Template Haskell panic when splicing an infix expression with a non-variable middle bit
Simple enough:
```
import Language.Haskell.TH
import Language.Haskell.TH.Syntax
panic = $(let var = varE . mkName in infixApp (var "x") (appE (var "f") (var "y")) (var "z"))
```
```
[1 of 1] Compiling Panic ( Panic.hs, interpreted )
ghc: panic! (the 'impossible' happened)
(GHC version 7.0.1 for i386-unknown-linux):
rnExpr: unexpected expression
{6:11-92}
f{v} y{v}
x{v} z{v}
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
```
Of course the expression being spliced doesn't make any sense. In fact, I'd think that the only things that make sense in the centre of an infix expression were a single variable or constructor, so that field being of type Exp is arguably way too permissiveP. haskell-src seems to have a data type especially for this purpose - `HsQOp` with constructors `HsQVarOp HsQName` and `HsQConOp HsQName`.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ---------------- |
| Version | 7.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Template Haskell |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Template Haskell panic when splicing an infix expression with a non-variable middle bit","status":"New","operating_system":"","component":"Template Haskell","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"7.0.1","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"Simple enough:\r\n\r\n{{{\r\nimport Language.Haskell.TH\r\nimport Language.Haskell.TH.Syntax\r\npanic = $(let var = varE . mkName in infixApp (var \"x\") (appE (var \"f\") (var \"y\")) (var \"z\"))\r\n}}}\r\n\r\n{{{\r\n[1 of 1] Compiling Panic ( Panic.hs, interpreted )\r\nghc: panic! (the 'impossible' happened)\r\n (GHC version 7.0.1 for i386-unknown-linux):\r\n\trnExpr: unexpected expression\r\n {6:11-92}\r\n f{v} y{v}\r\n x{v} z{v}\r\n\r\nPlease report this as a GHC bug: http://www.haskell.org/ghc/reportabug\r\n}}}\r\n\r\nOf course the expression being spliced doesn't make any sense. In fact, I'd think that the only things that make sense in the centre of an infix expression were a single variable or constructor, so that field being of type Exp is arguably way too permissiveP. haskell-src seems to have a data type especially for this purpose - `HsQOp` with constructors `HsQVarOp HsQName` and `HsQConOp HsQName`.","type_of_failure":"OtherFailure","blocking":[]} -->
issue