ast: duplicateFeatureDeclaration, deduplicate if a,b are switched around

src/dev/flang/ast/AstErrors.java

@@ -944,23 +944,28 @@ public static void repeatedInheritanceCannotBeResolved(SourcePosition pos, Abstr
           "To solve this, you could add a redefinition of " + sbnf(f1) + " to " + s(heir) + ".");
   }
 
-  public static void duplicateFeatureDeclaration(SourcePosition pos, AbstractFeature f, AbstractFeature existing)
+  public static void duplicateFeatureDeclaration(AbstractFeature a, AbstractFeature b)
   {
     // suppress error message if errors were reported already and any feature
     // involved is f_ERROR
-    if (!any() || (f                != Types.f_ERROR &&
-                         f       .outer() != Types.f_ERROR &&
-                         existing         != Types.f_ERROR &&
-                         existing.outer() != Types.f_ERROR    ))
+    if (!any() || (a                != Types.f_ERROR &&
+                   a       .outer() != Types.f_ERROR &&
+                   b         != Types.f_ERROR &&
+                   b.outer() != Types.f_ERROR    ))
       {
-        var of = f.isTypeFeature() ? f.typeFeatureOrigin() : f;
-        error(pos,
+        // report in source code order to avoid symmetric error with exchanged roles of f and existing
+        var cmpRes = a.pos().show().compareTo(b.pos().show())>0;
+        var aa = cmpRes ? a : b;
+        var bb = cmpRes ? b : a;
+
+        var of = aa.isTypeFeature() ? aa.typeFeatureOrigin() : aa;
+        error(bb.pos(),
               "Duplicate feature declaration",
               "Feature that was declared repeatedly: " + s(of) + "\n" +
-              "originally declared at " + existing.pos().show() + "\n" +
+              "originally declared at " + aa.pos().show() + "\n" +
               "To solve this, consider renaming one of these two features, e.g., as " + sbn(of.featureName().baseNameHuman() + "ʼ") +
               " (using a unicode modifier letter apostrophe " + sbn("ʼ")+ " U+02BC) "+
-              (f.isTypeFeature()
+              (aa.isTypeFeature()
                ? ("or changing it into a routine by returning a " +
                   sbn("unit") + " result, i.e., adding " + sbn("unit") + " before " + code("is") + " or using " + code("=>") +
                   " instead of "+ code("is") + ".")

src/dev/flang/fe/SourceModule.java

@@ -1020,7 +1020,7 @@ void addDeclaredInnerFeature(AbstractFeature outer, Feature f)
               }
             else
               {
-                AstErrors.duplicateFeatureDeclaration(f.pos(), f, existing);
+                AstErrors.duplicateFeatureDeclaration(f, existing);
               }
           }
       }
@@ -1901,7 +1901,7 @@ private void checkDuplicateFeatures(AbstractFeature outer, FeatureName fn, List<
                 else
                   {
                     // NYI: if (!isInherited && !sameModule(f, outer))
-                    AstErrors.duplicateFeatureDeclaration(f1.pos(), f1, f2);
+                    AstErrors.duplicateFeatureDeclaration(f1, f2);
                   }
               }
           }

tests/reg_issue3647/reg_issue3647.fz

@@ -21,8 +21,6 @@
 #
 # -----------------------------------------------------------------------
 
-# NYI: BUG: should not display error twice.
-
 reg_issue3647 =>
 
   Node(T Type, a Node T) ref is

tests/reg_issue3647/reg_issue3647.fz.expected_err

@@ -1,21 +1,11 @@
 
---CURDIR--/reg_issue3647.fz:28:23: error 1: Duplicate feature declaration
+--CURDIR--/reg_issue3647.fz:26:23: error 1: Duplicate feature declaration
   Node(T Type, a Node T) ref is
 ----------------------^
 Feature that was declared repeatedly: 'reg_issue3647.Node.T'
-originally declared at --CURDIR--/reg_issue3647.fz:28:8:
+originally declared at --CURDIR--/reg_issue3647.fz:26:8:
   Node(T Type, a Node T) ref is
 -------^
 To solve this, consider renaming one of these two features, e.g., as 'Tʼ' (using a unicode modifier letter apostrophe 'ʼ' U+02BC) or adding an additional argument (e.g. '_ unit' for an ignored unit argument used only to disambiguate these two).
 
-
---CURDIR--/reg_issue3647.fz:28:8: error 2: Duplicate feature declaration
-  Node(T Type, a Node T) ref is
--------^
-Feature that was declared repeatedly: 'reg_issue3647.Node.T'
-originally declared at --CURDIR--/reg_issue3647.fz:28:23:
-  Node(T Type, a Node T) ref is
-----------------------^
-To solve this, consider renaming one of these two features, e.g., as 'Tʼ' (using a unicode modifier letter apostrophe 'ʼ' U+02BC) or adding an additional argument (e.g. '_ unit' for an ignored unit argument used only to disambiguate these two).
-
-2 errors.
+one error.