fixes compiler crash by preventing exportc on generics (#22731)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
(cherry picked from commit fefde3a735)

compiler/semstmts.nim

@@ -1483,9 +1483,12 @@ proc typeSectionRightSidePass(c: PContext, n: PNode) =
       # final pass
       if a[2].kind in nkCallKinds:
         incl a[2].flags, nfSem # bug #10548
-    if sfExportc in s.flags and s.typ.kind == tyAlias:
-      localError(c.config, name.info, "{.exportc.} not allowed for type aliases")
-
+    if sfExportc in s.flags:
+      if s.typ.kind == tyAlias:
+        localError(c.config, name.info, "{.exportc.} not allowed for type aliases")
+      elif s.typ.kind == tyGenericBody:
+        localError(c.config, name.info, "{.exportc.} not allowed for generic types")
+    
     if tfBorrowDot in s.typ.flags:
       let body = s.typ.skipTypes({tyGenericBody})
       if body.kind != tyDistinct:

tests/types/texportgeneric.nim

@@ -0,0 +1,8 @@
+discard """
+  errormsg: "{.exportc.} not allowed for generic types"
+  line: 6
+"""
+
+type Struct[T] {.exportc.} = object
+  a:int
+  b: T