fix(sem): crash on unmatched immediate routines (#929)

## Summary

Unmatched `immediate` `macro`s and `template`s within a generic context
no longer result in a compiler error.

## Details

Immediate macros and templates are not dispatched via `sigmatch` and
instead `semgnrc` directly calls `evaltempl.evalTemplate`, in turn
`evalTemplateArgs` is called which can produce an error if an incorrect
number of arguments are provided. `evalTemplateArgs` is also used by
`semMacroExpr`, so similar issues would also exist for macros.

Now the error from `evalTemplateArgs` is immediately returned by
`evalTemplate` and treated as a mismatch in `semgnrc.semGenericStmt`.
Doing so prevents a compiler crash, attempt to access the sons of an
error
node. A test has been added as part of this change, which was
originally
from: zevv/npeg#68 (comment)

compiler/sem/evaltempl.nim

@@ -211,6 +211,8 @@ proc evalTemplate*(n: PNode, tmpl, genSymOwner: PSym;
     instID: instID[],
     idgen: idgen
   )
+  if args.kind == nkError:
+    return args # xxx: not the best way to do it, but this signals a mismatch
   let body = tmpl.ast[bodyPos]
   if isAtom(body):
     result = newNodeI(nkPar, body.info)

compiler/sem/semgnrc.nim

@@ -278,6 +278,9 @@ proc semGenericStmt(c: PContext, n: PNode,
       of skMacro:
         if macroToExpand(s) and sc.safeLen <= 1:
           result = semMacroExpr(c, n, s, {efNoSemCheck})
+          result =
+            if result.kind == nkError: n # an error means a "mismatch"
+            else:                      semGenericStmt(c, result, flags, ctx)
           result = semGenericStmt(c, result, flags, ctx)
           if result.isError: return
         else:
@@ -287,7 +290,9 @@ proc semGenericStmt(c: PContext, n: PNode,
       of skTemplate:
         if macroToExpand(s) and sc.safeLen <= 1:
           result = semTemplateExpr(c, n, s, {efNoSemCheck})
-          result = semGenericStmt(c, result, flags, ctx)
+          result =
+            if result.kind == nkError: n # an error means a "mismatch"
+            else:                      semGenericStmt(c, result, flags, ctx)
           if result.isError: return
         else:
           n[0] = sc

tests/lang_callable/overload/timmediate_meta_routine_mismatch.nim

@@ -0,0 +1,16 @@
+discard """
+action: reject
+description: "Ensure immediate macros/templates mismatches are reported"
+"""
+
+template unique(x, y: untyped) =
+  ## A template (macro would work too) that is not overloaded. 
+  # it's important that an argument is actually used within the template, in
+  # order to force traveral of the actual arguments (it's lazy and driven by
+  # the body).
+  y
+
+proc test[T]() =
+  unique("test")
+
+test()