produce more helpful error messages when overloading resolution fails

compiler/commands.nim

@@ -739,6 +739,8 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     of "hint": conf.globalOptions = conf.globalOptions + {optStyleHint}
     of "error": conf.globalOptions = conf.globalOptions + {optStyleError}
     else: localError(conf, info, errOffHintsError % arg)
+  of "showallmismatches":
+    processOnOffSwitchG(conf, {optShowAllMismatches}, arg, pass, info)
   of "cppcompiletonamespace":
     if arg.len > 0:
       conf.cppCustomNamespace = arg

compiler/options.nim

@@ -74,6 +74,7 @@ type                          # please make sure we have under 32 options
     optIdeTerse               # idetools: use terse descriptions
     optNoCppExceptions        # use C exception handling even with CPP
     optExcessiveStackTrace    # fully qualified module filenames
+    optShowAllMismatches      # show all overloading resolution candidates
     optWholeProject           # for 'doc2': output any dependency
     optMixedMode              # true if some module triggered C++ codegen
     optListFullPaths

compiler/semcall.nim

@@ -166,20 +166,22 @@ proc presentFailedCandidates(c: PContext, n: PNode, errors: CandidateErrors):
       prefer = preferModuleInfo
       break
 
-  when false:
   # we pretend procs are attached to the type of the first
   # argument in order to remove plenty of candidates. This is
   # comparable to what C# does and C# is doing fine.
   var filterOnlyFirst = false
+  if optShowAllMismatches notin c.config.globalOptions:
     for err in errors:
       if err.firstMismatch > 1:
         filterOnlyFirst = true
         break
 
   var candidates = ""
+  var skipped = 0
   for err in errors:
-    when false:
+    if filterOnlyFirst and err.firstMismatch == 1:
-      if filterOnlyFirst and err.firstMismatch == 1: continue
+      inc skipped
+      continue
     if err.sym.kind in routineKinds and err.sym.ast != nil:
       add(candidates, renderTree(err.sym.ast,
             {renderNoBody, renderNoComments, renderNoPragmas}))
@@ -216,7 +218,9 @@ proc presentFailedCandidates(c: PContext, n: PNode, errors: CandidateErrors):
                       "' is immutable\n")
     for diag in err.diagnostics:
       candidates.add(diag & "\n")
-
+  if skipped > 0:
+    candidates.add($skipped & " other mismatching symbols have been " &
+        " suppressed; compile with --showAllMismatches:on to see them\n")
   result = (prefer, candidates)
 
 const

doc/advopt.txt

@@ -29,6 +29,9 @@ Advanced options:
                             produce hints or errors for Nim identifiers that
                             do not adhere to Nim's official style guide
                             https://nim-lang.org/docs/nep1.html
+  --showAllMismatches:on|off
+                            show all mismatching candidates in overloading
+                            resolution
   --lib:PATH                set the system library path
   --import:PATH             add an automatically imported module
   --include:PATH            add an automatically included module