lattice: Don't forget to pass down lattice in Conditional tmerge

Also add some extra cases that before were taken care of by the
tmerge_fast_path at the entry to the tmerge code. I briefly
considered splitting an extra slow path function to avoid the
one `===` in the ConstsLattice, but in the non-equality case
that check should be quite fast, so it didn't seem worth it.

base/compiler/typelimits.jl

@@ -429,8 +429,8 @@ function tmerge(lattice::ConditionalsLattice, @nospecialize(typea), @nospecializ
     end
     if isa(typea, Conditional) && isa(typeb, Conditional)
         if is_same_conditionals(typea, typeb)
-            thentype = tmerge(typea.thentype, typeb.thentype)
-            elsetype = tmerge(typea.elsetype, typeb.elsetype)
+            thentype = tmerge(widenlattice(lattice), typea.thentype, typeb.thentype)
+            elsetype = tmerge(widenlattice(lattice), typea.elsetype, typeb.elsetype)
             if thentype !== elsetype
                 return Conditional(typea.slot, thentype, elsetype)
             end
@@ -464,8 +464,8 @@ function tmerge(lattice::InterConditionalsLattice, @nospecialize(typea), @nospec
     end
     if isa(typea, InterConditional) && isa(typeb, InterConditional)
         if is_same_conditionals(typea, typeb)
-            thentype = tmerge(typea.thentype, typeb.thentype)
-            elsetype = tmerge(typea.elsetype, typeb.elsetype)
+            thentype = tmerge(widenlattice(lattice), typea.thentype, typeb.thentype)
+            elsetype = tmerge(widenlattice(lattice), typea.elsetype, typeb.elsetype)
             if thentype !== elsetype
                 return InterConditional(typea.slot, thentype, elsetype)
             end
@@ -506,6 +506,9 @@ function tmerge_partial_struct(lattice::PartialsLattice, @nospecialize(typea), @
         for i = 1:type_nfields
             ai = getfield_tfunc(lattice, typea, Const(i))
             bi = getfield_tfunc(lattice, typeb, Const(i))
+            # N.B.: We're assuming here that !isType(aty), because that case
+            # only arises when typea === typeb, which should have been caught
+            # before calling this.
             ft = fieldtype(aty, i)
             if is_lattice_equal(lattice, ai, bi) || is_lattice_equal(lattice, ai, ft)
                 # Since ai===bi, the given type has no restrictions on complexity.
@@ -551,6 +554,7 @@ function tmerge(lattice::PartialsLattice, @nospecialize(typea), @nospecialize(ty
     acp = aps || isa(typea, Const)
     bcp = bps || isa(typeb, Const)
     if acp && bcp
+        typea === typeb && return typea
         psrt = tmerge_partial_struct(lattice, typea, typeb)
         psrt !== nothing && return psrt
     end
@@ -586,9 +590,9 @@ function tmerge(lattice::PartialsLattice, @nospecialize(typea), @nospecialize(ty
     return tmerge(wl, typea, typeb)
 end
 
+
 function tmerge(lattice::ConstsLattice, @nospecialize(typea), @nospecialize(typeb))
-    # the equality of the constants can be checked here, but the equivalent check is usually
-    # done by `tmerge_fast_path` at earlier lattice stage
+    typea === typeb && return typea
     wl = widenlattice(lattice)
     (isa(typea, Const) || isa(typea, PartialTypeVar)) && (typea = widenlattice(wl, typea))
     (isa(typeb, Const) || isa(typeb, PartialTypeVar)) && (typeb = widenlattice(wl, typeb))