c++: consteval and explicit instantiation [PR96905]

Normally, an explicit instantiation means we want to write out the
instantiation.  But not for a consteval function.

gcc/cp/ChangeLog:

	PR c++/96905
	* pt.c (mark_decl_instantiated): Exit early if consteval.

gcc/testsuite/ChangeLog:

	PR c++/96905
	* g++.dg/cpp2a/consteval-expinst1.C: New test.

gcc/cp/pt.c

@@ -24154,6 +24154,11 @@ mark_decl_instantiated (tree result, int extern_p)
   if (TREE_ASM_WRITTEN (result))
     return;
 
+  /* consteval functions are never emitted.  */
+  if (TREE_CODE (result) == FUNCTION_DECL
+      && DECL_IMMEDIATE_FUNCTION_P (result))
+    return;
+
   /* For anonymous namespace we don't need to do anything.  */
   if (decl_anon_ns_mem_p (result))
     {

gcc/testsuite/g++.dg/cpp2a/consteval-expinst1.C

@@ -0,0 +1,20 @@
+// PR c++/96905
+// { dg-do compile { target c++20 } }
+
+template<typename Rep>
+struct duration
+{
+  static consteval int
+  gcd(int m, int n) noexcept
+  {
+    while (m != 0 && n != 0)
+    {
+      int rem = m % n;
+      m = n;
+      n = rem;
+    }
+    return m + n;
+  }
+};
+
+template class duration<int>;