fix 26147; new-style concepts: broken generic (Case B) (#26151)

ref #26147

(cherry picked from commit 0be9b4f3f6)

compiler/concepts.nim

@@ -11,7 +11,8 @@
 ## for details. Note this is a first implementation and only the "Concept matching"
 ## section has been implemented.
 
-import ast, semdata, lookups, lineinfos, idents, msgs, renderer, types, layeredtable
+import ast, semdata, lookups, lineinfos, idents, msgs, renderer, types,
+  layeredtable, semtypinst
 
 import std/sets
 
@@ -71,7 +72,8 @@ proc semConceptDeclaration*(c: PContext; n: PNode): PNode =
 
 type
   MatchFlags* = enum
-    mfDontBind  # Do not bind generic parameters
+    mfDontBind  # Do not export bindings from the concept match
+    mfBindGenericParam  # Export inferred invocation parameters despite mfDontBind
     mfCheckGeneric  # formal <- formal comparison as opposed to formal <- operand
 
   ConceptTypePair = tuple[conceptId, typeId: ItemId]
@@ -578,7 +580,17 @@ proc conceptMatchNode(c: PContext; n: PNode; m: var MatchCon): bool =
     # error was reported earlier.
     result = false
 
-proc fixBindings(bindings: var LayeredIdTable; concpt: PType; invocation: PType; m: var MatchCon) =
+proc resolvedBinding(c: PContext; t: PType; m: MatchCon): PType =
+  ## An inferred concept parameter can refer to an implementation-local
+  ## generic parameter, for example `Elem[Impl.T]`. Resolve it while the
+  ## matcher's private bindings (`Impl.T -> int`) are still available.
+  if t.containsUnresolvedType:
+    prepareMetatypeForSigmatch(c, m.bindings, m.concpt.sym.info, t)
+  else:
+    t
+
+proc fixBindings(c: PContext; bindings: var LayeredIdTable; concpt: PType;
+                 invocation: PType; m: var MatchCon) =
   # invocation != nil means we have a non-atomic concept:
   if invocation != nil and invocation.kind == tyGenericInvocation:
     assert concpt.sym.typ.kind == tyGenericBody
@@ -590,8 +602,9 @@ proc fixBindings(bindings: var LayeredIdTable; concpt: PType; invocation: PType;
         continue
       let found = m.bindings.lookup(thisSym)
       if found != nil:
-        when logBindings: echo "Invocation bind: ", thisSym, " ", found
-        bindings.put(thisSym, found)
+        let resolved = resolvedBinding(c, found, m)
+        when logBindings: echo "Invocation bind: ", thisSym, " ", resolved
+        bindings.put(thisSym, resolved)
     
     # bind even more generic parameters
     let genBody = invocation.base
@@ -607,6 +620,20 @@ proc fixBindings(bindings: var LayeredIdTable; concpt: PType; invocation: PType;
         bindings.put(invocation[i], boundV)
   bindings.put(concpt, m.potentialImplementation)
 
+proc fixConstraintBindings(c: PContext; bindings: var LayeredIdTable;
+                           invocation: PType; m: MatchCon) =
+  ## Propagates only the dependent parameters of a concept constraint. The
+  ## concept itself and its private matcher bindings must remain unbound so
+  ## that independent constraints using the same concept don't get coupled.
+  if invocation != nil and invocation.kind == tyGenericInvocation:
+    let genBody = invocation.base
+    assert genBody.kind == tyGenericBody
+    for i in FirstGenericParamAt ..< invocation.kidsLen:
+      if lookup(bindings, invocation[i]) == nil:
+        let boundValue = m.bindings.lookup(genBody[i - 1])
+        if boundValue != nil:
+          bindings.put(invocation[i], resolvedBinding(c, boundValue, m))
+
 proc processConcept(c: PContext; concpt, invocation: PType, bindings: var LayeredIdTable; m: var MatchCon): bool =
   m.bindings = m.bindings.newTypeMapLayer()
   if invocation != nil and invocation.kind == tyGenericInst:
@@ -616,8 +643,11 @@ proc processConcept(c: PContext; concpt, invocation: PType, bindings: var Layere
       if invocation[i].kind != tyVoid:
         bindParam(c, m, genericBody[i-1], invocation[i])
   result = conceptMatchNode(c, concpt.conceptBody, m)
-  if result and mfDontBind notin m.flags:
-    fixBindings(bindings, concpt, invocation, m)
+  if result:
+    if mfDontBind notin m.flags:
+      fixBindings(c, bindings, concpt, invocation, m)
+    elif mfBindGenericParam in m.flags:
+      fixConstraintBindings(c, bindings, invocation, m)
 
