case macro now can be used inside generic. Fixes #20435 (#22752)

fixes #20435

---------

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>
Co-authored-by: Jake Leahy <jake@leahy.dev>
Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
(cherry picked from commit 0c179db657)

compiler/semgnrc.nim

@@ -187,6 +187,18 @@ proc addTempDecl(c: PContext; n: PNode; kind: TSymKind) =
   styleCheckDef(c, n.info, s, kind)
   onDef(n.info, s)
 
+proc addTempDeclToIdents(c: PContext; n: PNode; kind: TSymKind; inCall: bool) =
+  case n.kind 
+  of nkIdent:
+    if inCall:
+      addTempDecl(c, n, kind)
+  of nkCallKinds:
+    for s in n:
+      addTempDeclToIdents(c, s, kind, true)  
+  else:
+    for s in n:
+      addTempDeclToIdents(c, s, kind, inCall)
+
 proc semGenericStmt(c: PContext, n: PNode,
                     flags: TSemGenericFlags, ctx: var GenericCtx): PNode =
   result = n
@@ -359,7 +371,9 @@ proc semGenericStmt(c: PContext, n: PNode,
       var a = n[i]
       checkMinSonsLen(a, 1, c.config)
       for j in 0..<a.len-1:
-        a[j] = semGenericStmt(c, a[j], flags, ctx)
+        a[j] = semGenericStmt(c, a[j], flags+{withinMixin}, ctx)
+        addTempDeclToIdents(c, a[j], skVar, false)
+
       a[^1] = semGenericStmtScope(c, a[^1], flags, ctx)
     closeScope(c)
   of nkForStmt, nkParForStmt:

tests/macros/t20435.nim

@@ -0,0 +1,30 @@
+
+#[
+  A better test requires matching, so the use of @ working can be showcased
+  For example:
+
+  proc regularCase[T]() = 
+    case [(1, 3), (3, 4)]:
+    of [(1, @a), (_, @b)]:
+      echo a, b
+    else: discard
+]#
+
+{.experimental: "caseStmtMacros".}
+
+import macros
+
+type Foo = object
+
+macro `case`(obj: Foo) = quote do: discard
+
+proc notGeneric() =
+  case Foo()
+  of a b c d: discard
+
+proc generic[T]() =
+  case Foo()
+  of a b c d: discard
+
+notGeneric()
+generic[int]()