all the bodies in an artificial
do ... while (0); stmt.
* g++.dg/cpp26/expansion-stmt43.C: New test.
--- gcc/cp/pt.cc.jj 2026-06-03 11:49:04.897785724 +0200
+++ gcc/cp/pt.cc 2026-06-04 18:12:39.870465887 +0200
@@ -33631,6 +33631,7 @@ finish_expansion_stmt (tree expansion_st
DECL_NAME (decl) = NULL_TREE;
}
+ tree stmt_list = push_stmt_list ();
expansion_stmt_bc bc_data = { NULL_TREE, NULL_TREE, NULL, loc, false };
for (unsigned HOST_WIDE_INT i = 0; i < n; ++i)
@@ -33830,7 +33831,19 @@ finish_expansion_stmt (tree expansion_st
}
}
if (bc_data.break_label)
- add_stmt (build1 (LABEL_EXPR, void_type_node, bc_data.break_label));
+ {
+ /* If break; is seen, wrap all the expansion stmt bodies in
+ a single artificial do ... while (0); statement, so that
+ constant evaluation handles break; correctly. */
+ tree do_stmt
+ = build_stmt (loc, DO_STMT, NULL_TREE, NULL_TREE, NULL_TREE);
+ DO_COND (do_stmt) = boolean_false_node;
+ DO_BODY (do_stmt) = pop_stmt_list (stmt_list);
+ add_stmt (do_stmt);
+ add_stmt (build1 (LABEL_EXPR, void_type_node, bc_data.break_label));
+ }
+ else
+ add_stmt (pop_stmt_list (stmt_list));
if (args == NULL_TREE)
{
TREE_TYPE (range_decl) = error_mark_node;
--- gcc/testsuite/g++.dg/cpp26/expansion-stmt43.C.jj 2026-06-04
18:17:40.123509169 +0200
+++ gcc/testsuite/g++.dg/cpp26/expansion-stmt43.C 2026-06-04
18:17:03.575503867 +0200
@@ -0,0 +1,64 @@
+// PR c++/125601
+// { dg-do run { target c++14 } }
+// { dg-options "-O2" }
+
+constexpr int
+foo (int x)
+{
+ int a = 0, b = 3;
+ while (b > 0)
+ {
+ ++a;
+ --b;
+ template for (constexpr int value : { 10, 20, 30 }) // { dg-warning
"'template for' only available with" "" { target c++23_down } }
+ {
+ a += value;
+ if (x == 0)
+ break;
+ else if (x == 1)
+ continue;
+ a += 42;
+ }
+ }
+ return a;
+}
+
+constexpr int
+bar (int x)
+{
+ int a = 0;
+ template for (constexpr int value : { 10, 20, 30 }) // { dg-warning
"'template for' only available with" "" { target c++23_down } }
+ {
+ a += value;
+ if (x == 0)
+ break;
+ else if (x == 1)
+ continue;
+ a += 42;
+ }
+ return a;
+}
+
+static_assert (foo (0) == 3 * (1 + 10), "");
+static_assert (foo (1) == 3 * (1 + 10 + 20 + 30), "");
+static_assert (foo (2) == 3 * (1 + 10 + 20 + 30 + 3 * 42), "");
+static_assert (bar (0) == 10, "");
+static_assert (bar (1) == 10 + 20 + 30, "");
+static_assert (bar (2) == 10 + 20 + 30 + 3 * 42, "");
+
+int
+main ()
+{
+ if (foo (0) != 3 * (1 + 10))
+ __builtin_abort ();
+ if (foo (1) != 3 * (1 + 10 + 20 + 30))
+ __builtin_abort ();
+ if (foo (2) != 3 * (1 + 10 + 20 + 30 + 3 * 42))
+ __builtin_abort ();
+ if (bar (0) != 10)
+ __builtin_abort ();
+ if (bar (1) != 10 + 20 + 30)
+ __builtin_abort ();
+ if (bar (2) != 10 + 20 + 30 + 3 * 42)
+ __builtin_abort ();
+}
Jakub
