effects: Do not over-taint :terminates in abstract cycle (#49119)

We already infer non-termination as part of the conservative effects
we assume at the point in the call-graph that recursion is detected.

As a result, it should be sound to allow this to propagate through
the Effects system naturally rather than eagerly marking our callers
as non-terminating.

Doing this is important to avoid tainting non-termination from purely
abstract cycles, where inference is forced to analyze methods that do
not correspond to real calls at runtime, such as `Base._return_type`.

Resolves #48983.

base/compiler/typeinfer.jl

@@ -779,13 +779,9 @@ function merge_call_chain!(interp::AbstractInterpreter, parent::InferenceState,
  # then add all backedges of parent <- parent.parent
  # and merge all of the callers into ancestor.callers_in_cycle
  # and ensure that walking the parent list will get the same result (DAG) from everywhere
- # Also taint the termination effect, because we can no longer guarantee the absence
- # of recursion.
- merge_effects!(interp, parent, Effects(EFFECTS_TOTAL; terminates=false))
  while true
  add_cycle_backedge!(parent, child, parent.currpc)
  union_caller_cycle!(ancestor, child)
- merge_effects!(interp, child, Effects(EFFECTS_TOTAL; terminates=false))
  child = parent
  child === ancestor && break
  parent = child.parent::InferenceState

test/compiler/effects.jl

@@ -805,5 +805,16 @@ unknown_sparam_nothrow2(x::Ref{Ref{T}}) where T = (T; nothing)
 @test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_nothrow1, (Ref,)))
 @test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_nothrow2, (Ref{Ref{T}} where T,)))
 
+# purely abstract recursion should not taint :terminates
+# https://github.com/JuliaLang/julia/issues/48983
+abstractly_recursive1() = abstractly_recursive2()
+abstractly_recursive2() = (Core.Compiler._return_type(abstractly_recursive1, Tuple{}); 1)
+abstractly_recursive3() = abstractly_recursive2()
+@test Core.Compiler.is_terminates(Base.infer_effects(abstractly_recursive3, ()))
+actually_recursive1(x) = actually_recursive2(x)
+actually_recursive2(x) = (x <= 0) ? 1 : actually_recursive1(x - 1)
+actually_recursive3(x) = actually_recursive2(x)
+@test !Core.Compiler.is_terminates(Base.infer_effects(actually_recursive3, (Int,)))
+
 # https://github.com/JuliaLang/julia/issues/48856
 @test Base.ismutationfree(Vector{Any}) == false