effects: taint :nothrow effect on unknown :static_parameter (#46791)

* effects: taint `:nothrow` effect on unknown `:static_parameter` (conservatively)

With this commit, we taint `:nothrow` effect property correctly on
access to unknown `:static_parameter`, e.g.:
```julia
unknown_sparam_throw(::Union{Nothing, Type{T}}) where T = (T; nothing)
@test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, ((Type{Int},))))
@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, ((Nothing,))))
```

This commit implements a very conservative analysis, and thus there is a
room for improvement still, e.g.:
```julia
unknown_sparam_nothrow(x::Ref{T}) where {T} = (T; nothing)
@test_broken Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_nothrow, (Ref,)))
```

* inference: improve `:nothrow` modeling for `:static_parameter` (#46820)

* Fix test with free type params

* Test: Ignore ::Type{T} in detect_unbounded

These are only technically unbounded because of the existence of
ill-formed types. However, this function is supposed to be an API
sanity check and ordinary users should never have ill-formed types,
so for the purpose we want here, allow unboundedness in Type{T}.

---------

Co-authored-by: Keno Fischer <keno@juliacomputing.com>
Co-authored-by: Elliot Saba <staticfloat@gmail.com>

base/compiler/abstractinterpretation.jl

@@ -2189,9 +2189,17 @@ function abstract_eval_value_expr(interp::AbstractInterpreter, e::Expr, vtypes::
     head = e.head
     if head === :static_parameter
         n = e.args[1]::Int
+        nothrow = false
         if 1 <= n <= length(sv.sptypes)
             rt = sv.sptypes[n]
+            if is_maybeundefsp(rt)
+                rt = unwrap_maybeundefsp(rt)
+            else
+                nothrow = true
+            end
         end
+        merge_effects!(interp, sv, Effects(EFFECTS_TOTAL; nothrow))
+        return rt
     elseif head === :boundscheck
         if isa(sv, InferenceState)
             # If there is no particular `@inbounds` for this function, then we only taint `:noinbounds`,
@@ -2452,8 +2460,7 @@ function abstract_eval_statement_expr(interp::AbstractInterpreter, e::Expr, vtyp
         elseif isexpr(sym, :static_parameter)
             n = sym.args[1]::Int
             if 1 <= n <= length(sv.sptypes)
-                spty = sv.sptypes[n]
-                if isa(spty, Const)
+                if !is_maybeundefsp(sv.sptypes, n)
                     t = Const(true)
                 end
             end

base/compiler/inferencestate.jl

@@ -348,23 +348,109 @@ function InferenceState(result::InferenceResult, cache::Symbol, interp::Abstract
     return InferenceState(result, src, cache, interp)
 end
 
