bypass constraints for tyFromExpr in generic bodies

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

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