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

[Paolo Carlini]

Hi,

On 20/01/2018 02:59, Paolo Carlini wrote:
> Hi again,
>
> On 19/01/2018 23:55, Paolo Carlini wrote:
>> ...Therefore It seems to me that a way to more cleanly solve the bug 
>> would be moving something like || !DECL_NONTRIVIALLY_INITIALIZED_P to 
>> the the above check in check_for_uninitialized_const_var, and 
>> therefore remove completely the uninitialized case from 
>> potential_constant_expression_1, ...
> Of course this doesn't work. check_for_uninitialized_const would 
> really need to know that the VAR_DECL appears in a statement 
> expression which is initializing a constexpr variable, nothing to do 
> with DECL_NONTRIVIALLY_INITIALIZED_P. I'll give the issue more thought 
> over the we, but removing completely the check from 
> potential_constant_expression_1 seems very tough to me, assuming of 
> course we really care about diagnosing the uninitialized inner in 
> stmtexpr20.C, which is my invention, isn't in the testsuite yet ;)
Thus the below, carefully tested over the we, would be a change 
completely removing the problematic error_at call, plus some testcases 
checking that we would still do the right thing in a few cases (bug 
submitter added constexpr-83921-2.C). The updated stmtexpr20.C shows 
that we would reject anyway a statement expression using an 
uninitialized inner, simply because the whole initializer would still be 
flagged as non const. ICC does the same. CLANG diagnoses the 
uninitialized inner but then actually accepts something using 'static 
const int inner = 1' whereas we don't anyway, we (and ICC) don't accept 
anything similar to stmtexpr20.C. So, I would say this kind of change 
may actually make sense...

Thanks,
Paolo.

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-------------- next part --------------
Index: cp/constexpr.c
===================================================================
--- cp/constexpr.c	(revision 256939)
+++ cp/constexpr.c	(working copy)
@@ -5707,13 +5707,6 @@ potential_constant_expression_1 (tree t, bool want
 			  "%<thread_local%> in %<constexpr%> context", tmp);
 	      return false;
 	    }
-	  else if (!DECL_NONTRIVIALLY_INITIALIZED_P (tmp))
-	    {
-	      if (flags & tf_error)
-		error_at (DECL_SOURCE_LOCATION (tmp), "uninitialized "
-			  "variable %qD in %<constexpr%> context", tmp);
-	      return false;
-	    }
 	}
       return RECUR (tmp, want_rval);
 
Index: testsuite/g++.dg/cpp1y/constexpr-83921-1.C
===================================================================
--- testsuite/g++.dg/cpp1y/constexpr-83921-1.C	(nonexistent)
+++ testsuite/g++.dg/cpp1y/constexpr-83921-1.C	(working copy)
@@ -0,0 +1,5 @@
+// PR c++/83921
+// { dg-do compile { target c++14 } }
+
+struct Foo { };
+constexpr void test() { Foo f; }
Index: testsuite/g++.dg/cpp1y/constexpr-83921-2.C
===================================================================
--- testsuite/g++.dg/cpp1y/constexpr-83921-2.C	(nonexistent)
+++ testsuite/g++.dg/cpp1y/constexpr-83921-2.C	(working copy)
@@ -0,0 +1,5 @@
+// PR c++/83921
+// { dg-do compile { target c++14 } }
+
+struct Foo { Foo() = default; };
+constexpr void test() { Foo f; }
Index: testsuite/g++.dg/cpp1y/constexpr-83921-3.C
===================================================================
--- testsuite/g++.dg/cpp1y/constexpr-83921-3.C	(nonexistent)
+++ testsuite/g++.dg/cpp1y/constexpr-83921-3.C	(working copy)
@@ -0,0 +1,5 @@
+// PR c++/83921
+// { dg-do compile { target c++14 } }
+
+struct Foo { int m; };
+constexpr void test() { Foo f; }  // { dg-error "uninitialized" }
Index: testsuite/g++.dg/ext/stmtexpr20.C
===================================================================
--- testsuite/g++.dg/ext/stmtexpr20.C	(nonexistent)
+++ testsuite/g++.dg/ext/stmtexpr20.C	(working copy)
@@ -0,0 +1,13 @@
+// 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 "not a constant" }
+
+  return &atest;
+}