c++: limit instantiation with ill-formed class [PR96286]

I noticed that after the static_assert failures in lwg3466.cc, we got
various follow-on errors because we went ahead and tried to instantiate the
promise<T> member functions even after instantiating the class itself ran
into problems.  Interrupting instantiation of the class itself seems likely
to cause error-recovery problems, but preventing instantiation of member
functions seems strictly better for error-recovery.

This doesn't fix any of the specific testcases in PR96286, but addresses
part of that problem space.

	PR c++/96286

gcc/cp/ChangeLog:

	* cp-tree.h (struct lang_type): Add erroneous bit-field.
	(CLASSTYPE_ERRONEOUS): New.
	* pt.c (limit_bad_template_recursion): Check it.
	(instantiate_class_template_1): Set it.

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/promise/requirements/lwg3466.cc:
	Remove dg-prune-outputs.

gcc/testsuite/ChangeLog:

	* g++.dg/template/access2.C: Split struct A.

3 files changed, 24 insertions, 3 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 6a17937..ce7ca53 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -2336,6 +2336,7 @@ struct GTY(()) lang_type {
   unsigned has_constexpr_ctor : 1;
   unsigned unique_obj_representations : 1;
   unsigned unique_obj_representations_set : 1;
+  bool erroneous : 1;
 
   /* When adding a flag here, consider whether or not it ought to
      apply to a template instance if it applies to the template.  If
@@ -2344,7 +2345,7 @@ struct GTY(()) lang_type {
   /* There are some bits left to fill out a 32-bit word.  Keep track
      of this by updating the size of this bitfield whenever you add or
      remove a flag.  */
-  unsigned dummy : 5;
+  unsigned dummy : 4;
 
   tree primary_base;
   vec<tree_pair_s, va_gc> *vcall_indices;
@@ -2660,6 +2661,10 @@ struct GTY(()) lang_type {
 /* Nonzero if a _DECL node requires us to output debug info for this class.  */
 #define CLASSTYPE_DEBUG_REQUESTED(NODE) \
   (LANG_TYPE_CLASS_CHECK (NODE)->debug_requested)
+
+/* True if we saw errors while instantiating this class.  */
+#define CLASSTYPE_ERRONEOUS(NODE) \
+  (LANG_TYPE_CLASS_CHECK (NODE)->erroneous)
 
 /* Additional macros for inheritance information.  */
 
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d7d0dce..fcf3ac3 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10885,9 +10885,14 @@ limit_bad_template_recursion (tree decl)
 {
   struct tinst_level *lev = current_tinst_level;
   int errs = errorcount + sorrycount;
-  if (lev == NULL || errs == 0 || !neglectable_inst_p (decl))
+  if (errs == 0 || !neglectable_inst_p (decl))
     return false;
 
+  /* Avoid instantiating members of an ill-formed class.  */
+  if (DECL_CLASS_SCOPE_P (decl)
+      && CLASSTYPE_ERRONEOUS (DECL_CONTEXT (decl)))
+    return true;
+
   for (; lev; lev = lev->next)
     if (neglectable_inst_p (lev->maybe_get_node ()))
       break;
@@ -12212,6 +12217,13 @@ instantiate_class_template_1 (tree type)
   finish_struct_1 (type);
   TYPE_BEING_DEFINED (type) = 0;
 
+  /* Remember if instantiating this class ran into errors, so we can avoid
+     instantiating member functions in limit_bad_template_recursion.  We set
+     this flag even if the problem was in another instantiation triggered by
+     this one, as that will likely also cause trouble for member functions.  */
+  if (errorcount + sorrycount > current_tinst_level->errors)
+    CLASSTYPE_ERRONEOUS (type) = true;
+
   /* We don't instantiate default arguments for member functions.  14.7.1:
 
      The implicit instantiation of a class template specialization causes
diff --git a/gcc/testsuite/g++.dg/template/access2.C b/gcc/testsuite/g++.dg/template/access2.C
index 0620c10..4a80bb4 100644
--- a/gcc/testsuite/g++.dg/template/access2.C
+++ b/gcc/testsuite/g++.dg/template/access2.C
@@ -5,6 +5,9 @@
 
 template <class T> struct A {
   typename T::X x;			// { dg-error "this context" }
+};
+
+template <class T> struct A2 {
   int f() { return T::i; }		// { dg-error "this context" }
 };
 
@@ -16,5 +19,6 @@ class B {
 int main()
 {
   A<B> ab;				// { dg-message "required" }
-  ab.f();				// { dg-message "required" }
+  A2<B> a2b;
+  a2b.f();				// { dg-message "required" }
 }