Demand: Clear distinction between Call SubDmd and eval Dmd (#21717)wip/T21717

In #21717 we saw a reportedly unsound strictness signature due to an unsound
definition of plusSubDmd on Calls. This patch contains a description and the fix
to the unsoundness as outlined in `Note [Call SubDemand vs. evaluation Demand]`.

This fix means we also get rid of the special handling of `-fpedantic-bottoms`
in eta-reduction. Thanks to less strict and actually sound strictness results,
we will no longer eta-reduce the problematic cases in the first place, even
without `-fpedantic-bottoms`.

So fixing the unsoundness also makes our eta-reduction code simpler with less
hacks to explain. But there is another, more unfortunate side-effect:
We *unfix* #21085, but fortunately we have a new fix ready:
See `Note [mkCall and plusSubDmd]`.

There's another change:
I decided to make `Note [SubDemand denotes at least one evaluation]` a lot
simpler by using `plusSubDmd` (instead of `lubPlusSubDmd`) even if both argument
demands are lazy. That leads to less precise results, but in turn rids ourselves
from the need for 4 different `OpMode`s and the complication of
`Note [Manual specialisation of lub*Dmd/plus*Dmd]`. The result is simpler code
that is in line with the paper draft on Demand Analysis.

I left the abandoned idea in `Note [Unrealised opportunity in plusDmd]` for
posterity. The fallout in terms of regressions is negligible, as the testsuite
and NoFib shows.

```
        Program         Allocs    Instrs
--------------------------------------------------------------------------------
         hidden          +0.2%     -0.2%
         linear          -0.0%     -0.7%
--------------------------------------------------------------------------------
            Min          -0.0%     -0.7%
            Max          +0.2%     +0.0%
 Geometric Mean          +0.0%     -0.0%
```

Fixes #21717.

7 files changed, 35 insertions, 7 deletions

diff --git a/testsuite/tests/stranal/sigs/T20746.stderr b/testsuite/tests/stranal/sigs/T20746.stderr
index 2b54d3b8ff..65c3e5e296 100644
--- a/testsuite/tests/stranal/sigs/T20746.stderr
+++ b/testsuite/tests/stranal/sigs/T20746.stderr
@@ -1,6 +1,6 @@
 
 ==================== Strictness signatures ====================
-Foo.f: <LP(A,SCS(L),A)><L>
+Foo.f: <LP(A,L,A)><L>
 Foo.foogle: <L><L>
 
 
diff --git a/testsuite/tests/stranal/sigs/T21081.stderr b/testsuite/tests/stranal/sigs/T21081.stderr
index ec7e776ca8..7cf5f7cdd8 100644
--- a/testsuite/tests/stranal/sigs/T21081.stderr
+++ b/testsuite/tests/stranal/sigs/T21081.stderr
@@ -4,7 +4,7 @@ T21081.blah: <LCL(C1(L))><1!P(1L)>
 T21081.blurg: <S!P(SL)>
 T21081.blurg2: <S!P(SL)>
 T21081.call1: <MP(1L,A)>
-T21081.call2: <LP(SL,A)>
+T21081.call2: <LP(L,A)>
 T21081.call3: <LP(ML,A)>
 T21081.call4: <MP(1L,A)><1A>
 T21081.call5: <MP(1L,A)><MA>
@@ -50,7 +50,7 @@ T21081.blah: <LCL(C1(L))><1!P(1L)>
 T21081.blurg: <1!P(SL)>
 T21081.blurg2: <1!P(SL)>
 T21081.call1: <MP(1L,A)>
-T21081.call2: <LP(SL,A)>
+T21081.call2: <LP(L,A)>
 T21081.call3: <LP(ML,A)>
 T21081.call4: <MP(1L,A)><1A>
 T21081.call5: <MP(1L,A)><MA>
diff --git a/testsuite/tests/stranal/sigs/T21119.stderr b/testsuite/tests/stranal/sigs/T21119.stderr
index ca60a36995..c20b876677 100644
--- a/testsuite/tests/stranal/sigs/T21119.stderr
+++ b/testsuite/tests/stranal/sigs/T21119.stderr
@@ -4,7 +4,7 @@ T21119.$fMyShow(,): <1!A>
 T21119.$fMyShowInt: <1!A>
 T21119.get: <1!P(1!P(L),1!P(L))><1!P(L)><1L>
 T21119.getIO: <1P(1L,ML)><1L><ML><L>
-T21119.indexError: <1C1(S)><1!B><S!S><S>b
+T21119.indexError: <1C1(L)><1!B><S!S><S>b
 T21119.throwIndexError: <MC1(L)><MA><L><L><L>x
 
