commit 882a90dbfdd776ce33749ae4a44dc89588417a63
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Mar 27 09:56:13 2020 -0400

    set def_p and idxguide based on cdlguide

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 2f539570aae..3ca8eb9baf8 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -31024,6 +31024,12 @@ private:
     return locvec[i].class_key;
   }
 
+  /* True if a definition for the class has been seen.  */
+  bool def_p () const
+  {
+    return idxdef < locvec.length ();
+  }
+
   /* The location of a single mention of a class type with the given
      class-key.  */
   struct class_key_loc_t
@@ -31273,11 +31279,6 @@ class_decl_loc_t::diag_mismatched_tags (tree type_decl)
 
   /* Number of uses of the class.  */
   const unsigned ndecls = locvec.length ();
-  if (ndecls < 2)
-    return;
-
-  /* Set if a definition for the class has been seen.  */
-  const bool def_p = idxdef < ndecls;
 
   /* The class (or template) declaration guiding the decisions about
      the diagnostic.  For ordinary classes it's the same as THIS.  For
@@ -31305,17 +31306,13 @@ class_decl_loc_t::diag_mismatched_tags (tree type_decl)
 	return;
     }
 
+  /* Set if a definition for the class has been seen.  */
+  const bool def_p = cdlguide->def_p ();
+
   /* The index of the declaration whose class-key this declaration
      is expected to match.  It's either the class-key of the class
      definition if one exists or the first declaration otherwise.  */
-  const unsigned idxguide = def_p ? idxdef : 0;
-  unsigned idx = 0;
-  /* Advance IDX to the first declaration that either is not
-     a definition or that doesn't match the first declaration
-     if no definition is provided.  */
-  while (class_key (idx) == cdlguide->class_key (idxguide))
-    if (++idx == ndecls)
-      return;
+  const unsigned idxguide = def_p ? cdlguide->idxdef : 0;
 
   /* The class-key the class is expected to be declared with: it's
      either the key used in its definition or the first declaration
@@ -31323,10 +31320,15 @@ class_decl_loc_t::diag_mismatched_tags (tree type_decl)
      For implicit instantiations of a primary template it's
      the class-key used to declare the primary with.  The primary
      must be at index zero.  */
-  const tag_types xpect_key
-    = cdlguide->class_key (cdlguide == this ? idxguide : 0);
-  if (cdlguide != this && xpect_key == class_key (idx))
-    return;
+  const tag_types xpect_key = cdlguide->class_key (idxguide);
+
+  unsigned idx = 0;
+  /* Advance IDX to the first declaration that either is not
+     a definition or that doesn't match the first declaration
+     if no definition is provided.  */
+  while (class_key (idx) == xpect_key)
+    if (++idx == ndecls)
+      return;
 
   /* Save the current function before changing it below.  */
   tree save_func = current_function_decl;