+"""
+    constrains_param(var::TypeVar, sig, covariant::Bool, type_constrains::Bool)
+
+Check if `var` will be constrained to have a definite value
+in any concrete leaftype subtype of `sig`.
+
+It is used as a helper to determine whether type intersection is guaranteed to be able to
+find a value for a particular type parameter.
+A necessary condition for type intersection to not assign a parameter is that it only
+appears in a `Union[All]` and during subtyping some other union component (that does not
+constrain the type parameter) is selected.
+
+The `type_constrains` flag determines whether Type{T} is considered to be constraining
+`T`. This is not true in general, because of the existence of types with free type
+parameters, however, some callers would like to ignore this corner case.
+"""
+function constrains_param(var::TypeVar, @nospecialize(typ), covariant::Bool, type_constrains::Bool=false)
+    typ === var && return true
+    while typ isa UnionAll
+        covariant && constrains_param(var, typ.var.ub, covariant, type_constrains) && return true
+        # typ.var.lb doesn't constrain var
+        typ = typ.body
+    end
+    if typ isa Union
+        # for unions, verify that both options would constrain var
+        ba = constrains_param(var, typ.a, covariant, type_constrains)
+        bb = constrains_param(var, typ.b, covariant, type_constrains)
+        (ba && bb) && return true
+    elseif typ isa DataType
+        # return true if any param constrains var
+        fc = length(typ.parameters)
+        if fc > 0
+            if typ.name === Tuple.name
+                # vararg tuple needs special handling
+                for i in 1:(fc - 1)
+                    p = typ.parameters[i]
+                    constrains_param(var, p, covariant, type_constrains) && return true
+                end
+                lastp = typ.parameters[fc]
+                vararg = unwrap_unionall(lastp)
+                if vararg isa Core.TypeofVararg && isdefined(vararg, :N)
+                    constrains_param(var, vararg.N, covariant, type_constrains) && return true
+                    # T = vararg.parameters[1] doesn't constrain var
+                else
+                    constrains_param(var, lastp, covariant, type_constrains) && return true
+                end
+            else
+                if typ.name === typename(Type) && typ.parameters[1] === var && var.ub === Any
+                    # Types with free type parameters are <: Type cause the typevar
+                    # to be unconstrained because Type{T} with free typevars is illegal
+                    return type_constrains
+                end
+                for i in 1:fc
+                    p = typ.parameters[i]
+                    constrains_param(var, p, false, type_constrains) && return true
+                end
+            end
+        end
+    end
+    return false
+end
+
+"""
+    MaybeUndefSP(typ)
+    is_maybeundefsp(typ) -> Bool
+    unwrap_maybeundefsp(typ) -> Any
+
+A special wrapper that represents a static parameter that could be undefined at runtime.
+This does not participate in the native type system nor the inference lattice,
+and it thus should be always unwrapped when performing any type or lattice operations on it.
+"""
+struct MaybeUndefSP
+    typ
+    MaybeUndefSP(@nospecialize typ) = new(typ)
+end
+is_maybeundefsp(@nospecialize typ) = isa(typ, MaybeUndefSP)
+unwrap_maybeundefsp(@nospecialize typ) = isa(typ, MaybeUndefSP) ? typ.typ : typ
+is_maybeundefsp(sptypes::Vector{Any}, idx::Int) = is_maybeundefsp(sptypes[idx])
+unwrap_maybeundefsp(sptypes::Vector{Any}, idx::Int) = unwrap_maybeundefsp(sptypes[idx])
+
+const EMPTY_SPTYPES = Any[]
+
 function sptypes_from_meth_instance(linfo::MethodInstance)
-    toplevel = !isa(linfo.def, Method)
-    if !toplevel && isempty(linfo.sparam_vals) && isa(linfo.def.sig, UnionAll)
+    def = linfo.def
+    isa(def, Method) || return EMPTY_SPTYPES # toplevel
+    sig = def.sig
+    if isempty(linfo.sparam_vals)
+        isa(sig, UnionAll) || return EMPTY_SPTYPES
         # linfo is unspecialized
         sp = Any[]
-        sig = linfo.def.sig
-        while isa(sig, UnionAll)
-            push!(sp, sig.var)
-            sig = sig.body
+        sig′ = sig
+        while isa(sig′, UnionAll)
+            push!(sp, sig′.var)
+            sig′ = sig′.body
         end
     else
         sp = collect(Any, linfo.sparam_vals)
     end
     for i = 1:length(sp)
         v = sp[i]
         if v isa TypeVar
-            temp = linfo.def.sig
+            maybe_undef = !constrains_param(v, linfo.specTypes, #=covariant=#true)
+            temp = sig
             for j = 1:i-1
                 temp = temp.body
             end
@@ -402,12 +488,13 @@ function sptypes_from_meth_instance(linfo::MethodInstance)
                 tv = TypeVar(v.name, lb, ub)
                 ty = UnionAll(tv, Type{tv})
             end
+            @label ty_computed
+            maybe_undef && (ty = MaybeUndefSP(ty))
         elseif isvarargtype(v)
             ty = Int
         else
             ty = Const(v)
         end
-        @label ty_computed
         sp[i] = ty
     end
     return sp

base/compiler/optimize.jl

@@ -267,9 +267,9 @@ function stmt_effect_flags(𝕃ₒ::AbstractLattice, @nospecialize(stmt), @nospe
     if isa(stmt, Expr)
         (; head, args) = stmt
         if head === :static_parameter
-            etyp = (isa(src, IRCode) ? src.sptypes : src.ir.sptypes)[args[1]::Int]
             # if we aren't certain enough about the type, it might be an UndefVarError at runtime
-            nothrow = isa(etyp, Const)
+            sptypes = isa(src, IRCode) ? src.sptypes : src.ir.sptypes
+            nothrow = !is_maybeundefsp(sptypes, args[1]::Int)
             return (true, nothrow, nothrow)
         end
         if head === :call
@@ -377,7 +377,7 @@ function argextype(
     sptypes::Vector{Any}, slottypes::Vector{Any})
     if isa(x, Expr)
         if x.head === :static_parameter
-            return sptypes[x.args[1]::Int]
+            return unwrap_maybeundefsp(sptypes, x.args[1]::Int)
         elseif x.head === :boundscheck
             return Bool
         elseif x.head === :copyast

base/compiler/ssair/slot2ssa.jl

@@ -216,7 +216,7 @@ end
 function typ_for_val(@nospecialize(x), ci::CodeInfo, sptypes::Vector{Any}, idx::Int, slottypes::Vector{Any})
     if isa(x, Expr)
         if x.head === :static_parameter
-            return sptypes[x.args[1]::Int]
+            return unwrap_maybeundefsp(sptypes, x.args[1]::Int)
         elseif x.head === :boundscheck
             return Bool
         elseif x.head === :copyast

base/compiler/ssair/verify.jl

@@ -268,7 +268,7 @@ function verify_ir(ir::IRCode, print::Bool=true,
                 elseif stmt.head === :foreigncall
                     isforeigncall = true
                 elseif stmt.head === :isdefined && length(stmt.args) == 1 &&
-                        (stmt.args[1] isa GlobalRef || (stmt.args[1] isa Expr && stmt.args[1].head === :static_parameter))
+                        (stmt.args[1] isa GlobalRef || isexpr(stmt.args[1], :static_parameter))
                     # a GlobalRef or static_parameter isdefined check does not evaluate its argument
                     continue
                 elseif stmt.head === :call

stdlib/Test/src/Test.jl

@@ -1970,54 +1970,11 @@ function detect_unbound_args(mods...;
     return collect(ambs)
 end
 
