fix error messages for wrongly typed generic param default values (#24006)

fixes #21258

When a generic proc is instantiated, if one of the default values
doesn't match the type of the parameter, `seminst` sets the default
parameter node to an `nkEmpty` node with `tyError` type. `sigmatch`
checks for this to give an error message if the default param is
actually used, but only while actively matching the proc signature,
before the proc is even instantiated. The error message also gives very
little information.

Now, we check for this in `updateDefaultParams` at the end of
`semResolvedCall`, after the proc has been instantiated. The `nkEmpty`
node also is given the original mismatching type instead rather than
`tyError`, only setting `tyError` after erroring to prevent cascading
errors. The error message is changed to the standard type mismatch error
also giving the instantiation info of the routine.

1 files changed, 25 insertions, 0 deletions

diff --git a/tests/proc/twrongdefaultvalue.nim b/tests/proc/twrongdefaultvalue.nim
new file mode 100644
index 000000000..2c36c2247
--- /dev/null
+++ b/tests/proc/twrongdefaultvalue.nim
@@ -0,0 +1,25 @@
+discard """
+  cmd: "nim check $file"
+  action: reject
+  nimout: '''
+twrongdefaultvalue.nim(20, 12) template/generic instantiation of `doit` from here
+twrongdefaultvalue.nim(17, 37) Error: type mismatch: got <proc (p: int): Item[initItem.T]> but expected 'Item[system.string]'
+twrongdefaultvalue.nim(25, 3) template/generic instantiation of `foo` from here
+twrongdefaultvalue.nim(23, 33) Error: type mismatch: got <string> but expected 'int'
+'''
+"""
+
+block: # issue #21258
+  type Item[T] = object
+    pos: int
+  proc initItem[T](p:int=10000) : Item[T] = 
+    result = Item[T](p)
+  proc doit[T](x:Item[T], s:Item[T]=initItem) : string = 
+    return $x.pos
+  let x = Item[string](pos:100)
+  echo doit(x)
+
+block: # issue #21258, reduced case
+  proc foo[T](x: seq[T], y: T = "foo") =
+    discard
+  foo @[1, 2, 3]