c++: -fimplicit-constexpr diagnostic improvement [PR116696]

PR116696 expressed surprise that explicit 'constexpr' was needed on one
function; this was because the function isn't 'inline', and
-fimplicit-constexpr doesn't try to promote non-inline functions.  Let's be
more helpful in that situation, and also help trace through functions that
were promoted.

	PR c++/116696

gcc/cp/ChangeLog:

	* constexpr.cc (explain_invalid_constexpr_fn): When
	-fimplicit-constexpr, also explain inline functions, and point out
	non-inline functions.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2478.C: Prune extra diagnostic.
	* g++.dg/ext/fimplicit-constexpr1.C: New test.

gcc/cp/constexpr.cc

@@ -1057,9 +1057,16 @@ explain_invalid_constexpr_fn (tree fun)
   /* Only diagnose defaulted functions, lambdas, or instantiations.  */
   else if (!DECL_DEFAULTED_FN (fun)
 	   && !LAMBDA_TYPE_P (CP_DECL_CONTEXT (fun))
+	   && !(flag_implicit_constexpr
+		&& !DECL_DECLARED_CONSTEXPR_P (fun)
+		&& DECL_DECLARED_INLINE_P (fun))
 	   && !is_instantiation_of_constexpr (fun))
     {
       inform (DECL_SOURCE_LOCATION (fun), "%qD declared here", fun);
+      if (flag_implicit_constexpr && !maybe_constexpr_fn (fun)
+	  && decl_defined_p (fun))
+	inform (DECL_SOURCE_LOCATION (fun),
+		"%<-fimplicit-constexpr%> only affects %<inline%> functions");
       return;
     }
   if (diagnosed == NULL)

gcc/testsuite/g++.dg/DRs/dr2478.C

@@ -2,7 +2,7 @@
 // { dg-do compile { target c++20 } }
 
 // Defeat -fimplicit-constexpr
-int ii;
+int ii; // { dg-prune-output "value of 'ii' is not usable in a constant expr" }
 
 template <typename T>
 struct S {

gcc/testsuite/g++.dg/ext/fimplicit-constexpr1.C

@@ -0,0 +1,8 @@
+// { dg-additional-options -fimplicit-constexpr }
+// { dg-do compile { target c++14 } }
+
+void f() { } // { dg-message "'-fimplicit-constexpr' only affects 'inline' functions" }
+
+inline int g() { f(); return 42; } // { dg-error {non-'constexpr' function 'void f\(\)'} }
+
+constexpr int i = g(); // { dg-error {'int g\(\)' called in a constant expression} }