[v3,5/5] c++/modules: Validate external linkage definitions in header units [PR116401]

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 07fb9855cd2..572a4e7d98b 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -10833,6 +10833,10 @@  grokfndecl (tree ctype,
 	 have one: the restriction that you can't repeat a deduction guide
 	 makes them more like a definition anyway.  */
       DECL_INITIAL (decl) = void_node;
+      /* But to ensure that external-linkage deduction guides in header units
+	 don't fall afoul of [module.import] p6, mark them as inline to give
+	 the same effect as them not having a definition.  */
+      DECL_DECLARED_INLINE_P (decl) = true;
       break;
     default:
       break;
diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 456b73d59b6..09256bf843a 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -19943,6 +19943,24 @@  check_module_decl_linkage (tree decl)
   if (!module_has_cmi_p ())
     return;
 
+  /* A header unit shall not contain a definition of a non-inline function
+     or variable (not template) whose name has external linkage.  */
+  if (header_module_p ()
+      && !processing_template_decl
+      && ((TREE_CODE (decl) == FUNCTION_DECL
+	   && !DECL_DECLARED_INLINE_P (decl)
+	   && DECL_INITIAL (decl))
+	  || (TREE_CODE (decl) == VAR_DECL
+	      && !DECL_INLINE_VAR_P (decl)
+	      && DECL_INITIALIZED_P (decl)
+	      && !DECL_IN_AGGR_P (decl)))
+      && !(DECL_LANG_SPECIFIC (decl)
+	   && DECL_TEMPLATE_INSTANTIATION (decl))
+      && decl_linkage (decl) == lk_external)
+    error_at (DECL_SOURCE_LOCATION (decl),
+	      "external linkage definition of %qD in header module must "
+	      "be declared %<inline%>", decl);
+
   /* An internal-linkage declaration cannot be generally be exported.
      But it's OK to export any declaration from a header unit, including
      internal linkage declarations.  */
diff --git a/gcc/testsuite/g++.dg/modules/hdr-2.H b/gcc/testsuite/g++.dg/modules/hdr-2.H
new file mode 100644
index 00000000000..097546d5667
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/hdr-2.H
@@ -0,0 +1,172 @@ 
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi !{} }
+// external linkage variables or functions in header units must
+// not have non-inline definitions
+
+int x_err;  // { dg-error "external linkage definition" }
+int y_err = 123;  // { dg-error "external linkage definition" }
+void f_err() {}  // { dg-error "external linkage definition" }
+
+struct Err {
+  Err();
+  void m();
+  static void s();
+  static int x;
+  static int y;
+};
+Err::Err() = default;  // { dg-error "external linkage definition" }
+void Err::m() {}  // { dg-error "external linkage definition" }
+void Err::s() {}  // { dg-error "external linkage definition" }
+int Err::x;  // { dg-error "external linkage definition" }
+int Err::y = 123;  // { dg-error "external linkage definition" }
+
+// No definition, OK
+extern int y_decl;
+void f_decl();
+
+template <typename> struct DeductionGuide {};
+DeductionGuide() -> DeductionGuide<int>;
+
+struct NoDefStatics {
+  enum E { V };
+  static const int x = 123;
+  static const E e = V;
+};
+
+// But these have definitions again (though the error locations aren't great)
+struct YesDefStatics {
+  enum E { V };
+  static const int x = 123;  // { dg-error "external linkage definition" }
+  static const E e = V;  // { dg-error "external linkage definition" }
+};
+const int YesDefStatics::x;
+const YesDefStatics::E YesDefStatics::e;
+
+// Inline decls are OK
+inline int x_inl;
+inline int y_inl = 123;
+inline void f_inl() {}
+constexpr void g_inl() {}
+void h_inl() = delete;
+
+struct Inl {
+  void m() {}
+  static void s() {}
+  static inline int x;
+  static inline int y = 123;
+};
+
+// Internal linkage decls are OK
+static int x_internal;
+static int y_internal = 123;
+static void f_internal() {}
+
+namespace {
+  struct Internal {
+    void m();
+    static void s();
+    static int x;
+    static int y;
+  };
+  void Internal::m() {}
+  void Internal::s() {}
+  int Internal::x;
+  int Internal::y = 123;
+}
+
+// Function-scope entities are OK
+inline void f_static() {
+  static int x_static;
+  static int y_static = 123;
+  thread_local int x_thread_local;
+  thread_local int y_thread_local = 123;
+
+  x_static = y_static;
+  x_thread_local = y_thread_local;
+}
+
+// Templates (not functions or variables) are OK
+template <typename> int x_tpl;
+template <typename> int y_tpl = 123;
+template <typename> void f_tpl() {}
+
+struct Template_Body {
+  template <typename> void m();
+  template <typename> static void s();
+  template <typename> static int x;
+  template <typename> static int y;
+};
+template <typename> void Template_Body::m() {}
+template <typename> void Template_Body::s() {}
+template <typename> int Template_Body::x;
+template <typename> int Template_Body::y = 123;
+
+template <typename> struct Template_Type {
+  void m();
+  static void s();
+  static int x;
+  static int y;
+};
+template <typename T> void Template_Type<T>::m() {}
+template <typename T> void Template_Type<T>::s() {}
+template <typename T> int Template_Type<T>::x;
+template <typename T> int Template_Type<T>::y = 123;
+
+// Implicit instantiations are OK
+inline void instantiate_tmpls() {
+  x_tpl<int> = y_tpl<int>;
+  f_tpl<int>();
+
+  Template_Body{}.m<int>();
+  Template_Body::s<int>();
+  Template_Body::x<int> = Template_Body::y<int>;
+
+  using TT = Template_Type<int>;
+  TT{}.m();
+  TT::s();
+  TT::x = TT::y;
+}
+
+// Explicit instantiations are also OK (extern or otherwise)
+template int x_tpl<char>;
+template int y_tpl<char>;
+template void f_tpl<char>();
+
+template void Template_Body::m<char>();
+template void Template_Body::s<char>();
+template int Template_Body::x<char>;
+template int Template_Body::y<char>;
+
+template void Template_Type<char>::m();
+template void Template_Type<char>::s();
+template int Template_Type<char>::x;
+template int Template_Type<char>::y;
+
+extern template int x_tpl<double>;
+extern template int y_tpl<double>;
+extern template void f_tpl<double>();
+
+extern template void Template_Body::m<double>();
+extern template void Template_Body::s<double>();
+extern template int Template_Body::x<double>;
+extern template int Template_Body::y<double>;
+
+extern template void Template_Type<double>::m();
+extern template void Template_Type<double>::s();
+extern template int Template_Type<double>::x;
+extern template int Template_Type<double>::y;
+
+// But explicit specialisations are not (though note [temp.expl.spec] p13)
+template <> int x_tpl<short>;  // { dg-error "inline" }
+template <> int y_tpl<short> = 123;  // { dg-error "inline" }
+template <> void f_tpl<short>() {}  // { dg-error "inline" }
+
+template <> void Template_Body::m<short>() {}  // { dg-error "inline" }
+template <> void Template_Body::s<short>() {}  // { dg-error "inline" }
+template <> int Template_Body::x<short>;  // { dg-bogus "inline" "not a definition" }
+template <> int Template_Body::y<short> = 123;  // { dg-error "inline" }
+
+template <> void Template_Type<short>::m() {}  // { dg-error "inline" }
+template <> void Template_Type<short>::s() {}  // { dg-error "inline" }
+template <> int Template_Type<short>::x;  // { dg-bogus "inline" "not a definition" }
+template <> int Template_Type<short>::y = 123;  // { dg-error "inline" }
diff --git a/gcc/testsuite/g++.dg/modules/macro-4_c.H b/gcc/testsuite/g++.dg/modules/macro-4_c.H
index ec2bed91ccd..5692e8e41d2 100644
--- a/gcc/testsuite/g++.dg/modules/macro-4_c.H
+++ b/gcc/testsuite/g++.dg/modules/macro-4_c.H
@@ -6,7 +6,7 @@ 
 
