fixes #3998

compiler/semtypes.nim

@@ -831,9 +831,11 @@ proc liftParamType(c: PContext, procKind: TSymKind, genericParams: PNode,
       result.rawAddSon paramType.lastSon
       return addImplicitGeneric(result)
 
-    result = instGenericContainer(c, paramType.sym.info, result,
+    let x = instGenericContainer(c, paramType.sym.info, result,
                                   allowMetaTypes = true)
-    result = newTypeWithSons(c, tyCompositeTypeClass, @[paramType, result])
+    result = newTypeWithSons(c, tyCompositeTypeClass, @[paramType, x])
+    #result = newTypeS(tyCompositeTypeClass, c)
+    #for i in 0..<x.len: result.rawAddSon(x.sons[i])
     result = addImplicitGeneric(result)
 
   of tyGenericInst:

compiler/sigmatch.nim

@@ -847,7 +847,10 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
           inc(c.inheritancePenalty, depth)
           result = isSubtype
   of tyDistinct:
-    if a.kind == tyDistinct and sameDistinctTypes(f, a): result = isEqual
+    if a.kind == tyDistinct:
+      if sameDistinctTypes(f, a): result = isEqual
+      elif f.base.kind == tyAnything: result = isGeneric
+      elif c.coerceDistincts: result = typeRel(c, f.base, a)
     elif c.coerceDistincts: result = typeRel(c, f.base, a)
   of tySet:
     if a.kind == tySet:
@@ -922,19 +925,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
     if a.kind == tyEmpty: result = isEqual
 
   of tyGenericInst:
-    let roota = a.skipGenericAlias
-    let rootf = f.skipGenericAlias
-    if a.kind == tyGenericInst and roota.base == rootf.base:
-      for i in 1 .. rootf.sonsLen-2:
-        let ff = rootf.sons[i]
-        let aa = roota.sons[i]
-        result = typeRel(c, ff, aa)
-        if result == isNone: return
-        if ff.kind == tyRange and result != isEqual: return isNone
-      #result = isGeneric
-      # XXX See bug #2220. A[int] should match A[int] better than some generic X
-    else:
-      result = typeRel(c, lastSon(f), a)
+    result = typeRel(c, lastSon(f), a)
 
   of tyGenericBody:
     considerPreviousT:
@@ -1035,12 +1026,20 @@ proc typeRel(c: var TCandidate, f, aOrig: PType, doBind = true): TTypeRelation =
 
   of tyCompositeTypeClass:
     considerPreviousT:
-      if typeRel(c, f.sons[1], a) != isNone:
-        put(c.bindings, f, a)
-        return isGeneric
+      let roota = a.skipGenericAlias
+      let rootf = f.lastSon
+      if a.kind == tyGenericInst and roota.base == rootf.base:
+        for i in 1 .. rootf.sonsLen-2:
+          let ff = rootf.sons[i]
+          let aa = roota.sons[i]
+          result = typeRel(c, ff, aa)
+          if result == isNone: return
+          if ff.kind == tyRange and result != isEqual: return isNone
       else:
-        return isNone
-
+        result = typeRel(c, rootf.lastSon, a)
+      if result != isNone:
+        put(c.bindings, f, a)
+        result = isGeneric
   of tyGenericParam:
     var x = PType(idTableGet(c.bindings, f))
     if x == nil:

tests/generics/tcritical.nim

@@ -0,0 +1,19 @@
+discard """
+  errormsg: "type mismatch"
+  line: 18
+"""
+
+# bug #3998
+
+type Vec3[T] = array[3, T]
+
+var vg: Vec3[float32] = Vec3([1.0f, 2.0f, 3.0f])
+
+echo "vg[0]: " & $vg[0]  # prints 1.0    OK
+echo "vg[1]: " & $vg[1]  # prints 2.0    OK
+echo "vg[2]: " & $vg[2]  # prints 3.0    OK
+echo ""
+
+var ve: Vec3[float64]
+ve = vg     # compiles, this MUST NOT be allowed!
+