Fix #14690 - :steplocal panics after break-on-error

`:steplocal` enables only breakpoints in the current top-level binding.

When a normal breakpoint is hit, then the module name and the break id from the `BRK_FUN` byte code
allow us to access the corresponding entry in a ModBreak table. From this entry we then get the SrcSpan
(see compiler/main/InteractiveEval.hs:bindLocalsAtBreakpoint).
With this source-span we can then determine the current top-level binding, needed for the steplocal command.

However, if we break at an exception or at an error, we don't have an BRK_FUN byte-code, so we don't have any source information.
The function `bindLocalsAtBreakpoint` creates an `UnhelpfulSpan`, which doesn't allow us to determine the current top-level binding.
To avoid a `panic`, we have to check for `UnhelpfulSpan` in the function `ghc/GHCi/UI.hs:stepLocalCmd`.
Hence a :steplocal command after a break-on-exception or a break-on-error is not possible.

1 files changed, 4 insertions, 0 deletions

diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index e2f51be65f..34bb627238 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -3460,6 +3460,10 @@ stepLocalCmd arg = withSandboxOnly ":steplocal" $ step arg
       mb_span <- getCurrentBreakSpan
       case mb_span of
         Nothing  -> stepCmd []
+        Just (UnhelpfulSpan _) -> liftIO $ putStrLn (            -- #14690
+           ":steplocal is not possible." ++
+           "\nCannot determine current top-level binding after " ++
+           "a break on error / exception.\nUse :stepmodule.")
         Just loc -> do
            md <- fromMaybe (panic "stepLocalCmd") <$> getCurrentBreakModule
            current_toplevel_decl <- enclosingTickSpan md loc