when widening tuple types in tmerge, only widen the complex parts

Author: oscarddssmith

base/compiler/typelimits.jl

@@ -764,23 +764,26 @@ end
         return u
     end
     # don't let the slow widening of Tuple cause the whole type to grow too fast
+    # Specifically widen Tuple{..., Union{lots of stuff}...} to Tuple{..., Any, ...}
     for i in 1:length(types)
+        issimpleenoughtype(types[i]) && continue
+        # this Tuple is too complicated, so widen any of it's complicated parameters
         if typenames[i] === Tuple.name
-            widen = unwrap_unionall(types[i])
-            if isa(widen, DataType) && !isvatuple(widen)
-                widen = NTuple{length(widen.parameters), Any}
-            else
-                widen = Tuple
-            end
-            types[i] = widen
-            u = Union{types...}
-            if issimpleenoughtype(u)
-                return u
+            ti = types[i]
+            tip = (unwrap_unionall(types[i])::DataType).parameters
+            lt = length(tip)
+            p = Vector{Any}(undef, lt)
+            for j = 1:lt
+                ui = tip[j]
+                p[j] = issimpleenoughtype(ui) ? ui : isvarargtype(ui) ? Vararg : Any
             end
-            break
+            types[i] = rewrap_unionall(Tuple{p...}, ti)
         end
     end
-    # finally, just return the widest possible type
+    u = Union{types...}
+    if issimpleenoughtype(u)
+        return u
+    end
     return Any
 end
 

test/compiler/inference.jl

@@ -213,7 +213,9 @@ tmerge_test(Tuple{}, Tuple{Complex, Vararg{Union{ComplexF32, ComplexF64}}},
 @test Core.Compiler.tmerge(Base.BitIntegerType, Union{}) === Base.BitIntegerType
 @test Core.Compiler.tmerge(Union{}, Base.BitIntegerType) === Base.BitIntegerType
 @test Core.Compiler.tmerge(Core.Compiler.fallback_ipo_lattice, Core.Compiler.InterConditional(1, Int, Union{}), Core.Compiler.InterConditional(2, String, Union{})) === Core.Compiler.Const(true)
-
+# test issue behind https://github.com/JuliaLang/julia/issues/50458
+@test Core.Compiler.tmerge(Nothing, Tuple{Base.BitInteger, Int}) == Union{Nothing, Tuple{Any, Int}}
+@test Core.Compiler.tmerge(Nothing, Tuple{Integer, Int}) == Union{Nothing, Tuple{Integer, Int}}
 struct SomethingBits
     x::Base.BitIntegerType
 end