c++: C++17 constexpr static data member linkage [PR99901]

C++17 makes constexpr static data members implicitly inline variables.  In
C++14, a subsequent out-of-class declaration is the definition.  We want to
continue emitting a symbol for such a declaration in C++17 mode, for ABI
compatibility with C++14 code that wants to refer to it.

Normally I'd distinguish in- and out-of-class declarations by looking at
DECL_IN_AGGR_P, but we never set DECL_IN_AGGR_P on inline variables.  I
think that's wrong, but don't want to mess with it so close to release.
Conveniently, we already have a test for in-class declaration earlier in the
function.

gcc/cp/ChangeLog:

	PR c++/99901
	* decl.c (cp_finish_decl): mark_needed an implicitly inline
	static data member with an out-of-class redeclaration.

gcc/testsuite/ChangeLog:

	PR c++/99901
	* g++.dg/cpp1z/inline-var9.C: New test.

2 files changed, 54 insertions, 4 deletions

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 565f5711fd5..7fb669fd48f 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -7010,10 +7010,13 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
   if (asmspec_tree && asmspec_tree != error_mark_node)
     asmspec = TREE_STRING_POINTER (asmspec_tree);
 
-  if (current_class_type
-      && CP_DECL_CONTEXT (decl) == current_class_type
-      && TYPE_BEING_DEFINED (current_class_type)
-      && !CLASSTYPE_TEMPLATE_INSTANTIATION (current_class_type)
+  bool in_class_decl
+    = (current_class_type
+       && CP_DECL_CONTEXT (decl) == current_class_type
+       && TYPE_BEING_DEFINED (current_class_type)
+       && !CLASSTYPE_TEMPLATE_INSTANTIATION (current_class_type));
+
+  if (in_class_decl
       && (DECL_INITIAL (decl) || init))
     DECL_INITIALIZED_IN_CLASS_P (decl) = 1;
 
@@ -7345,6 +7348,13 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	{
 	  layout_var_decl (decl);
 	  maybe_commonize_var (decl);
+	  /* A class-scope constexpr variable with an out-of-class declaration.
+	     C++17 makes them implicitly inline, but still force it out.  */
+	  if (DECL_INLINE_VAR_P (decl)
+	      && !DECL_VAR_DECLARED_INLINE_P (decl)
+	      && !DECL_TEMPLATE_INSTANTIATION (decl)
+	      && !in_class_decl)
+	    mark_needed (decl);
 	}
 
       /* This needs to happen after the linkage is set. */
diff --git a/gcc/testsuite/g++.dg/cpp1z/inline-var9.C b/gcc/testsuite/g++.dg/cpp1z/inline-var9.C
new file mode 100644
index 00000000000..43c9748877b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/inline-var9.C
@@ -0,0 +1,40 @@
+// PR c++/99901
+// { dg-do compile { target c++11 } }
+// { dg-final { scan-assembler-not "_ZN1A1aE" } }
+// { dg-final { scan-assembler-not "_ZN2A21aE" } }
+// { dg-final { scan-assembler-not "_ZN1CIiE1cE" } }
+// { dg-final { scan-assembler "_ZN1B1bE" } }
+// { dg-final { scan-assembler "_ZN2B21bE" } }
+// { dg-final { scan-assembler "_ZN2B31bE" } }
+
+struct A {
+  static const int a = 5;
+};
+
+struct A2 {
+  static constexpr int a = 5;
+};
+
+struct B {
+  static const int b;
+};
+constexpr int B::b = 5;
+
+struct B2 {
+  static const int b = 5;
+};
+constexpr int B2::b;
+
+struct B3 {
+  static constexpr int b = 5;
+};
+const int B3::b;
+
+template <class T>
+struct C {
+  static constexpr int c = 5;
+};
+template <class T>
+constexpr int C<T>::c;
+
+int i = C<int>::c;