Fix fixAbstractType for user defined typeclasses, fixes #19730 & #18409 (#19732)

compiler/semexprs.nim

@@ -645,7 +645,7 @@ proc fixAbstractType(c: PContext, n: PNode) =
         skipTypes(it.typ, abstractVar).kind notin {tyOpenArray, tyVarargs}:
       if skipTypes(it[1].typ, abstractVar).kind in
             {tyNil, tyTuple, tySet} or it[1].isArrayConstr:
-        var s = skipTypes(it.typ, abstractVar)
+        var s = skipTypes(it.typ, abstractVar + tyUserTypeClasses)
         if s.kind != tyUntyped:
           changeType(c, it[1], s, check=true)
         n[i] = it[1]

tests/concepts/t18409.nim

@@ -0,0 +1,37 @@
+discard """
+  action: "compile"
+"""
+
+# A vector space over a field F concept.
+type VectorSpace*[F] = concept x, y, type V
+  vector_add(x, y) is V
+  scalar_mul(x, F) is V
+  dimension(V) is Natural
+
+# Real numbers (here floats) form a vector space.
+func vector_add*(v: float, w: float): float =  v + w
+func scalar_mul*(v: float, s: float): float =   v * s
+func dimension*(x: typedesc[float]): Natural = 1
+
+# 2-tuples of real numbers form a vector space.
+func vector_add*(v, w: (float, float)): (float, float) =
+  (vector_add(v[0], w[0]), vector_add(v[1], w[1]))
+
+func scalar_mul*(v: (float, float), s: float): (float, float) =
+  (scalar_mul(v[0], s), scalar_mul(v[1], s))
+
+func dimension*(x: typedesc[(float, float)]): Natural = 2
+
+# Check concept requirements.
+assert float is VectorSpace
+assert (float, float) is VectorSpace
+
+# Commutivity axiom for vector spaces over the same field.
+func axiom_commutivity*[F](u, v: VectorSpace[F]): bool =
+  vector_add(u, v) == vector_add(v, u)
+
+# This is okay.
+assert axiom_commutivity(2.2, 3.3)
+
+# This is not.
+assert axiom_commutivity((2.2, 3.3), (4.4, 5.5))

tests/concepts/t19730.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''1.01.01.01.0
+1.01.01.01.0
+'''
+"""
+
+type
+  Color = concept c
+    c.r is SomeFloat
+    c.g is SomeFloat
+    c.b is SomeFloat
+    c.a is SomeFloat
+
+proc useColor(color: Color) =
+  echo(color.r, color.g, color.b, color.a)
+
+let color = (r: 1.0, g: 1.0, b: 1.0, a: 1.0)
+useColor(color)
+
+useColor((r: 1.0, g: 1.0, b: 1.0, a: 1.0))