fix nim-lang#13443

compiler/ccgtypes.nim

@@ -54,6 +54,9 @@ proc mangleParamName(m: BModule; s: PSym): Rope =
   ## we cannot use 'sigConflicts' here since we have a BModule, not a BProc.
   ## Fortunately C's scoping rules are sane enough so that that doesn't
   ## cause any trouble.
+  if s.name.s == "_":
+    result = getTempName(m)
+    return
   result = s.loc.r
   if result == nil:
     var res = s.name.s.mangle

tests/proc/tunderscore.nim

@@ -0,0 +1,23 @@
+discard """
+  targets: "c cpp js"
+"""
+
+var x = 0
+
+block:
+  proc foo(_, _: int) = x += 5
+
+  foo(1, 2)
+  doAssert x == 5
+
+block:
+  proc foo(_: int, _: float) = x += 5
+
+  foo(1, 2)
+  doAssert x == 10
+
+block:
+  proc foo(_: int, _: float, _: string) = x += 5
+
+  foo(1, 2, "5")
+  doAssert x == 15