Don't skip generic aliases when reading a generic parameter

compiler/semexprs.nim

@@ -1361,7 +1361,8 @@ const
   tyDotOpTransparent = {tyVar, tyLent, tyPtr, tyRef, tyOwned, tyAlias, tySink}
 
 proc readTypeParameter(c: PContext, typ: PType,
-                       paramName: PIdent, info: TLineInfo): PNode =
+                       paramName: PIdent, info: TLineInfo, skip = true): PNode =
+  ## - **skip**: Skips generic aliases and tries to get the parameter from the aliased type.
   # Note: This function will return emptyNode when attempting to read
   # a static type parameter that is not yet resolved (e.g. this may
   # happen in proc signatures such as `proc(x: T): array[T.sizeParam, U]`
@@ -1387,8 +1388,12 @@ proc readTypeParameter(c: PContext, typ: PType,
         discard
 
   if typ.kind != tyUserTypeClass:
-    let ty = if typ.kind == tyCompositeTypeClass: typ.firstGenericParam.skipGenericAlias
-             else: typ.skipGenericAlias
+    let ty = block:
+      let ty = if typ.kind == tyCompositeTypeClass: typ.firstGenericParam
+               else: typ
+      if skip: typ.skipGenericAlias()
+      else: typ
+
     let tbody = ty[0]
     for s in 0..<tbody.len-1:
       let tParam = tbody[s]
@@ -1519,7 +1524,8 @@ proc semSym(c: PContext, n: PNode, sym: PSym, flags: TExprFlags): PNode =
 proc tryReadingGenericParam(c: PContext, n: PNode, i: PIdent, t: PType): PNode =
   case t.kind
   of tyGenericInst:
-    result = readTypeParameter(c, t, i, n.info)
+    # Parameter could exist on the alias, don't skip past it
+    result = readTypeParameter(c, t, i, n.info, skip=false)
     if result == c.graph.emptyNode:
       if c.inGenericContext > 0:
         result = semGenericStmt(c, n)

tests/generics/t24791.nim

@@ -0,0 +1,14 @@
+import std/options
+
+type
+  Foo[D: static int; T] = T
+  Bar[D] = Option[D]
+  Bizz = Foo[1, string]
+  Buzz = Foo[2, Option[string]]
+
+  CheckPriority[T: static int; D] = Option[D]
+
+assert Bar[string].D is string
+assert Bizz.D == 1
+assert Buzz.D == 2
+assert CheckPriority[3, string].T is string