[GHC] #14397: For type error involving inferred types, show source of the type

[GHC]

#14397: For type error involving inferred types, show source of the type
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        Reporter:  max630            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Old description:

> Consider the code:
>
> {{{#!hs
> module TestTypes where
>
> a = 42
>
> b = a ++ "hello"
> }}}
>
> The type Error is (ghc-8.2.1):
>
> {{{
> TestTypes.hs:3:5: error:
>     • No instance for (Num [Char]) arising from the literal ‘42’
>     • In the expression: 42
>       In an equation for ‘a’: a = 42
>   |
> 3 | a = 42
>   |     ^^
> }}}
>
> Actually, the real mistake is in line 3, where `a` is inferred as of type
> `[Char]`, while it was not meant to. But that location is not reported to
> the error. As you can guess, in real code location where the unexpected
> type is inferred for a binding is not that obvious to find.
>
> Suggestion: track locations where bindings' types are inferred, and in
> case of type error involvin the bindings, add them to the "Relevant
> bindings" section, or otherwise show it.

New description:

 Consider the code:

 {{{#!hs
 module TestTypes where

 a = 42

 b = a ++ "hello"
 }}}

 The type Error is (ghc-8.2.1):

 {{{
 TestTypes.hs:3:5: error:
     • No instance for (Num [Char]) arising from the literal ‘42’
     • In the expression: 42
       In an equation for ‘a’: a = 42
   |
 3 | a = 42
   |     ^^
 }}}

 Actually, the real mistake is in line 5, where `a` is inferred as of type
 `[Char]`, while it was not meant to. But that location is not reported to
 the error. As you can guess, in real code location where the unexpected
 type is inferred for a binding is not that obvious to find.

 Suggestion: track locations where bindings' types are inferred, and in
 case of type error involvin the bindings, add them to the "Relevant
 bindings" section, or otherwise show it.

--

Comment (by max630):

 Fix the line reference in description

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14397#comment:3>
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