c++: Fix thinko in enum_min_precision [PR61414]

When backporting the PR61414 fix to 8.4, I've noticed that the caching
of prec is actually broken, as it would fail to actually store the computed
precision into the hash_map's value and so next time we'd think the enum needs
0 bits.

2020-02-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/61414
	* class.c (enum_min_precision): Change prec type from int to int &.

	* g++.dg/cpp0x/enum39.C: New test.

gcc/cp/ChangeLog

 2020-02-14  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c++/61414
+	* class.c (enum_min_precision): Change prec type from int to int &.
+
 	PR libstdc++/92906
 	* cp-tree.h (enum cp_tree_index): Add CPTI_FALLBACK_DFLOAT32_TYPE,
 	CPTI_FALLBACK_DFLOAT64_TYPE and CPTI_FALLBACK_DFLOAT128_TYPE.

gcc/cp/class.c

@@ -3289,7 +3289,7 @@ enum_min_precision (tree type)
     enum_to_min_precision = hash_map<tree, int>::create_ggc (37);
 
   bool existed;
-  int prec = enum_to_min_precision->get_or_insert (type, &existed);
+  int &prec = enum_to_min_precision->get_or_insert (type, &existed);
   if (existed)
     return prec;
 

gcc/testsuite/ChangeLog

+2020-02-14  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/61414
+	* g++.dg/cpp0x/enum39.C: New test.
+
 2020-02-14  Martin Jambor  <mjambor@suse.cz>
 
 	PR tree-optimization/93516

gcc/testsuite/g++.dg/cpp0x/enum39.C

+// PR c++/61414
+// { dg-do compile { target c++11 } }
+
+enum class E { E0 = -4, E1 = 3 };
+enum F : unsigned { F0 = 0, F1 = 15 };
+
+struct S
+{
+  E a : 2;	// { dg-warning "'S::a' is too small to hold all values of 'enum class E'" }
+  E b : 2;	// { dg-warning "'S::b' is too small to hold all values of 'enum class E'" }
+  E c : 3;	// { dg-bogus "'S::c' is too small to hold all values of 'enum class E'" }
+  F d : 3;	// { dg-warning "'S::d' is too small to hold all values of 'enum F'" }
+  F e : 3;	// { dg-warning "'S::e' is too small to hold all values of 'enum F'" }
+  F f : 4;	// { dg-bogus "'S::f' is too small to hold all values of 'enum F'" }
+};