diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 07500fa2b21..6f0f5cc14a2 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8623,6 +8623,7 @@ extern void mangle_module_substitution		(int);
 extern int mangle_module_component		(tree id, bool partition);
 extern tree mangle_module_global_init		(int);
 extern unsigned HOST_WIDE_INT range_expr_nelts	(tree);
+extern bool equal_abi_tags			(tree, tree);
 
 /* in dump.cc */
 extern bool cp_dump_tree			(void *, tree);
diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 9ca5cf6eecd..02129c6b89a 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -238,7 +238,6 @@ static void write_local_name (tree, const tree, const tree);
 static void dump_substitution_candidates (void);
 static tree mangle_decl_string (const tree);
 static void maybe_check_abi_tags (tree, tree = NULL_TREE, int = 10);
-static bool equal_abi_tags (tree, tree);
 
 /* Control functions.  */
 
@@ -1673,7 +1672,7 @@ write_source_name (tree identifier)
 
 /* Compare two TREE_STRINGs like strcmp.  */
 
-int
+static int
 tree_string_cmp (const void *p1, const void *p2)
 {
   if (p1 == p2)
@@ -1728,7 +1727,7 @@ write_abi_tags (tree tags)
 
 /* True iff the TREE_LISTS T1 and T2 of ABI tags are equivalent.  */
 
-static bool
+bool
 equal_abi_tags (tree t1, tree t2)
 {
   releasing_vec v1 = sorted_abi_tags (t1);
@@ -1738,7 +1737,8 @@ equal_abi_tags (tree t1, tree t2)
   if (len1 != v2->length())
     return false;
   for (unsigned i = 0; i < len1; ++i)
-    if (tree_string_cmp (v1[i], v2[i]) != 0)
+    if (strcmp (TREE_STRING_POINTER (v1[i]),
+		TREE_STRING_POINTER (v2[i])) != 0)
       return false;
   return true;
 }
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index f2d64bf3b5f..214fb918e8d 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -3012,6 +3012,7 @@ public:
   bool read_definition (tree decl);
 
 private:
+  void check_abi_tags (tree existing, tree decl, tree &eattr, tree &dattr);
   bool is_matching_decl (tree existing, tree decl, bool is_typedef);
   static bool install_implicit_member (tree decl);
   bool read_function_def (tree decl, tree maybe_template);
@@ -8832,8 +8833,11 @@ trees_in::decl_value ()
 	  tree etype = TREE_TYPE (existing);
 
 	  /* Handle separate declarations with different attributes.  */
+	  tree &dattr = TYPE_ATTRIBUTES (type);
 	  tree &eattr = TYPE_ATTRIBUTES (etype);
-	  eattr = merge_attributes (eattr, TYPE_ATTRIBUTES (type));
+	  check_abi_tags (existing, decl, eattr, dattr);
+	  // TODO: handle other conflicting type attributes
+	  eattr = merge_attributes (eattr, dattr);
 
 	  /* When merging a partial specialisation, the existing decl may have
 	     had its TYPE_CANONICAL adjusted.  If so we should use structural
@@ -12011,6 +12015,51 @@ trees_in::binfo_mergeable (tree *type)
   return u ();
 }
 
+/* DECL is a just streamed declaration with attributes DATTR that should
+   have matching ABI tags as EXISTING's attributes EATTR.  Check that the
+   ABI tags match, and report an error if not.  */
+
+void
+trees_in::check_abi_tags (tree existing, tree decl, tree &eattr, tree &dattr)
+{
+  tree etags = lookup_attribute ("abi_tag", eattr);
+  tree dtags = lookup_attribute ("abi_tag", dattr);
+  if ((etags == nullptr) != (dtags == nullptr)
+      || (etags && !attribute_value_equal (etags, dtags)))
+    {
+      if (etags)
+	etags = TREE_VALUE (etags);
+      if (dtags)
+	dtags = TREE_VALUE (dtags);
+
+      /* We only error if mangling wouldn't consider the tags equivalent.  */
+      if (!equal_abi_tags (etags, dtags))
+	{
+	  auto_diagnostic_group d;
+	  if (dtags)
+	    error_at (DECL_SOURCE_LOCATION (decl),
+		      "mismatching abi tags for %qD with tags %qE",
+		      decl, dtags);
+	  else
+	    error_at (DECL_SOURCE_LOCATION (decl),
+		      "mismatching abi tags for %qD with no tags", decl);
+	  if (etags)
+	    inform (DECL_SOURCE_LOCATION (existing),
+		    "existing declaration here with tags %qE", etags);
+	  else
+	    inform (DECL_SOURCE_LOCATION (existing),
+		    "existing declaration here with no tags");
+	}
+
+      /* Always use the existing abi_tags as the canonical set so that
+	 later processing doesn't get confused.  */
+      if (dtags)
+	dattr = remove_attribute ("abi_tag", dattr);
+      if (etags)
+	duplicate_one_attribute (&dattr, eattr, "abi_tag");
+    }
+}
+
 /* DECL is a just streamed mergeable decl that should match EXISTING.  Check
    it does and issue an appropriate diagnostic if not.  Merge any
    bits from DECL to EXISTING.  This is stricter matching than
@@ -12222,6 +12271,10 @@ trees_in::is_matching_decl (tree existing, tree decl, bool is_typedef)
   if (!DECL_EXTERNAL (d_inner))
     DECL_EXTERNAL (e_inner) = false;
 
