warn against a, b = val in routine arguments (#21604)

closes https://github.com/nim-lang/RFCs/issues/480

compiler/lineinfos.nim

@@ -88,6 +88,7 @@ type
     warnUnnamedBreak = "UnnamedBreak",
     warnStmtListLambda = "StmtListLambda",
     warnBareExcept = "BareExcept",
+    warnImplicitDefaultValue = "ImplicitDefaultValue",
     warnUser = "User",
     # hints
     hintSuccess = "Success", hintSuccessX = "SuccessX",
@@ -189,6 +190,7 @@ const
     warnUnnamedBreak: "Using an unnamed break in a block is deprecated; Use a named block with a named break instead",
     warnStmtListLambda: "statement list expression assumed to be anonymous proc; this is deprecated, use `do (): ...` or `proc () = ...` instead",
     warnBareExcept: "$1",
+    warnImplicitDefaultValue: "$1",
     warnUser: "$1",
     hintSuccess: "operation successful: $#",
     # keep in sync with `testament.isSuccess`

compiler/semtypes.nim

@@ -1311,6 +1311,16 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
 
     if hasDefault:
       def = a[^1]
+      if a.len > 3:
+        var msg = ""
+        for j in 0 ..< a.len - 2:
+          if msg.len != 0: msg.add(", ")
+          msg.add($a[j])
+        msg.add(" all have default value '")
+        msg.add(def.renderTree)
+        msg.add("', this may be unintentional, " &
+          "either use ';' (semicolon) or explicitly write each default value")
+        message(c.config, a.info, warnImplicitDefaultValue, msg)
       block determineType:
         var defTyp = typ
         if genericParams != nil and genericParams.len > 0:

doc/manual.md

@@ -3816,15 +3816,6 @@ every time the function is called.
   proc foo(a: int, b: int = 47): int
   ```
 
-Just as the comma propagates the types from right to left until the
-first parameter or until a semicolon is hit, it also propagates the
-default value starting from the parameter declared with it.
-
-  ```nim
-  # Both a and b are optional with 47 as their default values.
-  proc foo(a, b: int = 47): int
-  ```
-
 Parameters can be declared mutable and so allow the proc to modify those
 arguments, by using the type modifier `var`.
 

tests/proc/t15949.nim

@@ -1,14 +1,9 @@
-# bug #15949
+# bug #15949 and RFC #480
 
-discard """
-errormsg: "parameter 'a' requires a type"
-nimout: '''
-t15949.nim(20, 14) Error: parameter 'a' requires a type'''
-"""
+proc procWarn(a, b = 1): (int, int) = (a, b) #[tt.Warning
+              ^ a, b all have default value '1', this may be unintentional, either use ';' (semicolon) or explicitly write each default value [ImplicitDefaultValue]]#
 
-
-# line 10
-proc procGood(a, b = 1): (int, int) = (a, b)
+proc procGood(a = 1, b = 1): (int, int) = (a, b)
 
 doAssert procGood() == (1, 1)
 doAssert procGood(b = 3) == (1, 3)
@@ -17,4 +12,5 @@ doAssert procGood(a = 5, b = 6) == (5, 6)
 
 # The type (and default value propagation breaks in the below example
 # as semicolon is used instead of comma.
-proc procBad(a; b = 1): (int, int) = (a, b)
+proc procBad(a; b = 1): (int, int) = (a, b) #[tt.Error
+             ^ parameter 'a' requires a type]#