allow referencing other parameters in default parameter values

fix #7756
fix #1201
fix #7000
fix #3002
fix #1046

tests/misc/tparamsindefault.nim

@@ -0,0 +1,114 @@
+discard """
+output: '''
+@[1, 2, 3]@[1, 2, 3]
+a
+a
+1
+3 is an int
+2 is an int
+miau is a string
+f1 1 1 1
+f1 2 3 3
+f1 10 20 30
+f2 100 100 100
+f2 200 300 300
+f2 300 400 400
+f3 10 10 20
+f3 10 15 25
+true true
+false true
+world
+'''
+"""
+
+template reject(x) =
+  assert(not compiles(x))
+
+block:
+  # https://github.com/nim-lang/Nim/issues/7756
+  proc foo[T](x: seq[T], y: seq[T] = x) =
+    echo x, y
+
+  let a = @[1, 2, 3]
+  foo(a)
+
+block:
+  # https://github.com/nim-lang/Nim/issues/1201
+  proc issue1201(x: char|int = 'a') = echo x
+
+  issue1201()
+  issue1201('a')
+  issue1201(1)
+
+  # https://github.com/nim-lang/Nim/issues/7000
+  proc test(a: int|string = 2) =
+    when a is int:
+        echo a, " is an int"
+    elif a is string:
+        echo a, " is a string"
+
+  test(3) # works
+  test() # works
+  test("miau")
+
+block:
+  # https://github.com/nim-lang/Nim/issues/3002 and similar
+  proc f1(a: int, b = a, c = b) =
+    echo "f1 ", a, " ", b, " ", c
+
+  proc f2(a: int, b = a, c: int = b) =
+    echo "f2 ", a, " ", b, " ", c
+
+  proc f3(a: int, b = a, c = a + b) =
+    echo "f3 ", a, " ", b, " ", c
+
+  f1 1
+  f1(2, 3)
+  f1 10, 20, 30
+  100.f2
+  200.f2 300
+  300.f2(400)
+
+  10.f3()
+  10.f3(15)
+
+  reject:
+    # This is a type mismatch error:
+    proc f4(a: int, b = a, c: float = b) = discard
+
+  reject:
+    # undeclared identifier
+    proc f5(a: int, b = c, c = 10) = discard
+
+  reject:
+    # undeclared identifier
+    proc f6(a: int, b = b) = discard
+
+  reject:
+    # undeclared identifier
+    proc f7(a = a) = discard
+
+block:
+  proc f(a: var int, b: ptr int, c = addr(a)) =
+    echo addr(a) == b, " ",  b == c
+
+  var x = 10
+  f(x, addr(x))
+  f(x, nil, nil)
+
+block:
+  # https://github.com/nim-lang/Nim/issues/1046
+  proc pySubstr(s: string, start: int, endd = s.len()): string =
+    var
+      revStart = start
+      revEnd = endd
+
+    if start < 0:
+      revStart = s.len() + start
+    if endd < 0:
+      revEnd = s.len() + endd
+
+    return s[revStart ..  revEnd-1]
+
+  echo pySubstr("Hello world", -5)
+