[committed] c++: Partially revert fix for PR c++/95497 [PR96132]

[Patrick Palka]

I was mistaken to assume that a dependent type is necessarily
incomplete, and indeed there are multiple places in the frontend where
we check a type for both dependency and completeness.  So this patch
partially reverts the fix for PR95497, restoring the dependent_type_p
check that guarded the call to is_really_empty_class below.

gcc/cp/ChangeLog:

	PR c++/96132
	* constexpr.c (potential_constant_expression_1) <case PARM_DECL>:
	Restore dependent_type_p check that guarded the call to
	is_really_empty_class.

gcc/testsuite/ChangeLog:

	PR c++/96132
	* g++.dg/template/incomplete12.C: New test.
---
 gcc/cp/constexpr.c                           | 1 +
 gcc/testsuite/g++.dg/template/incomplete12.C | 9 +++++++++
 2 files changed, 10 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/template/incomplete12.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index ff78ebda2dc..97dcc1b1d10 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -7444,6 +7444,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	{
 	  tree type = TREE_TYPE (t);
 	  if ((processing_template_decl && !COMPLETE_TYPE_P (type))
+	      || dependent_type_p (type)
 	      || is_really_empty_class (type, /*ignore_vptr*/false))
 	    /* An empty class has no data to read.  */
 	    return true;
diff --git a/gcc/testsuite/g++.dg/template/incomplete12.C b/gcc/testsuite/g++.dg/template/incomplete12.C
new file mode 100644
index 00000000000..335e5356874
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/incomplete12.C
@@ -0,0 +1,9 @@
+// PR c++/96132
+// { dg-do compile }
+
+template <int> class a;
+
+template <int b> class c {
+  a<b> e;
+  void operator=(c d) { e = d; }
+};
-- 
2.27.0.343.g4a0fcf9f76