+  if (VAR_OR_FUNCTION_DECL_P (d_inner))
+    check_abi_tags (existing, decl,
+		    DECL_ATTRIBUTES (e_inner), DECL_ATTRIBUTES (d_inner));
+
   if (TREE_CODE (decl) == TEMPLATE_DECL)
     {
       /* Merge default template arguments.  */
diff --git a/gcc/testsuite/g++.dg/modules/attrib-3_a.H b/gcc/testsuite/g++.dg/modules/attrib-3_a.H
new file mode 100644
index 00000000000..d1f672ae64d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/attrib-3_a.H
@@ -0,0 +1,16 @@
+// PR c++/118920
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+struct [[gnu::abi_tag("a", "b")]] A;
+struct [[gnu::abi_tag("a", "b")]] B;
+struct [[gnu::abi_tag("c")]] C;
+struct D;
+struct E;
+
+[[gnu::abi_tag("f")]] void f();
+void g();
+[[gnu::abi_tag("x", "y")]] void h();
+
+[[gnu::abi_tag("test", "some", "more")]] extern int i;
+[[gnu::abi_tag("test", "some", "more")]] extern int j;
diff --git a/gcc/testsuite/g++.dg/modules/attrib-3_b.C b/gcc/testsuite/g++.dg/modules/attrib-3_b.C
new file mode 100644
index 00000000000..557c18bb31f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/attrib-3_b.C
@@ -0,0 +1,30 @@
+// PR c++/118920
+// { dg-additional-options "-fmodules -fno-module-lazy" }
+
+struct A {};  // { dg-message "existing declaration here" }
+struct [[gnu::abi_tag("b", "a")]] B;  // OK
+struct [[gnu::abi_tag("x")]] C;  // { dg-message "existing declaration here" }
+struct [[gnu::abi_tag("d")]] D;  // { dg-message "existing declaration here" }
+
+void f();  // { dg-message "existing declaration here" }
+[[gnu::abi_tag("g")]] void g();  // { dg-message "existing declaration here" }
+[[gnu::abi_tag("y", "x")]] void h();  // OK
+
+[[gnu::abi_tag("more", "test")]] extern int i;  // { dg-message "existing declaration here" }
+[[gnu::abi_tag("more", "test", "really", "some")]] extern int j;  // { dg-message "existing declaration here" }
+
+import "attrib-3_a.H";
+
+struct [[gnu::abi_tag("e")]] E;  // { dg-error "adds abi tag" }
+
+// { dg-error "mismatching abi tags for .struct A." "" { target *-*-* } 0 }
+// B is OK
+// { dg-error "mismatching abi tags for .struct C." "" { target *-*-* } 0 }
+// { dg-error "mismatching abi tags for .struct D." "" { target *-*-* } 0 }
+
+// { dg-error "mismatching abi tags for .void f()." "" { target *-*-* } 0 }
+// { dg-error "mismatching abi tags for .void g()." "" { target *-*-* } 0 }
+// h is OK
+
+// { dg-error "mismatching abi tags for .i." "" { target *-*-* } 0 }
+// { dg-error "mismatching abi tags for .j." "" { target *-*-* } 0 }
diff --git a/gcc/testsuite/g++.dg/modules/pr118920.h b/gcc/testsuite/g++.dg/modules/pr118920.h
new file mode 100644
index 00000000000..73d38b56adb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pr118920.h
@@ -0,0 +1,3 @@
+template <typename T> struct out_ptr_t {
+  operator int() const;
+};
diff --git a/gcc/testsuite/g++.dg/modules/pr118920_a.H b/gcc/testsuite/g++.dg/modules/pr118920_a.H
new file mode 100644
index 00000000000..75023b37c16
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pr118920_a.H
@@ -0,0 +1,4 @@
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+#include "pr118920.h"
diff --git a/gcc/testsuite/g++.dg/modules/pr118920_b.H b/gcc/testsuite/g++.dg/modules/pr118920_b.H
new file mode 100644
index 00000000000..23e37a6ee77
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pr118920_b.H
@@ -0,0 +1,12 @@
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+template <typename T> struct __shared_ptr {
+  template <typename> friend struct out_ptr_t;
+};
+inline namespace __cxx11 __attribute__((__abi_tag__)) {
+  struct _Impl;
+}
+struct path {
+  __shared_ptr<_Impl> _M_impl;
+};
diff --git a/gcc/testsuite/g++.dg/modules/pr118920_c.C b/gcc/testsuite/g++.dg/modules/pr118920_c.C
new file mode 100644
index 00000000000..b1b6c711ac3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/pr118920_c.C
@@ -0,0 +1,5 @@
+// { dg-additional-options "-fmodules -fno-module-lazy" }
+
+#include "pr118920.h"
+import "pr118920_b.H";
+import "pr118920_a.H";