fix 25778; concept coerces incompatible types

compiler/sigmatch.nim

@@ -1155,6 +1155,10 @@ proc enterConceptMatch(c: var TCandidate; f,a: PType, flags: TTypeRelFlags): TTy
   if concpt.kind != tyConcept:
     container = concpt
     concpt = container.reduceToBase
+  # considerPreviousT-like behavior
+  let prev = lookup(c.bindings, concpt)
+  if prev != nil:
+    return typeRel(c, prev, a, flags)
   if trDontBind in flags:
     conceptFlags.incl mfDontBind
   if trCheckGeneric in flags:

tests/concepts/conceptv2negative/tconsistentparams.nim

@@ -0,0 +1,28 @@
+discard """
+action: "reject"
+errormsg: "type mismatch"
+"""
+
+type
+  Dollarable = concept
+    proc `$`(x: Self): string
+
+proc checkEqual(x, y: Dollarable) =
+  if x != y:
+    echo $x
+    echo $y
+
+type
+  StateFlags = enum
+    sfMatch
+    sfSoft
+
+  MatchKind = enum
+    NoFurtherMatch
+    NoMatch
+    Match
+    AllFurtherMatch
+
+proc `==`(a: set[StateFlags]; b: MatchKind): bool = true
+
+checkEqual({sfMatch, sfSoft}, Match)