fix error messages for wrongly typed generic param default values (#24006)

fixes #21258

When a generic proc is instantiated, if one of the default values
doesn't match the type of the parameter, `seminst` sets the default
parameter node to an `nkEmpty` node with `tyError` type. `sigmatch`
checks for this to give an error message if the default param is
actually used, but only while actively matching the proc signature,
before the proc is even instantiated. The error message also gives very
little information.

Now, we check for this in `updateDefaultParams` at the end of
`semResolvedCall`, after the proc has been instantiated. The `nkEmpty`
node also is given the original mismatching type instead rather than
`tyError`, only setting `tyError` after erroring to prevent cascading
errors. The error message is changed to the standard type mismatch error
also giving the instantiation info of the routine.

compiler/semcall.nim

@@ -595,7 +595,7 @@ proc inferWithMetatype(c: PContext, formal: PType,
     result = copyTree(arg)
     result.typ = formal
 
-proc updateDefaultParams(call: PNode) =
+proc updateDefaultParams(c: PContext, call: PNode) =
   # In generic procs, the default parameter may be unique for each
   # instantiation (see tlateboundgenericparams).
   # After a call is resolved, we need to re-assign any default value
@@ -605,8 +605,18 @@ proc updateDefaultParams(call: PNode) =
   let calleeParams = call[0].sym.typ.n
   for i in 1..<call.len:
     if nfDefaultParam in call[i].flags:
-      let def = calleeParams[i].sym.ast
+      let formal = calleeParams[i].sym
+      let def = formal.ast
       if nfDefaultRefsParam in def.flags: call.flags.incl nfDefaultRefsParam
+      # mirrored with sigmatch:
+      if def.kind == nkEmpty:
+        # The default param value is set to empty in `instantiateProcType`
+        # when the type of the default expression doesn't match the type
+        # of the instantiated proc param:
+        pushInfoContext(c.config, call.info, call[0].sym.detailedInfo)
+        typeMismatch(c.config, def.info, formal.typ, def.typ, formal.ast)
+        popInfoContext(c.config)
+        def.typ = errorType(c)
       call[i] = def
 
 proc getCallLineInfo(n: PNode): TLineInfo =
@@ -727,7 +737,7 @@ proc semResolvedCall(c: PContext, x: var TCandidate,
   result[0] = newSymNode(finalCallee, getCallLineInfo(result[0]))
   if finalCallee.magic notin {mArrGet, mArrPut}:
     result.typ = finalCallee.typ.returnType
-  updateDefaultParams(result)
+  updateDefaultParams(c, result)
 
 proc canDeref(n: PNode): bool {.inline.} =
   result = n.len >= 2 and (let t = n[1].typ;

compiler/seminst.nim

@@ -253,9 +253,13 @@ proc instantiateProcType(c: PContext, pt: TypeMapping,
     var typeToFit = resulti
 
     let needsStaticSkipping = resulti.kind == tyFromExpr
+    let needsTypeDescSkipping = resulti.kind == tyTypeDesc and tfUnresolved in resulti.flags
     result[i] = replaceTypeVarsT(cl, resulti)
     if needsStaticSkipping:
       result[i] = result[i].skipTypes({tyStatic})
+    if needsTypeDescSkipping:
+      result[i] = result[i].skipTypes({tyTypeDesc})
+      typeToFit = result[i]
 
     # ...otherwise, we use the instantiated type in `fitNode`
     if (typeToFit.kind != tyTypeDesc or typeToFit.base.kind != tyNone) and
@@ -292,6 +296,8 @@ proc instantiateProcType(c: PContext, pt: TypeMapping,
         # the user calls an explicit instantiation of the proc (this is
         # the only way the default value might be inserted).
         param.ast = errorNode(c, def)
+        # we know the node is empty, we need the actual type for error message
+        param.ast.typ = def.typ
       else:
         param.ast = fitNodePostMatch(c, typeToFit, converted)
       param.typ = result[i]

compiler/sigmatch.nim

@@ -2790,14 +2790,16 @@ proc matches*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
           m.firstMismatch.formal = formal
           break
       else:
+        # mirrored with updateDefaultParams:
         if formal.ast.kind == nkEmpty:
           # The default param value is set to empty in `instantiateProcType`
           # when the type of the default expression doesn't match the type
           # of the instantiated proc param:
-          localError(c.config, m.call.info,
-                     ("The default parameter '$1' has incompatible type " &
-                      "with the explicitly requested proc instantiation") %
-                      formal.name.s)
+          pushInfoContext(c.config, m.call.info,
+            if m.calleeSym != nil: m.calleeSym.detailedInfo else: "")
+          typeMismatch(c.config, formal.ast.info, formal.typ, formal.ast.typ, formal.ast)
+          popInfoContext(c.config)
+          formal.ast.typ = errorType(c)
         if nfDefaultRefsParam in formal.ast.flags:
           m.call.flags.incl nfDefaultRefsParam
         var defaultValue = copyTree(formal.ast)

tests/proc/twrongdefaultvalue.nim

@@ -0,0 +1,25 @@
+discard """
+  cmd: "nim check $file"
+  action: reject
+  nimout: '''
+twrongdefaultvalue.nim(20, 12) template/generic instantiation of `doit` from here
+twrongdefaultvalue.nim(17, 37) Error: type mismatch: got <proc (p: int): Item[initItem.T]> but expected 'Item[system.string]'
+twrongdefaultvalue.nim(25, 3) template/generic instantiation of `foo` from here
+twrongdefaultvalue.nim(23, 33) Error: type mismatch: got <string> but expected 'int'
+'''
+"""
+
+block: # issue #21258
+  type Item[T] = object
+    pos: int
+  proc initItem[T](p:int=10000) : Item[T] = 
+    result = Item[T](p)
+  proc doit[T](x:Item[T], s:Item[T]=initItem) : string = 
+    return $x.pos
+  let x = Item[string](pos:100)
+  echo doit(x)
+
+block: # issue #21258, reduced case
+  proc foo[T](x: seq[T], y: T = "foo") =
+    discard
+  foo @[1, 2, 3]