 #undef FIVE // no effect
 import "macro-4_a.H";
-int a;
+inline int a;
 #undef THREE
 #undef FOUR
 #define FOUR 4c
diff --git a/gcc/testsuite/g++.dg/modules/pr106761.h b/gcc/testsuite/g++.dg/modules/pr106761.h
index 9f22a22a45d..5c13fc0f118 100644
--- a/gcc/testsuite/g++.dg/modules/pr106761.h
+++ b/gcc/testsuite/g++.dg/modules/pr106761.h
@@ -19,4 +19,4 @@  struct tuple {
     = typename _TupleConstraints<Ts...>::template __constructible<Us...>;
 };
 
-tuple<int, int> t;
+inline tuple<int, int> t;
diff --git a/gcc/testsuite/g++.dg/modules/pr98843_b.H b/gcc/testsuite/g++.dg/modules/pr98843_b.H
index d6734bd382d..af504a840c7 100644
--- a/gcc/testsuite/g++.dg/modules/pr98843_b.H
+++ b/gcc/testsuite/g++.dg/modules/pr98843_b.H
@@ -6,7 +6,7 @@  template<int I> int Fn ()
   return I;
 }
 
-template<> int Fn<1> () 
+template<> inline int Fn<1> () 
 {
   return 0;
 }
diff --git a/gcc/testsuite/g++.dg/modules/pr99468.H b/gcc/testsuite/g++.dg/modules/pr99468.H
index d7da3a83e1c..df63c613b2c 100644
--- a/gcc/testsuite/g++.dg/modules/pr99468.H
+++ b/gcc/testsuite/g++.dg/modules/pr99468.H
@@ -3,4 +3,4 @@ 
 
 module M; // { dg-error "module-declaration not permitted" }
 
-int i;
+inline int i;
diff --git a/gcc/testsuite/g++.dg/modules/pragma-1_a.H b/gcc/testsuite/g++.dg/modules/pragma-1_a.H
index 6eb0a5900a7..a69be2fd05e 100644
--- a/gcc/testsuite/g++.dg/modules/pragma-1_a.H
+++ b/gcc/testsuite/g++.dg/modules/pragma-1_a.H
@@ -1,4 +1,4 @@ 
 // { dg-additional-options -fmodule-header }
 
 // { dg-module-cmi {} }
-int i;
+inline int i;
diff --git a/gcc/testsuite/g++.dg/modules/tpl-ary-1.h b/gcc/testsuite/g++.dg/modules/tpl-ary-1.h
index 2f745afba36..2f9a8106412 100644
--- a/gcc/testsuite/g++.dg/modules/tpl-ary-1.h
+++ b/gcc/testsuite/g++.dg/modules/tpl-ary-1.h
@@ -1,5 +1,5 @@ 
 
-int ary[4];
+inline int ary[4];
 extern int unb[];
 typedef int z[0];