EnzymeRules: bind free TypeVars in has_{f,r}rule_from_sig query type

[Keno]

_annotate_tt synthesizes a method-table query like Tuple{<:FwdConfig, <:Annotation{ft}, Type{<:Annotation}, tt...} from the caller's signature. The tt... entries came from _annotate, which builds fresh TypeVars but never wraps them in a UnionAll; ft (the first parameter of unwrap_unionall(TT0)) likewise could contain TypeVars bound by TT0's outer where clauses. Both sources left the constructed query with free TypeVars, which used to be tolerated by Julia's subtype/intersection but no longer is under JuliaLang/julia#61876 (free TypeVars now compare by identity, so e.g. typeintersect(Tuple{…,Const{Float64}}, Tuple{…,V<:Annotation{Float64}}) collapses to Union{}).

_annotate_tt now returns the new TypeVars alongside ft, tt, with ft rewrapped in TT0's UnionAll chain. Each has_*_from_sig caller wraps the constructed query in a UnionAll per fresh TypeVar via foldr(UnionAll, …). The internal callers in Enzyme.Compiler.tfunc are updated to the new return shape.

The behavior of free TypeVars in subtyping was previously undefined (in effect a bug); JuliaLang/julia#61876 makes them well-defined singleton identities, which is what surfaces this latent issue.

This change was AI-generated and should be reviewed carefully before merging.

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diff --git a/lib/EnzymeCore/src/rules.jl b/lib/EnzymeCore/src/rules.jl
index d3a87f3a..440a6d25 100644
--- a/lib/EnzymeCore/src/rules.jl
+++ b/lib/EnzymeCore/src/rules.jl
@@ -355,7 +355,7 @@ function has_frule_from_sig(@nospecialize(TT);
                             method_table::Union{Nothing,Core.Compiler.MethodTableView}=nothing,
                             caller::Union{Nothing,Core.MethodInstance}=nothing)::Bool
     ft, tt, tvars = _annotate_tt(TT)
-    TT = foldr(UnionAll, tvars; init=Tuple{<:FwdConfig, <:Annotation{ft}, Type{<:Annotation}, tt...})
+    TT = foldr(UnionAll, tvars; init = Tuple{<:FwdConfig, <:Annotation{ft}, Type{<:Annotation}, tt...})
     return isapplicable(forward, TT; world, method_table, caller)
 end
 
@@ -364,7 +364,7 @@ function has_rrule_from_sig(@nospecialize(TT);
                             method_table::Union{Nothing,Core.Compiler.MethodTableView}=nothing,
                             caller::Union{Nothing,Core.MethodInstance}=nothing)::Bool
     ft, tt, tvars = _annotate_tt(TT)
-    TT = foldr(UnionAll, tvars; init=Tuple{<:RevConfig, <:Annotation{ft}, Type{<:Annotation}, tt...})
+    TT = foldr(UnionAll, tvars; init = Tuple{<:RevConfig, <:Annotation{ft}, Type{<:Annotation}, tt...})
     return isapplicable(augmented_primal, TT; world, method_table, caller)
 end
 
diff --git a/src/compiler/tfunc.jl b/src/compiler/tfunc.jl
index 34ba158d..982b198a 100644
--- a/src/compiler/tfunc.jl
+++ b/src/compiler/tfunc.jl
@@ -4,7 +4,7 @@ import EnzymeCore.EnzymeRules: FwdConfig, RevConfig, forward, augmented_primal,
 function has_frule_from_sig(@nospecialize(interp::Core.Compiler.AbstractInterpreter),
     @nospecialize(TT::Type), sv::Core.Compiler.AbsIntState, partialedge::Bool=true)::Bool
     ft, tt, tvars = _annotate_tt(TT)
-    TT = foldr(UnionAll, tvars; init=Tuple{<:FwdConfig,<:Annotation{ft},Type{<:Annotation},tt...})
+    TT = foldr(UnionAll, tvars; init = Tuple{<:FwdConfig, <:Annotation{ft}, Type{<:Annotation}, tt...})
     fwd_sig = Tuple{typeof(EnzymeRules.forward), <:EnzymeRules.FwdConfig, <:Enzyme.EnzymeCore.Annotation, Type{<:Enzyme.EnzymeCore.Annotation},Vararg{Enzyme.EnzymeCore.Annotation}}
     return isapplicable(interp, forward, TT, sv, fwd_sig)
 end
@@ -12,7 +12,7 @@ end
 function has_rrule_from_sig(@nospecialize(interp::Core.Compiler.AbstractInterpreter),
     @nospecialize(TT::Type), sv::Core.Compiler.AbsIntState, partialedge::Bool=true)::Bool
     ft, tt, tvars = _annotate_tt(TT)
-    TT = foldr(UnionAll, tvars; init=Tuple{<:RevConfig,<:Annotation{ft},Type{<:Annotation},tt...})
+    TT = foldr(UnionAll, tvars; init = Tuple{<:RevConfig, <:Annotation{ft}, Type{<:Annotation}, tt...})
     rev_sig = Tuple{typeof(EnzymeRules.augmented_primal), <:EnzymeRules.RevConfig, <:Enzyme.EnzymeCore.Annotation, Type{<:Enzyme.EnzymeCore.Annotation},Vararg{Enzyme.EnzymeCore.Annotation}}
     return isapplicable(interp, augmented_primal, TT, sv, rev_sig)
 end

[wsmoses]

we changed this to an explicit loop earlier for performance reasons, any reason this needs to be a comprehension?

[Keno]

No

[wsmoses]

let's keep the loop then

[wsmoses]

@vchuravy @kshyatt I think you guys should review this one

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.62%. Comparing base (000acf6) to head (75b47f3).
⚠️ Report is 122 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/EnzymeCore/src/rules.jl	71.42%	4 Missing ⚠️
src/compiler/tfunc.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3121   +/-   ##
=======================================
  Coverage   70.61%   70.62%           
=======================================
  Files          66       66           
  Lines       21334    21332    -2     
=======================================
  Hits        15066    15066           
+ Misses       6268     6266    -2     

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[wsmoses]

bump @vchuravy