-# find if var will be constrained to have a definite value
-# in any concrete leaftype subtype of typ
-function constrains_param(var::TypeVar, @nospecialize(typ), covariant::Bool)
-    typ === var && return true
-    while typ isa UnionAll
-        covariant && constrains_param(var, typ.var.ub, covariant) && return true
-        # typ.var.lb doesn't constrain var
-        typ = typ.body
-    end
-    if typ isa Union
-        # for unions, verify that both options would constrain var
-        ba = constrains_param(var, typ.a, covariant)
-        bb = constrains_param(var, typ.b, covariant)
-        (ba && bb) && return true
-    elseif typ isa DataType
-        # return true if any param constrains var
-        fc = length(typ.parameters)
-        if fc > 0
-            if typ.name === Tuple.name
-                # vararg tuple needs special handling
-                for i in 1:(fc - 1)
-                    p = typ.parameters[i]
-                    constrains_param(var, p, covariant) && return true
-                end
-                lastp = typ.parameters[fc]
-                vararg = Base.unwrap_unionall(lastp)
-                if vararg isa Core.TypeofVararg && isdefined(vararg, :N)
-                    constrains_param(var, vararg.N, covariant) && return true
-                    # T = vararg.parameters[1] doesn't constrain var
-                else
-                    constrains_param(var, lastp, covariant) && return true
-                end
-            else
-                for i in 1:fc
-                    p = typ.parameters[i]
-                    constrains_param(var, p, false) && return true
-                end
-            end
-        end
-    end
-    return false
-end
-
 function has_unbound_vars(@nospecialize sig)
     while sig isa UnionAll
         var = sig.var
         sig = sig.body
-        if !constrains_param(var, sig, true)
+        if !Core.Compiler.constrains_param(var, sig, #=covariant=#true, #=type_constrains=#true)
             return true
         end
     end

test/compiler/effects.jl

@@ -770,3 +770,18 @@ gotoifnot_throw_check_48583(x) = x ? x : 0
 @test !Core.Compiler.is_nothrow(Base.infer_effects(gotoifnot_throw_check_48583, (Missing,)))
 @test !Core.Compiler.is_nothrow(Base.infer_effects(gotoifnot_throw_check_48583, (Any,)))
 @test Core.Compiler.is_nothrow(Base.infer_effects(gotoifnot_throw_check_48583, (Bool,)))
+
+
+# unknown :static_parameter should taint :nothrow
+# https://github.com/JuliaLang/julia/issues/46771
+unknown_sparam_throw(::Union{Nothing, Type{T}}) where T = (T; nothing)
+unknown_sparam_nothrow1(x::Ref{T}) where T = (T; nothing)
+unknown_sparam_nothrow2(x::Ref{Ref{T}}) where T = (T; nothing)
+@test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Type{Int},)))
+@test Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Type{<:Integer},)))
+@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Type,)))
+@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Nothing,)))
+@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Union{Type{Int},Nothing},)))
+@test !Core.Compiler.is_nothrow(Base.infer_effects(unknown_sparam_throw, (Any,)))
+@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,)))

test/compiler/inference.jl

@@ -4761,3 +4761,16 @@ g_no_bail_effects_any(x::Any) = f_no_bail_effects_any(x)
 
 # issue #48374
 @test (() -> Union{<:Nothing})() == Nothing
+
+# :static_parameter accuracy
+unknown_sparam_throw(::Union{Nothing, Type{T}}) where T = @isdefined(T) ? T::Type : nothing
+unknown_sparam_nothrow1(x::Ref{T}) where T = @isdefined(T) ? T::Type : nothing
+unknown_sparam_nothrow2(x::Ref{Ref{T}}) where T = @isdefined(T) ? T::Type : nothing
+@test only(Base.return_types(unknown_sparam_throw, (Type{Int},))) == Type{Int}
+@test only(Base.return_types(unknown_sparam_throw, (Type{<:Integer},))) == Type{<:Integer}
+@test only(Base.return_types(unknown_sparam_throw, (Type,))) == Union{Nothing, Type}
+@test_broken only(Base.return_types(unknown_sparam_throw, (Nothing,))) === Nothing
+@test_broken only(Base.return_types(unknown_sparam_throw, (Union{Type{Int},Nothing},))) === Union{Nothing,Type{Int}}
+@test only(Base.return_types(unknown_sparam_throw, (Any,))) === Union{Nothing,Type}
+@test only(Base.return_types(unknown_sparam_nothrow1, (Ref,))) === Type
+@test only(Base.return_types(unknown_sparam_nothrow2, (Ref{Ref{T}} where T,))) === Type