propagate used typevar count in nested invoke (#1171)

fixes #1167

Invocations track if semchecking their arguments adds to a counter of
used typevar symbols, to decide if the invocation is concrete or not so
that it can be instantiated. However it does this by swapping the
counter with a local variable, which isolates the typevar count in the
arguments from the typevar count of the invocation itself. So
invocations with generic parameters do not count as generic when nested
inside another invocation, which wrongly instantiates it. To fix this,
the typevar counter is not isolated, only the counter changing is
tracked.

Worst case `containsGenericParams` can be used on each of the arguments.

Author: metagn

src/nimony/semtypes.nim

@@ -323,8 +323,7 @@ proc semInvoke(c: var SemContext; n: var Cursor) =
   var paramCount = 0
   var argCount = 0
   var m = createMatch(addr c)
-  var genericArgs = 0
-  swap c.usedTypevars, genericArgs
+  let usedTypevarsInitial = c.usedTypevars
   let beforeArgs = c.dest.len
   while n.kind != ParRi:
     inc argCount
@@ -349,15 +348,16 @@ proc semInvoke(c: var SemContext; n: var Cursor) =
           addArg = false
     if addArg:
       c.dest.add argBuf
-  swap c.usedTypevars, genericArgs
+  let usedTypevarsFinal = c.usedTypevars
+  let isConcrete = usedTypevarsInitial == usedTypevarsFinal # no generic params were used
   takeParRi c, n
   if ok and paramCount != argCount:
     c.dest.shrink typeStart
     c.buildErr info, "wrong amount of generic parameters for type " & pool.syms[headId] &
       ", expected " & $paramCount & " but got " & $argCount
     return
 
-  if ok and (genericArgs == 0 or
+  if ok and (isConcrete or
       # structural types are inlined even with generic arguments
       not isNominal(decl.body.typeKind)):
     # we have to be eager in generic type instantiations so that type-checking
@@ -374,7 +374,7 @@ proc semInvoke(c: var SemContext; n: var Cursor) =
       let instSuffix = instToSuffix(c.dest, typeStart)
       let targetSym = newInstSymId(c, headId, instSuffix)
       c.instantiatedTypes[key] = targetSym
-      if genericArgs == 0:
+      if isConcrete:
         c.typeInstDecls.add targetSym
       var sub = createTokenBuf(30)
       subsGenericTypeFromArgs c, sub, info, instSuffix, headId, targetSym, decl, args

tests/nimony/generics/tnestedgenericinvoke.nim

@@ -0,0 +1,7 @@
+type
+  Foo[T] = object
+  Bar[T] = object
+
+proc foo[T](x: Foo[Bar[T]]) = discard
+var x = Foo[Bar[int]]()
+foo x