Emit a warning with -gno-strict-parameter-declaration

When a parameter is used before declaration, a warning is printed,
unless `-Wno-anachronisms`.

Documentation/usage/command_line_flags.rst

@@ -135,9 +135,8 @@ These flags affect the general behavior of the compiler.
 
     The standards requires that parameters must be declared lexically
     before they are used.  Using -gno-strict-parameter-declaration
-    will allow using a parameter before declaration, e.g., in a port
-    declaration, with the parameter declared in the body of the
-    module.
+    will allow using a parameter before declaration, with a
+    warning. The warning can be suppressed with -Wno-anachronisms.
 
   * shared-loop-index/no-shared-loop-index
 

compiler.h

@@ -211,8 +211,9 @@ extern bool gn_strict_expr_width_flag;
 extern bool gn_shared_loop_index_flag;
 
 /* If this flag is true (default), then parameters must be declared before
-   use.
-   use `-gno-strict-parameter-declaration` for pre version 13 behaviour.
+   use. `-gno-strict-parameter-declaration` allows to use parameters before
+   declaration, as prior to version 13.
+   A warning is emited with -Wanachronisms (default).
  */
 extern bool gn_strict_parameter_declaration;
 

driver/iverilog.man.in

@@ -153,8 +153,8 @@ numbers are not truncated to integer width.
 .B -gstrict-parameter-declaration\fI|\fP-gno-strict-parameter-declaration
 The standards requires that parameters must be declared lexically
 before they are used.  Using \fB\-gno\-strict\-parameter\-declaration\fP
-will allow using a parameter before declaration, e.g., in a port
-declaration, with the parameter declared in the body of the module.
+will allow using a parameter before declaration, with a warning.
+The warning can be suppressed with -Wno-anachronisms.
 .TP 8
 .B -gshared-loop-index\fI|\fP-gno-shared-loop-index
 Enable (default) or disable the exclusion of for-loop control variables

symbol_search.cc

@@ -195,6 +195,12 @@ bool symbol_search(const LineInfo*li, Design*des, NetScope*scope,
 			      res->scope = scope;
 			      res->par_val = par;
 			      res->path_head = path;
+			      if (warn_anachronisms && !prefix_scope
+				  && !(scope->get_parameter_lexical_pos(path_tail.name) <= lexical_pos)) {
+				      cerr << li->get_fileline()
+					   << ": warning: parameter `" << path_tail.name
+					   << "` used before declaration." << endl;
+			      }
 			      return true;
 			} else if (!res->decl_after_use) {
 			      res->decl_after_use = par;