c++: explicit() ignored on deduction guide [PR100065]

When we have explicit() with a value-dependent argument, we can't
evaluate it at parsing time, so cp_parser_function_specifier_opt stashes
the argument into the decl-specifiers and grokdeclarator then stores it
into explicit_specifier_map, which is then used when substituting the
function decl.  grokdeclarator stores it for constructors and conversion
functions, but we also need to do it for deduction guides, otherwise
we'll forget that we've seen an explicit-specifier as in the attached
test.

	PR c++/100065

gcc/cp/ChangeLog:

	* decl.c (grokdeclarator): Store a value-dependent
	explicit-specifier even for deduction guides.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/explicit18.C: New test.

gcc/cp/decl.c

@@ -14043,6 +14043,8 @@ grokdeclarator (const cp_declarator *declarator,
 		storage_class = sc_none;
 	      }
 	  }
+	if (declspecs->explicit_specifier)
+	  store_explicit_specifier (decl, declspecs->explicit_specifier);
       }
     else
       {

gcc/testsuite/g++.dg/cpp2a/explicit18.C

@@ -0,0 +1,23 @@
+// PR c++/100065
+// { dg-do compile { target c++20 } }
+
+template<bool B>
+struct bool_constant {
+  static constexpr bool value = B;
+  constexpr operator bool() const { return value; }
+};
+
+using true_type = bool_constant<true>;
+using false_type = bool_constant<false>;
+
+template<bool>
+struct X {
+    template<typename T>
+    X(T);
+};
+
+template<bool b>
+explicit(b) X(bool_constant<b>) -> X<b>;
+
+X false_ = false_type{}; // OK
+X true_  = true_type{};  // { dg-error "explicit deduction guide" }