Pattern match translation suboptimal

Compile the following code and look at the (sad) intermediate code:

baz :: Integer -> Int -> Int
baz 10 1 = 1
baz 20 1 = 2
baz 10 2 = 2
baz 20 2 = 3
baz 10 3 = 1
baz 20 3 = 2
baz 10 4 = 2
baz 20 4 = 3
baz _ _  = 0

The pattern match compiler has not rearranged the clauses, and so it produces an 8 level deep nested test.

Now change the type signature to

baz :: Int -> Int -> Int

Now the pattern match compiler does its job and rearranges the clauses to make the tests 2 levels deep.

The same phenomenon happens when matching string literals. For the predefined String type the right thing happens, but for some other string type (using OverloadedStrings) it doesn't.

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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