Converters to take into account constraints. Fixes #7520 (#8593)

compiler/sigmatch.nim

@@ -1809,6 +1809,10 @@ proc userConvMatch(c: PContext, m: var TCandidate, f, a: PType,
     # see tests/tgenericconverter:
     let srca = typeRel(m, src, a)
     if srca notin {isEqual, isGeneric, isSubtype}: continue
+   
+    let constraint = c.converters[i].typ.n[1].sym.constraint
+    if not constraint.isNil and not matchNodeKinds(constraint, arg):
+      continue
 
     let destIsGeneric = containsGenericType(dest)
     if destIsGeneric:

tests/converter/tconverter_with_constraint.nim

@@ -0,0 +1,20 @@
+
+discard """
+  file: "tconverter_with_constraint.nim"
+  line: 20
+  errormsg: "type mismatch: got <int>"
+"""
+
+type
+  MyType = distinct int
+
+converter to_mytype(m: int{lit}): MyType =
+  m.MyType
+ 
+proc myproc(m: MyType) =
+  echo m.int, ".MyType"
+
+myproc(1) # call by literal is ok 
+
+var x: int = 12
+myproc(x) # should fail