From 0448557bfec17f42acdccea2ad6a157ce620ce0d Mon Sep 17 00:00:00 2001
From: Ryan McConnell <rammcconnell@gmail.com>
Date: Thu, 11 Jun 2026 02:17:35 -0400
Subject: [PATCH] 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                         |  4 +++
 .../conceptv2negative/tconsistentparams.nim   | 28 +++++++++++++++++++
 tests/concepts/tconceptsv2.nim                | 14 ++++++++++
 3 files changed, 46 insertions(+)
 create mode 100644 tests/concepts/conceptv2negative/tconsistentparams.nim

diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index cb79823af5..9526d37290 100644
--- a/compiler/sigmatch.nim
+++ b/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:
diff --git a/tests/concepts/conceptv2negative/tconsistentparams.nim b/tests/concepts/conceptv2negative/tconsistentparams.nim
new file mode 100644
index 0000000000..c1a0c1290d
--- /dev/null
+++ b/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)
diff --git a/tests/concepts/tconceptsv2.nim b/tests/concepts/tconceptsv2.nim
index d861c51c75..13f8211f5b 100644
--- a/tests/concepts/tconceptsv2.nim
+++ b/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")