 proc conceptMatch*(c: PContext; concpt, arg: PType; bindings: var LayeredIdTable; invocation: PType, flags: set[MatchFlags] = {}): bool =
   ## Entry point from sigmatch. 'concpt' is the concept we try to match (here still a PType but

compiler/sigmatch.nim

@@ -1175,6 +1175,8 @@ proc enterConceptMatch(c: var TCandidate; f,a: PType, flags: TTypeRelFlags): TTy
     return typeRel(c, prev, a, flags)
   if trDontBind in flags:
     conceptFlags.incl mfDontBind
+  if trBindGenericParam in flags:
+    conceptFlags.incl mfBindGenericParam
   if trCheckGeneric in flags:
     conceptFlags.incl mfCheckGeneric
   let mres = concepts.conceptMatch(c.c, concpt, a, c.bindings, container, flags = conceptFlags)

tests/concepts/t26147.nim

@@ -0,0 +1,75 @@
+discard """
+  action: run
+"""
+
+type Indexable[T] = concept
+  proc `[]`(a: Self; index: int): T
+  proc len(a: Self): int
+
+iterator items[T; I: Indexable[T]](indexable: I): T =
+  for index in 0 ..< indexable.len:
+    yield indexable[index]
+
+type Dummy[T] = distinct seq[T]
+
+proc `[]`[T](d: Dummy[T], i: int): T = seq[T](d)[i]
+proc len[T](d: Dummy[T]): int = seq[T](d).len
+
+var acc = 0
+for x in Dummy(@[1, 2, 3]):
+  acc += x
+doAssert acc == 6
+
+# Inferred concept parameters are resolved through the implementation's own
+# generic bindings before being exported to the surrounding routine.
+type
+  Elem[T] = object
+    value: T
+  NestedDummy[T] = ref object
+    data: seq[T]
+
+proc `[]`[T](d: NestedDummy[T], i: int): Elem[T] =
+  Elem[T](value: d.data[i])
+proc len[T](d: NestedDummy[T]): int = d.data.len
+
+iterator directItems[T](indexable: Indexable[T]): T =
+  for index in 0 ..< indexable.len:
+    yield indexable[index]
+
+var nestedAcc = 0
+for x in NestedDummy[int](data: @[4, 5, 6]):
+  nestedAcc += x.value
+doAssert nestedAcc == 15
+
+var directNestedAcc = 0
+for x in directItems(NestedDummy[int](data: @[7, 8, 9])):
+  directNestedAcc += x.value
+doAssert directNestedAcc == 24
+
+# All dependent parameters inferred while checking a concept constraint must
+# be propagated to the constrained routine.
+type
+  KeyValue[K, V] = concept
+    proc key(x: Self): K
+    proc value(x: Self): V
+  Pair[K, V] = object
+    k: K
+    v: V
+
+proc key[K, V](x: Pair[K, V]): K = x.k
+proc value[K, V](x: Pair[K, V]): V = x.v
+
+proc unpack[K, V; P: KeyValue[K, V]](x: P): (K, V) =
+  (x.key, x.value)
+
+let pair = Pair[int, string](k: 7, v: "seven")
+doAssert unpack(pair) == (7, "seven")
+doAssert not compiles(unpack[string, int](pair))
+
+proc unpackBoth[K1, V1, K2, V2;
+                P1: KeyValue[K1, V1]; P2: KeyValue[K2, V2]](
+    x: P1; y: P2): ((K1, V1), (K2, V2)) =
+  (unpack(x), unpack(y))
+
+let otherPair = Pair[string, float](k: "eight", v: 8.0)
+doAssert unpackBoth(pair, otherPair) == ((7, "seven"), ("eight", 8.0))