fixes #4088

compiler/pragmas.nim

@@ -751,7 +751,9 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: int,
         noVal(it)
         incl(sym.flags, sfThread)
         incl(sym.flags, sfProcvar)
-        if sym.typ != nil: incl(sym.typ.flags, tfThread)
+        if sym.typ != nil:
+          incl(sym.typ.flags, tfThread)
+          if sym.typ.callConv == ccClosure: sym.typ.callConv = ccDefault
       of wGcSafe:
         noVal(it)
         if sym.kind != skType: incl(sym.flags, sfThread)

compiler/semtypes.nim

@@ -1141,14 +1141,12 @@ proc semProcTypeWithScope(c: PContext, n: PNode,
   checkSonsLen(n, 2)
   openScope(c)
   result = semProcTypeNode(c, n.sons[0], nil, prev, kind, isType=true)
+  # start with 'ccClosure', but of course pragmas can overwrite this:
+  result.callConv = ccClosure
   # dummy symbol for `pragma`:
   var s = newSymS(kind, newIdentNode(getIdent("dummy"), n.info), c)
   s.typ = result
-  if n.sons[1].kind == nkEmpty or n.sons[1].len == 0:
-    if result.callConv == ccDefault:
-      result.callConv = ccClosure
-      #Message(n.info, warnImplicitClosure, renderTree(n))
-  else:
+  if n.sons[1].kind != nkEmpty and n.sons[1].len > 0:
     pragma(c, s, n.sons[1], procTypePragmas)
     when useEffectSystem: setEffectsForProcType(result, n.sons[1])
   closeScope(c)

lib/system/threads.nim

@@ -419,7 +419,7 @@ when false:
 
 when hostOS == "windows":
   proc createThread*[TArg](t: var Thread[TArg],
-                           tp: proc (arg: TArg) {.thread.},
+                           tp: proc (arg: TArg) {.thread, nimcall.},
                            param: TArg) =
     ## creates a new thread `t` and starts its execution. Entry point is the
     ## proc `tp`. `param` is passed to `tp`. `TArg` can be ``void`` if you
@@ -441,7 +441,7 @@ when hostOS == "windows":
 
 else:
   proc createThread*[TArg](t: var Thread[TArg],
-                           tp: proc (arg: TArg) {.thread.},
+                           tp: proc (arg: TArg) {.thread, nimcall.},
                            param: TArg) =
     ## creates a new thread `t` and starts its execution. Entry point is the
     ## proc `tp`. `param` is passed to `tp`. `TArg` can be ``void`` if you
@@ -464,7 +464,7 @@ else:
     cpusetIncl(cpu.cint, s)
     setAffinity(t.sys, sizeof(s), s)
 
-proc createThread*(t: var Thread[void], tp: proc () {.thread.}) =
+proc createThread*(t: var Thread[void], tp: proc () {.thread, nimcall.}) =
   createThread[void](t, tp)
 
 proc threadId*[TArg](t: var Thread[TArg]): ThreadId[TArg] {.inline.} =