[gcc r9-10128] c++: Don't reject GOTO_EXPRs to cdtor_label in potential_constant_expression_1 [PR104513]

[gcc r9-10128] c++: Don't reject GOTO_EXPRs to cdtor_label in potential_constant_expression_1 [PR104513]

[Jakub Jelinek]

https://gcc.gnu.org/g:87cd4bc02f602226922e6c8099a2f4687664bed9

commit r9-10128-g87cd4bc02f602226922e6c8099a2f4687664bed9
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Feb 14 16:56:15 2022 +0100

    c++: Don't reject GOTO_EXPRs to cdtor_label in potential_constant_expression_1 [PR104513]
    
    return in ctors on targetm.cxx.cdtor_returns_this () target like arm
    is emitted as GOTO_EXPR cdtor_label where at cdtor_label it emits
    RETURN_EXPR with the this.
    Similarly, in all dtors regardless of targetm.cxx.cdtor_returns_this ()
    a return is emitted similarly.
    
    potential_constant_expression_1 was rejecting these gotos and so we
    incorrectly rejected these testcases, but actual cxx_eval* is apparently
    handling these just fine.  I was a little bit worried that for the
    destruction of bases we wouldn't evaluate something we should, but as the
    testcase shows, that is evaluated through try ... finally and there is
    nothing after the cdtor_label.  For arm there is RETURN_EXPR this; but we
    don't really care about the return value from ctors and dtors during the
    constexpr evaluation.
    
    I must say I don't see much the point of cdtor_labels at all, I'd think
    that with try ... finally around it for non-arm we could just RETURN_EXPR
    instead of the GOTO_EXPR and the try/finally gimplification would DTRT,
    and we could just add the right return value for the arm case.
    
    2022-02-14  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/104513
            * constexpr.c (potential_constant_expression_1) <case GOTO_EXPR>:
            Don't punt if returns (target).
    
            * g++.dg/cpp1y/constexpr-104513.C: New test.
    
    (cherry picked from commit 02a981a8e512934a990d1427d14e8e884409fade)

Diff:
---
 gcc/cp/constexpr.c                            |  4 ++--
 gcc/testsuite/g++.dg/cpp1y/constexpr-104513.C | 10 ++++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 3a35df5a1ee..5744537d8e6 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6928,8 +6928,8 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     case GOTO_EXPR:
       {
 	tree *target = &TREE_OPERAND (t, 0);
-	/* Gotos representing break and continue are OK.  */
-	if (breaks (target) || continues (target))
+	/* Gotos representing break, continue and cdtor return are OK.  */
+	if (breaks (target) || continues (target) || returns (target))
 	  {
 	    *jump_target = *target;
 	    return true;
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-104513.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-104513.C
new file mode 100644
index 00000000000..4fa78a3e025
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-104513.C
@@ -0,0 +1,10 @@
+// PR c++/104513
+// { dg-do compile { target c++14 } }
+
+struct A {
+  int a1;
+  short a2, a3;
+  long a4;
+  constexpr A() : a1(42), a2(42), a3(42), a4(42) { return; }
+};
+constexpr A a;