fix logic for `dcEqIgnoreDistinct` in `sameType` (#24197)

fixes #22523

There were 2 problems with the code in `sameType` for
`dcEqIgnoreDistinct`:

1. The code that skipped `{tyDistinct, tyGenericInst}` only ran if the
given types had different kinds. This is fixed by always performing this
skip.
2. The code block below that checks if `tyGenericInst`s have different
values still ran for `dcEqIgnoreDistinct` since it checks if the given
types are generic insts, not the skipped types (and also only the 1st
given type). This is fixed by only invoking this block for `dcEq`;
`dcEqOrDistinctOf` (which is unused) also skips the first given type.
Arguably there is another issue here that `skipGenericAlias` only ever
skips 1 type.

These combined fix the issue (`T` is `GenericInst(V, 1, distinct int)`
and `D[0]` is `GenericInst(D, 0, distinct int)`).

#24037 shouldn't be a dependency but the diff follows it.

1 files changed, 14 insertions, 13 deletions

diff --git a/compiler/types.nim b/compiler/types.nim
index ec310e248..a441b0ea2 100644
--- a/compiler/types.nim
+++ b/compiler/types.nim
@@ -1250,18 +1250,18 @@ proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
     b = skipTypes(b.last, aliasSkipSet)
   assert(a != nil)
   assert(b != nil)
-  if a.kind != b.kind:
-    case c.cmp
-    of dcEq: return false
-    of dcEqIgnoreDistinct:
-      let distinctSkipSet = maybeSkipRange({tyDistinct, tyGenericInst})
-      a = a.skipTypes(distinctSkipSet)
-      b = b.skipTypes(distinctSkipSet)
-      if a.kind != b.kind: return false
-    of dcEqOrDistinctOf:
-      let distinctSkipSet = maybeSkipRange({tyDistinct, tyGenericInst})
-      a = a.skipTypes(distinctSkipSet)
-      if a.kind != b.kind: return false
+  case c.cmp
+  of dcEq:
+    if a.kind != b.kind: return false
+  of dcEqIgnoreDistinct:
+    let distinctSkipSet = maybeSkipRange({tyDistinct, tyGenericInst})
+    a = a.skipTypes(distinctSkipSet)
+    b = b.skipTypes(distinctSkipSet)
+    if a.kind != b.kind: return false
+  of dcEqOrDistinctOf:
+    let distinctSkipSet = maybeSkipRange({tyDistinct, tyGenericInst})
+    a = a.skipTypes(distinctSkipSet)
+    if a.kind != b.kind: return false
 
   #[
     The following code should not run in the case either side is an generic alias,
@@ -1269,7 +1269,8 @@ proc sameTypeAux(x, y: PType, c: var TSameTypeClosure): bool =
     objects ie `type A[T] = SomeObject`
   ]#
   # this is required by tunique_type but makes no sense really:
-  if x.kind == tyGenericInst and IgnoreTupleFields notin c.flags and tyDistinct != y.kind:
+  if c.cmp == dcEq and x.kind == tyGenericInst and
+      IgnoreTupleFields notin c.flags and tyDistinct != y.kind:
     let
       lhs = x.skipGenericAlias
       rhs = y.skipGenericAlias