diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index db2a9c1543e..d0f61748141 100644
--- a/gcc/cp/constexpr.cc
+++ b/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)
diff --git a/gcc/testsuite/g++.dg/DRs/dr2478.C b/gcc/testsuite/g++.dg/DRs/dr2478.C
index 7f581cabb7b..b2292561381 100644
--- a/gcc/testsuite/g++.dg/DRs/dr2478.C
+++ b/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 {
diff --git a/gcc/testsuite/g++.dg/ext/fimplicit-constexpr1.C b/gcc/testsuite/g++.dg/ext/fimplicit-constexpr1.C
new file mode 100644
index 00000000000..fc4b2829b65
--- /dev/null
+++ b/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} }