undefined: Neater CallStack in error message

Users of `undefined` don’t want to see
```
files.hs: Prelude.undefined:
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/Err.hs:79:14 in base:GHC.Err
  undefined, called at file.hs:151:19 in main:Main
```
but want to see
```
files.hs: Prelude.undefined:
CallStack (from HasCallStack):
  undefined, called at file.hs:151:19 in main:Main
```
so let’s make that so.

The function for that is `withFrozenCallStack`, but that is not usable
here (module dependencies, and also not representation-polymorphic). And
even if it were, it could confuse GHC’s strictness analyzer, leading to
big regressions in some perf tests (T10421 in particular).

So after shuffling modules and definitions around, I eventually noticed
that the easiest way is to just not call `error` here.

Fixes #19886

1 files changed, 7 insertions, 1 deletions

diff --git a/libraries/base/GHC/Err.hs b/libraries/base/GHC/Err.hs
index 87f3e46bd7..07d263bf7d 100644
--- a/libraries/base/GHC/Err.hs
+++ b/libraries/base/GHC/Err.hs
@@ -71,7 +71,13 @@ errorWithoutStackTrace s = raise# (errorCallException s)
 -- appears.
 undefined :: forall (r :: RuntimeRep). forall (a :: TYPE r).
              HasCallStack => a
-undefined =  error "Prelude.undefined"
+-- This used to be
+--   undefined = error "Prelude.undefined"
+-- but that would add an extra call stack entry that is not actually helpful
+-- nor wanted (see #19886). We’d like to use withFrozenCallStack, but that
+-- is not available in this module yet, and making it so is hard. So let’s just
+-- use raise# directly.
+undefined = raise# (errorCallWithCallStackException "Prelude.undefined" ?callStack)
 
 -- | Used for compiler-generated error message;
 -- encoding saves bytes of string junk.