From 24e5b21c900811ae8d0a69e06fc35fdd884b3ddc Mon Sep 17 00:00:00 2001
From: metagn <metagngn@gmail.com>
Date: Mon, 9 Sep 2024 11:12:10 +0300
Subject: [PATCH] fix regression with generic params in static type (#24075)

Caught in https://github.com/metagn/applicates, I'm not sure which
commit causes this but it's also in the 2.0 branch (but not 2.0.2), so
it's not any recent PRs.

If a proc has a static parameter with type `static Foo[T]`, then another
parameter with type `static Bar[T, U]`, the generic instantiation for
`Bar` doesn't match `U` which has type `tyGenericParam`, but matches `T`
since it has type `tyTypeDesc`. The reason is that `concreteType`
returns the type itself for `tyTypeDesc` if `c.isNoCall` (i.e. matching
a generic invocation), but returns `nil` for `tyGenericParam`. I'm
guessing `tyGenericParam` is received here because of #22618, but that
doesn't explain why `T` is still `tyTypeDesc`. I'm not sure.

Regardless, we can just copy the behavior for `tyTypeDesc` to
`tyGenericParam` and also return the type itself when `c.isNoCall`. This
feels like it defeats the purpose of `concreteType` but the way it's
used doesn't make sense without it (generic param can't match another
generic param?). Alternatively we could loosen the `if concrete == nil:
return isNone` checks in some places for specific conditions, whether
`c.isNoCall` or `c.inGenericContext == 0` (though this would need
#24005).
---
 compiler/sigmatch.nim                          |  1 +
 tests/generics/tuninstantiatedgenericcalls.nim | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index e519f17c8c85..e1f197679852 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -485,6 +485,7 @@ proc concreteType(c: TCandidate, t: PType; f: PType = nil): PType =
     else: result = t
   of tyGenericParam, tyAnything, tyConcept:
     result = t
+    if c.isNoCall: return
     while true:
       result = idTableGet(c.bindings, t)
       if result == nil:
diff --git a/tests/generics/tuninstantiatedgenericcalls.nim b/tests/generics/tuninstantiatedgenericcalls.nim
index 9b43d18d9c5e..a349f0d2ad99 100644
--- a/tests/generics/tuninstantiatedgenericcalls.nim
+++ b/tests/generics/tuninstantiatedgenericcalls.nim
@@ -397,3 +397,15 @@ block: # `when`, test no constant semchecks
       {.error: "bad 2".}
   )
   var y: Bar[int]
+
+block: # weird regression
+  type
+    Foo[T] = distinct int
+    Bar[T, U] = distinct int
+  proc foo[T, U](x: static Foo[T], y: static Bar[T, U]): Foo[T] =
+    # signature gives:
+    # Error: cannot instantiate Bar
+    # got: <typedesc[T], U>
+    # but expected: <T, U>
+    x
+  doAssert foo(Foo[int](1), Bar[int, int](2)).int == 1