[commit: ghc] master: Make exprType work correctly in the presnce of type bindings (Trac #8522) (96e6edd)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/96e6eddc40c34a171f6261a7814a1e264457ce86/ghc

>---------------------------------------------------------------

commit 96e6eddc40c34a171f6261a7814a1e264457ce86
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Tue Nov 12 15:10:01 2013 +0000

    Make exprType work correctly in the presnce of type bindings (Trac #8522)


>---------------------------------------------------------------

96e6eddc40c34a171f6261a7814a1e264457ce86
 compiler/coreSyn/CoreUtils.lhs |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index f717049..264a9da 100644
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -86,7 +86,10 @@ exprType :: CoreExpr -> Type
 exprType (Var var)           = idType var
 exprType (Lit lit)           = literalType lit
 exprType (Coercion co)       = coercionType co
-exprType (Let _ body)        = exprType body
+exprType (Let bind body)     
+  | NonRec tv rhs <- bind    -- See Note [Type bindings]
+  , Type ty <- rhs           = substTyWith [tv] [ty] (exprType body)
+  | otherwise                = exprType body
 exprType (Case _ _ ty _)     = ty
 exprType (Cast _ co)         = pSnd (coercionKind co)
 exprType (Tick _ e)          = exprType e
@@ -113,6 +116,15 @@ coreAltsType (alt:_) = coreAltType alt
 coreAltsType []      = panic "corAltsType"
 \end{code}
 
+Note [Type bindings]
+~~~~~~~~~~~~~~~~~~~~
+Core does allow type bindings, although such bindings are
+not much used, except in the output of the desuguarer.
+Example:
+     let a = Int in (\x:a. x)
+Given this, exprType must be careful to substitute 'a' in the 
+result type (Trac #8522).
+
 Note [Existential variables and silly type synonyms]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Consider