fix 25778; concept coerces incompatible types (#25781)

I don't like it, but seems like this is correct. Concept type classes
have to behave like other "named" type classes and participate in "bind
once" mechanics or require some weird semantics. As a side note I'm
pretty sure the `tuple` example in the manual explaining this is either
wrong now or has regressed, but I don't think it matters because I doubt
anyone thinks about this feature much.
#25778

compiler/sigmatch.nim

@@ -1168,6 +1168,10 @@ 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)

tests/concepts/tconceptsv2.nim

@@ -14,6 +14,8 @@ b
 c
 1
 2
+5
+test
 '''
 """
 import conceptsv2_helper
@@ -600,3 +602,15 @@ block:
 
   let test = MemMapFileStream()
   spring(test)
+
+# explicit negative "bind once"
+
+type
+  Dollarable = concept
+    proc `$`(x: Self): string
+
+proc checkEqual2[T: Dollarable; S: Dollarable](x: T, y: S) =
+  echo $x
+  echo $y
+
+checkEqual2(5, "test")