From 31ee75f10e6482cd482d329507190ac850eee2cf Mon Sep 17 00:00:00 2001
From: metagn <metagngn@gmail.com>
Date: Sat, 20 Jul 2024 01:02:08 -0600
Subject: [PATCH] bypass constraints for tyFromExpr in generic bodies (#23863)

fixes #19819, fixes #23339

Since #22029 `tyFromExpr` does not match anything in overloading, so
generic bodies can know which call expressions to delay until the type
can be evaluated. However generic type invocations also run overloading
to check for generic constraints even in generic bodies. To prevent them
from failing early from the overload not matching, pretend that
`tyFromExpr` matches. This mirrors the behavior of the compiler in more
basic cases like:

```nim
type
  Foo[T: int] = object
    x: T
  Bar[T] = object
    y: Foo[T]
```

Unfortunately this case doesn't respect the constraint (#21181, some
other bugs) but `tyFromExpr` should easily use the same principle when
it does.
---
 compiler/sigmatch.nim                          | 13 +++++++++----
 tests/generics/tuninstantiatedgenericcalls.nim | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index 18170ecfcc..7a12b957ba 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -1196,10 +1196,15 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
         return isGeneric
   of tyFromExpr:
     if c.c.inGenericContext > 0:
-      # generic type bodies can sometimes compile call expressions
-      # prevent expressions with unresolved types from
-      # being passed as parameters
-      return isNone
+      if not c.isNoCall:
+        # generic type bodies can sometimes compile call expressions
+        # prevent expressions with unresolved types from
+        # being passed as parameters
+        return isNone
+      else:
+        # Foo[templateCall(T)] shouldn't fail early if Foo has a constraint
+        # and we can't evaluate `templateCall(T)` yet
+        return isGeneric
   else: discard
 
   case f.kind
diff --git a/tests/generics/tuninstantiatedgenericcalls.nim b/tests/generics/tuninstantiatedgenericcalls.nim
index 63ba3908f5..2163789e7b 100644
--- a/tests/generics/tuninstantiatedgenericcalls.nim
+++ b/tests/generics/tuninstantiatedgenericcalls.nim
@@ -145,3 +145,21 @@ block: # issue #23730
   proc test(M: static[int]): array[1 shl M, int] = discard
   doAssert len(test(3)) == 8
   doAssert len(test(5)) == 32
+
+block: # issue #19819
+  type
+    Example[N: static int] = distinct int
+    What[E: Example] = Example[E.N + E.N]
+
+block: # issue #23339
+  type
+    A = object
+    B = object
+  template aToB(t: typedesc[A]): typedesc = B
+  type
+    Inner[I] = object
+      innerField: I
+    Outer[O] = object
+      outerField: Inner[O.aToB]
+  var x: Outer[A]
+  doAssert typeof(x.outerField.innerField) is B