 
@@ -24,7 +24,7 @@ T21119.$fMyShow(,): <1!A>
 T21119.$fMyShowInt: <1!A>
 T21119.get: <1!P(1!P(L),1!P(L))><1!P(L)><1L>
 T21119.getIO: <1P(1L,ML)><1L><ML><L>
-T21119.indexError: <1C1(S)><1!B><S!S><S>b
+T21119.indexError: <1C1(L)><1!B><S!S><S>b
 T21119.throwIndexError: <MC1(L)><MA><L><L><L>x
 
 
diff --git a/testsuite/tests/stranal/sigs/T21717.hs b/testsuite/tests/stranal/sigs/T21717.hs
new file mode 100644
index 0000000000..f0d9b96a40
--- /dev/null
+++ b/testsuite/tests/stranal/sigs/T21717.hs
@@ -0,0 +1,12 @@
+module T21717 (g) where
+
+-- This is the original reproducer from #21717.
+-- See T21717b for a reproducer that exhibited a crash.
+
+f :: Bool -> (Int, Int)
+f True  = (42,error "m")
+f False = (error "m",42)
+
+g :: (Bool -> (Int, Int)) -> Int
+g h = fst (h True) + snd (h False)
+{-# NOINLINE g #-}
diff --git a/testsuite/tests/stranal/sigs/T21717.stderr b/testsuite/tests/stranal/sigs/T21717.stderr
new file mode 100644
index 0000000000..1dd0856f7b
--- /dev/null
+++ b/testsuite/tests/stranal/sigs/T21717.stderr
@@ -0,0 +1,15 @@
+
+==================== Strictness signatures ====================
+T21717.g: <SCS(P(ML,ML))>
+
+
+
+==================== Cpr signatures ====================
+T21717.g: 1
+
+
+
+==================== Strictness signatures ====================
+T21717.g: <SCS(P(ML,ML))>
+
+
diff --git a/testsuite/tests/stranal/sigs/T5075.stderr b/testsuite/tests/stranal/sigs/T5075.stderr
index 9bc8301440..5b3701ef56 100644
--- a/testsuite/tests/stranal/sigs/T5075.stderr
+++ b/testsuite/tests/stranal/sigs/T5075.stderr
@@ -1,6 +1,6 @@
 
 ==================== Strictness signatures ====================
-T5075.f: <S!P(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCS(C1(L)),A,A,A,LCS(L))><L>
+T5075.f: <S!P(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCS(C1(L)),A,A,A,L)><L>
 T5075.g: <1L><S!P(L)>
 T5075.h: <S!P(L)>
 
@@ -14,7 +14,7 @@ T5075.h:
 
 
 ==================== Strictness signatures ====================
-T5075.f: <1P(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCS(C1(L)),A,A,A,LCS(L))><L>
+T5075.f: <1P(A,A,SCS(C1(L)),A,A,A,A,A)><LP(A,A,LCS(C1(L)),A,A,A,L)><L>
 T5075.g: <1L><S!P(L)>
 T5075.h: <1!P(L)>
 
diff --git a/testsuite/tests/stranal/sigs/all.T b/testsuite/tests/stranal/sigs/all.T
index 211cbda94d..50b8176ce8 100644
--- a/testsuite/tests/stranal/sigs/all.T
+++ b/testsuite/tests/stranal/sigs/all.T
@@ -33,5 +33,6 @@ test('T20746', normal, compile, [''])
 test('T20746b', normal, compile, [''])
 test('T21081', normal, compile, [''])
 test('T21119', normal, compile, [''])
+test('T21717', normal, compile, [''])
 test('T21888', normal, compile, [''])
 test('T21888a', normal, compile, [''])