don't allow instantiations resolving to generic body types (#24273)

fixes #24091, refs #24092

Any instantiations resolving to a generic body type now gives an error.
Due to #24092, this does not error in cases like matching against `type
M` in generics because generic body type symbols are just not
instantiated. But this prevents parameters with type `type M` from being
used, although there doesn't seem to be any code which does this. Just
in case such code exists, we still allow `typedesc` types resolving to
generic body types.

(cherry picked from commit 2f904535d0)

compiler/seminst.nim

@@ -53,7 +53,9 @@ iterator instantiateGenericParamList(c: PContext, n: PNode, pt: LayeredIdTable):
           if q.typ.kind != tyCompositeTypeClass:
             localError(c.config, a.info, errCannotInstantiateX % s.name.s)
           t = errorType(c)
-      elif t.kind in {tyGenericParam, tyConcept, tyFromExpr}:
+      elif t.kind in {tyGenericParam, tyConcept, tyFromExpr} or
+          # generic body types are accepted as typedesc arguments
+          (t.kind == tyGenericBody and q.typ.kind != tyTypeDesc):
         localError(c.config, a.info, errCannotInstantiateX % q.name.s)
         t = errorType(c)
       elif isUnresolvedStatic(t) and (q.typ.kind == tyStatic or

tests/generics/tgenericbodyreturn1.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "cannot instantiate: 'T'"
+  file: "system.nim"
+"""
+
+# issue #24091
+
+type M[V] = object
+echo default(M)

tests/generics/tgenericbodyreturn2.nim

@@ -0,0 +1,11 @@
+discard """
+  errormsg: "cannot instantiate: 'T'"
+  file: "system.nim"
+"""
+
+# issue #24091
+
+type M[V] = object
+type Foo = object # notice not generic
+  x: typeof(default(M))
+echo Foo() # ()

tests/generics/tgenericbodyreturn3.nim

@@ -0,0 +1,13 @@
+discard """
+  errormsg: "cannot instantiate: 'ShouldNotResolve'"
+"""
+
+# issue #24091
+
+type Generic[U] = object
+proc foo[ShouldNotResolve](x: typedesc[ShouldNotResolve]): ShouldNotResolve =
+  echo ShouldNotResolve # Generic
+  echo declared(result) # true
+  echo typeof(result) # Generic
+echo typeof(foo(Generic)) # void
+foo(Generic)

tests/generics/tuninstantiatedgenericcalls.nim

@@ -421,17 +421,17 @@ block: # issue #24090
   doAssert a.x is M[int]
   var b: Foo[float]
   doAssert b.x is M[float]
+  # related to #24091:
   doAssert not (compiles do:
     type Bar[T] = object
-      x: typeof(M()) # actually fails here immediately
-    var bar: Bar[int])
+      # ideally fails here immediately since the inside of `typeof` does not
+      # depend on an unresolved parameter
+      # but if typechecking gets too lazy, then we need to instantiate to error
+      x: typeof(M()))
   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)
+      # again, ideally fails here immediately
+      x: typeof(default(M)))
   proc foo[T: M](x: T = default(T)) = discard x
   foo[M[int]]()
   doAssert not compiles(foo())