c++: bogus warning w/ deduction guide in anon ns [PR106604]

Here we're unintentionally issuing a "declared static but never defined"
warning from wrapup_namespace_globals for a deduction guide declared in
an anonymous namespace.  This patch fixes this by giving deduction guides
a dummy DECL_INITIAL, which suppresses the warning and also allows us to
simplify redeclaration checking for them.

Co-authored-by: Jason Merrill <jason@redhat.com>

	PR c++/106604

gcc/cp/ChangeLog:

	* decl.cc (redeclaration_error_message): Remove special handling
	for deduction guides.
	(grokfndecl): Give deduction guides a dummy DECL_INITIAL.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction74.C: Expect "defined" instead
	of "declared" in the repeated deduction guide diagnostics.
	* g++.dg/cpp1z/class-deduction116.C: New test.

gcc/cp/decl.cc

@@ -3297,10 +3297,6 @@ redeclaration_error_message (tree newdecl, tree olddecl)
 	    }
 	}
-      if (deduction_guide_p (olddecl)
-	  && deduction_guide_p (newdecl))
-	return G_("deduction guide %q+D redeclared");
-
       /* [class.compare.default]: A definition of a comparison operator as
 	 defaulted that appears in a class shall be the first declaration of
 	 that function.  */
@@ -3355,10 +3351,6 @@ redeclaration_error_message (tree newdecl, tree olddecl)
 	    }
 	}
-      if (deduction_guide_p (olddecl)
-	  && deduction_guide_p (newdecl))
-	return G_("deduction guide %q+D redeclared");
-
       /* Core issue #226 (C++11):
            If a friend function template declaration specifies a
@@ -10352,6 +10344,12 @@ grokfndecl (tree ctype,
       DECL_CXX_DESTRUCTOR_P (decl) = 1;
       DECL_NAME (decl) = dtor_identifier;
       break;
+    case sfk_deduction_guide:
+      /* Give deduction guides a definition even though they don't really
+	 have one: the restriction that you can't repeat a deduction guide
+	 makes them more like a definition anyway.  */
+      DECL_INITIAL (decl) = void_node;
+      break;
     default:
       break;
     }

gcc/testsuite/g++.dg/cpp1z/class-deduction116.C

+// PR c++/106604
+// { dg-do compile { target c++17 } }
+// { dg-additional-options "-Wunused-function" }
+
+namespace {
+  template<class T> struct A { A(...); };
+  A(bool) -> A<bool>; // { dg-bogus "never defined" }
+}

gcc/testsuite/g++.dg/cpp1z/class-deduction74.C

@@ -7,20 +7,20 @@
 template<typename> struct S { };
 template<typename> struct X { };
 
-S() -> S<int>; // { dg-message "previously declared here|old declaration" }
-S() -> S<int>; // { dg-error "redeclared" }
+S() -> S<int>; // { dg-message "previously defined here|old declaration" }
+S() -> S<int>; // { dg-error "redefinition" }
 X() -> X<int>;
 S() -> S<float>; // { dg-error "ambiguating new declaration of" }
 
-S(bool) -> S<int>; // { dg-message "previously declared here" }
-explicit S(bool) -> S<int>; // { dg-error "redeclared" }
+S(bool) -> S<int>; // { dg-message "previously defined here" }
+explicit S(bool) -> S<int>; // { dg-error "redefinition" }
 
-explicit S(char) -> S<int>; // { dg-message "previously declared here" }
-S(char) -> S<int>; // { dg-error "redeclared" }
+explicit S(char) -> S<int>; // { dg-message "previously defined here" }
+S(char) -> S<int>; // { dg-error "redefinition" }
 
 template<typename T> S(T, T) -> S<int>; // { dg-message "previously declared here" }
 template<typename T> X(T, T) -> X<int>;
-template<typename T> S(T, T) -> S<int>; // { dg-error "redeclared" }
+template<typename T> S(T, T) -> S<int>; // { dg-error "redefinition" }
 
 // OK: Use SFINAE.
 template<typename T> S(T) -> S<typename T::foo>;