allow referencing other parameters in default parameter values

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

compiler/transf.nim

@@ -780,6 +780,43 @@ proc commonOptimizations*(g: ModuleGraph; c: PSym, n: PNode): PNode =
     else:
       result = n
 
+proc hoistParamsUsedInDefault(c: PTransf, call, letSection, defExpr: PNode): PNode =
+  # This takes care of complicated signatures such as:
+  # proc foo(a: int, b = a)
+  # proc bar(a: int, b: int, c = a + b)
+  #
+  # The recursion may confuse you. It performs two duties:
+  #
+  # 1) extracting all referenced params from default expressions
+  #    into a let section preceeding the call
+  #
+  # 2) replacing the "references" within the default expression
+  #    with these extracted skLet symbols.
+  #
+  # The first duty is carried out directly in the code here, while the second
+  # duty is activated by returning a non-nil value. The caller is responsible
+  # for replacing the input to the function with the returned non-nil value.
+  # (which is the hoisted symbol)
+  if defExpr.kind == nkSym:
+    if defExpr.sym.kind == skParam and defExpr.sym.owner == call[0].sym:
+      let paramPos = defExpr.sym.position + 1
+
+      if call[paramPos].kind == nkSym and sfHoisted in call[paramPos].sym.flags:
+        # Already hoisted, we still need to return it in order to replace the
+        # placeholder expression in the default value.
+        return call[paramPos]
+
+      let hoistedVarSym = hoistExpr(letSection,
+                                    call[paramPos],
+                                    getIdent(c.graph.cache, genPrefix),
+                                    c.transCon.owner).newSymNode
+      call[paramPos] = hoistedVarSym
+      return hoistedVarSym
+  else:
+    for i in 0..<defExpr.safeLen:
+      let hoisted = hoistParamsUsedInDefault(c, call, letSection, defExpr[i])
+      if hoisted != nil: defExpr[i] = hoisted
+
 proc transform(c: PTransf, n: PNode): PTransNode =
   when false:
     var oldDeferAnchor: PNode
@@ -849,6 +886,15 @@ proc transform(c: PTransf, n: PNode): PTransNode =
   of nkBreakStmt: result = transformBreak(c, n)
   of nkCallKinds:
     result = transformCall(c, n)
+    var call = result.PNode
+    if nfDefaultRefsParam in call.flags:
+      # We've found a default value that references another param.
+      # See the notes in `hoistParamsUsedInDefault` for more details.
+      var hoistedParams = newNodeI(nkLetSection, call.info, 0)
+      for i in 1 ..< call.len:
+        let hoisted = hoistParamsUsedInDefault(c, call, hoistedParams, call[i])
+        if hoisted != nil: call[i] = hoisted
+      result = newTree(nkStmtListExpr, hoistedParams, call).PTransNode
   of nkAddr, nkHiddenAddr:
     result = transformAddrDeref(c, n, nkDerefExpr, nkHiddenDeref)
   of nkDerefExpr, nkHiddenDeref: