solc does not warn or error for duplicate @param tags (#1320)

Make it a warning and add note to previous doc tag, and fix the locations.

Signed-off-by: Sean Young <sean@mess.org>

src/sema/tags.rs

@@ -19,7 +19,7 @@ pub fn resolve_tags(
     for c in tags.iter().flat_map(DocComment::comments) {
         let tag_loc = pt::Loc::File(file_no, c.tag_offset, c.tag_offset + c.tag.len() + 1);
         let value_loc = pt::Loc::File(file_no, c.value_offset, c.value_offset + c.value.len());
-        let loc = pt::Loc::File(file_no, c.tag_offset, c.value_offset + c.value.len());
+        let loc = pt::Loc::File(file_no, c.tag_offset - 1, c.value_offset + c.value.len());
 
         match c.tag.as_str() {
             "notice" | "author" | "title" | "dev" => {
@@ -49,10 +49,13 @@ pub fn resolve_tags(
                 let value = v.get(1).unwrap_or(&"").to_string();
 
                 if let Some(no) = params.unwrap().iter().position(|p| p.name_as_str() == name) {
-                    if res.iter().any(|e| e.tag == "param" && e.no == no) {
-                        ns.diagnostics.push(Diagnostic::error(
-                            tag_loc,
+                    if let Some(other) = res.iter().find(|e| e.tag == "param" && e.no == no) {
+                        // Note: solc does not detect this problem
+                        ns.diagnostics.push(Diagnostic::warning_with_note(
+                            loc,
                             format!("duplicate tag '@param' for '{name}'"),
+                            other.loc,
+                            format!("previous tag '@param' for '{name}'"),
                         ));
                     } else {
                         res.push(Tag {

tests/contract_testcases/substrate/tags/event_tag_02.sol

@@ -5,4 +5,6 @@
             uint32 f
         );
 // ---- Expect: diagnostics ----
-// error: 3:14-20: duplicate tag '@param' for 'f'
+// warning: 3:13-25: duplicate tag '@param' for 'f'
+// 	note 2:13-27: previous tag '@param' for 'f'
+// warning: 4:15-16: event 'x' has never been emitted

tests/contract_testcases/substrate/tags/functions_02.sol

@@ -7,4 +7,7 @@
             function foo(int f) public {}
         }
 // ---- Expect: diagnostics ----
-// error: 5:15-21: duplicate tag '@param' for 'f'
+// warning: 5:14-27: duplicate tag '@param' for 'f'
+// 	note 3:17-25: previous tag '@param' for 'f'
+// warning: 7:13-39: function can be declared 'pure'
+// warning: 7:30-31: function parameter 'f' has never been read

tests/contract_testcases/substrate/tags/struct_tag_02.sol

@@ -5,4 +5,5 @@
             uint32 f;
         }
 // ---- Expect: diagnostics ----
-// error: 3:14-20: duplicate tag '@param' for 'f'
+// warning: 3:13-25: duplicate tag '@param' for 'f'
+// 	note 2:13-27: previous tag '@param' for 'f'