fixes #10912 (#11317)

* fixes #10912

* update the tutorial examples

3 files changed, 54 insertions, 6 deletions

diff --git a/compiler/cgmeth.nim b/compiler/cgmeth.nim
index db548ccc9..ab406bc4b 100644
--- a/compiler/cgmeth.nim
+++ b/compiler/cgmeth.nim
@@ -56,7 +56,7 @@ proc methodCall*(n: PNode; conf: ConfigRef): PNode =
 type
   MethodResult = enum No, Invalid, Yes
 
-proc sameMethodBucket(a, b: PSym): MethodResult =
+proc sameMethodBucket(a, b: PSym; multiMethods: bool): MethodResult =
   if a.name.id != b.name.id: return
   if sonsLen(a.typ) != sonsLen(b.typ):
     return
@@ -75,7 +75,7 @@ proc sameMethodBucket(a, b: PSym): MethodResult =
     if sameType(a.typ.sons[i], b.typ.sons[i]):
       if aa.kind == tyObject and result != Invalid:
         result = Yes
-    elif aa.kind == tyObject and bb.kind == tyObject:
+    elif aa.kind == tyObject and bb.kind == tyObject and (i == 1 or multiMethods):
       let diff = inheritanceDiff(bb, aa)
       if diff < 0:
         if result != Invalid:
@@ -162,7 +162,7 @@ proc methodDef*(g: ModuleGraph; s: PSym, fromCache: bool) =
   var witness: PSym
   for i in 0 ..< L:
     let disp = g.methods[i].dispatcher
-    case sameMethodBucket(disp, s)
+    case sameMethodBucket(disp, s, multimethods = optMultiMethods in g.config.globalOptions)
     of Yes:
       add(g.methods[i].methods, s)
       attachDispatcher(s, disp.ast[dispatcherPos])
diff --git a/doc/tut2.rst b/doc/tut2.rst
index fda953eda..44b76f5ac 100644
--- a/doc/tut2.rst
+++ b/doc/tut2.rst
@@ -277,7 +277,7 @@ Procedures always use static dispatch. For dynamic dispatch replace the
       a, b: Expression
 
   # watch out: 'eval' relies on dynamic binding
-  method eval(e: Expression): int =
+  method eval(e: Expression): int {.base.} =
     # override this base method
     quit "to override!"
 
@@ -300,7 +300,7 @@ In a multi-method all parameters that have an object type are used for the
 dispatching:
 
 .. code-block:: nim
-    :test: "nim c $1"
+    :test: "nim c --multiMethods:on $1"
 
   type
     Thing = ref object of RootObj
@@ -611,7 +611,7 @@ To pass a block of statements to a template, use 'untyped' for the last paramete
     :test: "nim c $1"
 
   template withFile(f: untyped, filename: string, mode: FileMode,
-                    body: untyped): typed =
+                    body: untyped) =
     let fn = filename
     var f: File
     if open(f, fn, mode):
diff --git a/tests/method/tsingle_methods.nim b/tests/method/tsingle_methods.nim
new file mode 100644
index 000000000..40269559a
--- /dev/null
+++ b/tests/method/tsingle_methods.nim
@@ -0,0 +1,48 @@
+discard """
+  cmd: "nim c --multimethods:off $file"
+  output: '''base
+base
+base
+base
+base
+base
+'''
+"""
+
+# bug #10912
+
+type
+  X = ref object of RootObj
+
+type
+  A* = ref object of RootObj
+  B* = ref object of A
+  C* = ref object of A
+  D* = ref object of A
+  E* = ref object of A
+  F* = ref object of A
+
+method resolve(self: var X, stmt: A) {.base.} = echo "base"
+
+proc resolveSeq*(self: var X, statements: seq[A]) =
+  for statement in statements:
+    resolve(self, statement)
+
+method resolve(self: var X, stmt: B) =
+  echo "B"
+
+method resolve(self: var X, stmt: D) =
+  echo "D"
+
+method resolve(self: var X, stmt: E) =
+  echo "E"
+
+method resolve(self: var X, stmt: C) =
+  echo "C"
+
+method resolve(self: var X, stmt: F) =
+  echo "F"
+
+var x = X()
+var a = @[A(), B(), C(), D(), E(), F()]
+resolveSeq(x, a)