fixes #23790; roll back instCounter properly in case of exceptions (#23802)

fixes #23790

(cherry picked from commit 841d30a213)

compiler/pragmas.nim

@@ -862,6 +862,7 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: var int,
 
     # number of pragmas increase/decrease with user pragma expansion
     inc c.instCounter
+    defer: dec c.instCounter
     if c.instCounter > 100:
       globalError(c.config, it.info, "recursive dependency: " & userPragma.name.s)
 
@@ -871,7 +872,6 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: var int,
     pragma(c, sym, userPragma.ast, validPragmas, isStatement)
     n.sons[i..i] = userPragma.ast.sons # expand user pragma with its content
     i.inc(userPragma.ast.len - 1) # inc by -1 is ok, user pragmas was empty
-    dec c.instCounter
   else:
     let k = whichKeyword(ident)
     if k in validPragmas:

compiler/seminst.nim

@@ -339,7 +339,8 @@ proc generateInstance(c: PContext, fn: PSym, pt: TIdTable,
   # generates an instantiated proc
   if c.instCounter > 50:
     globalError(c.config, info, "generic instantiation too nested")
-  inc(c.instCounter)
+  inc c.instCounter
+  defer: dec c.instCounter
   # careful! we copy the whole AST including the possibly nil body!
   var n = copyTree(fn.ast)
   # NOTE: for access of private fields within generics from a different module
@@ -416,7 +417,6 @@ proc generateInstance(c: PContext, fn: PSym, pt: TIdTable,
   popOwner(c)
   c.currentScope = oldScope
   discard c.friendModules.pop()
-  dec(c.instCounter)
   c.matchedConcept = oldMatchedConcept
   if result.kind == skMethod: finishMethod(c, result)
 

tests/generics/t23790.nim

@@ -0,0 +1,14 @@
+# bug #23790
+
+discard compiles($default(seq[seq[ref int]]))
+discard compiles($default(seq[seq[ref uint]]))
+discard compiles($default(seq[seq[ref int8]]))
+discard compiles($default(seq[seq[ref uint8]]))
+discard compiles($default(seq[seq[ref int16]]))
+discard compiles($default(seq[seq[ref uint16]]))
+discard compiles($default(seq[seq[ref int32]]))
+discard compiles($default(seq[seq[ref uint32]]))
+discard compiles($default(seq[seq[ref int64]]))
+discard compiles($default(seq[seq[ref uint64]]))
+proc s(_: int | string) = discard
+s(0)