[C++ Patch] PR 83921 ("[7/8 Regression] GCC rejects constexpr initialization of empty aggregate")

[Paolo Carlini]

Hi,

I'm finishing testing on x86_64-linux the below - which anyway seems 
very unlikely to cause regressions because we aren't really stress 
testing the relevant checks in potential_constant_expression_1 much, if 
at all (surely stmtexpr19.C tests static).

Anyway, the issue is the following. In 239268 aka "Implement C++17 
constexpr lambda" Jason added some checks to 
potential_constant_expression_1 covering static, thread_local and 
uninitialized var declaration in constexpr function context. Then 
extended to constexpr context more generally in 249382 aka "constexpr 
and static var in statement-expression", with ext/stmtexpr19.C covering 
the static case. Now, it looks like the check for uninitialized vars in 
constexpr functions context is more correctly carried out by 
check_for_uninitialized_const_var instead, because the simple check in 
potential_constant_expression_1 as-is causes the regression pointed out 
by this bug. Thus the fix below which just restricts the check in 
potential_constant_expression_1, and the testcases, one for this bug 
proper, plus one, very similar to stmtexpr19.C, double checking that we 
are still diagnosing in the statement-expression context. I also 
verified under the debugger how for constexpr-83921.C we are actually 
running check_for_uninitialized_const_var on 'f' - which obviously passes.

Thanks, Paolo.

////////////////////////


/cp
2018-01-18  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/83921
        * constexpr.c (potential_constant_expression_1): Do not handle
        uninitialized vars in constexpr function here.

/testsuite
2018-01-18  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/83921
        * g++.dg/cpp1y/constexpr-83921.C: New.
        * g++.dg/ext/stmtexpr20.C: Likewise.
Index: cp/constexpr.c
===================================================================
--- cp/constexpr.c      (revision 256865)
+++ cp/constexpr.c      (working copy)
@@ -5708,7 +5708,9 @@ potential_constant_expression_1 (tree t, bool want
                          "%<thread_local%> in %<constexpr%> context", tmp);
              return false;
            }
-         else if (!DECL_NONTRIVIALLY_INITIALIZED_P (tmp))
+         else if (!DECL_NONTRIVIALLY_INITIALIZED_P (tmp)
+                  /* Handled in check_for_uninitialized_const_var.  */
+                  && !var_in_constexpr_fn (tmp))
            {
              if (flags & tf_error)
                error_at (DECL_SOURCE_LOCATION (tmp), "uninitialized "
Index: testsuite/g++.dg/cpp1y/constexpr-83921.C
===================================================================
--- testsuite/g++.dg/cpp1y/constexpr-83921.C    (nonexistent)
+++ testsuite/g++.dg/cpp1y/constexpr-83921.C    (working copy)
@@ -0,0 +1,7 @@
+// PR c++/83921
+// { dg-do compile { target c++14 } }
+
+struct Foo {};
+constexpr void test() {
+  Foo f;
+}
Index: testsuite/g++.dg/ext/stmtexpr20.C
===================================================================
--- testsuite/g++.dg/ext/stmtexpr20.C   (nonexistent)
+++ testsuite/g++.dg/ext/stmtexpr20.C   (working copy)
@@ -0,0 +1,15 @@
+// PR c++/83921
+// { dg-options "" }
+// { dg-do compile { target c++11 } }
+
+struct test { const int *addr; };
+
+const test* setup()
+{
+  static constexpr test atest =
+    {
+      ({ int inner; &inner; }) // { dg-error "uninitialized" }
+    };
+
+  return &atest;
+}