don't instantiate generic body type symbols in generic expressions (#24092)

fixes #24090

Generic body types are normally a sign of an uninstantiated type, and so
give errors when trying to instantiate them. However when instantiating
free user expressions like the nodes of `tyFromExpr`, generic default
params, static values etc, they can be used as arguments to macros or
templates etc (as in the issue). So, we don't try to instantiate generic
body type symbols at all in free expressions such as these (but not in
for example type nodes), and avoid the error.

In the future there should be a "concrete type" check for generic body
types different from the check in type instantiation to deal with things
like #24091, if we do want to allow this use of them.

compiler/semtypinst.nim

@@ -119,6 +119,11 @@ proc replaceTypeVarsT*(cl: var TReplTypeVars, t: PType): PType =
   checkMetaInvariants(cl, result)
 
 proc prepareNode*(cl: var TReplTypeVars, n: PNode): PNode =
+  ## instantiates a given generic expression, not a type node
+  if n.kind == nkSym and n.sym.kind == skType and
+      n.sym.typ != nil and n.sym.typ.kind == tyGenericBody:
+    # generic body types are allowed as user expressions, see #24090
+    return n
   let t = replaceTypeVarsT(cl, n.typ)
   if t != nil and t.kind == tyStatic and t.n != nil:
     return if tfUnresolved in t.flags: prepareNode(cl, t.n)

tests/generics/tuninstantiatedgenericcalls.nim

@@ -409,3 +409,29 @@ block: # weird regression
     # but expected: <T, U>
     x
   doAssert foo(Foo[int](1), Bar[int, int](2)).int == 1
+
+block: # issue #24090
+  type M[V] = object
+  template y[V](N: type M, v: V): M[V] = default(M[V])
+  proc d(x: int | int, f: M[int] = M.y(0)) = discard
+  d(0, M.y(0))
+  type Foo[T] = object
+    x: typeof(M.y(default(T)))
+  var a: Foo[int]
+  doAssert a.x is M[int]
+  var b: Foo[float]
+  doAssert b.x is M[float]
+  doAssert not (compiles do:
+    type Bar[T] = object
+      x: typeof(M()) # actually fails here immediately
+    var bar: Bar[int])
+  doAssert not (compiles do:
+    type Bar[T] = object
+      x: typeof(default(M))
+    var bar: Bar[int]
+    # gives "undeclared identifier x" because of #24091,
+    # normally it should fail in the line above
+    echo bar.x)
+  proc foo[T: M](x: T = default(T)) = discard x
+  foo[M[int]]()
+  doAssert not compiles(foo())