Type 2M sentinel returns by all result-feeding inputs

docs/src/API.md

@@ -388,6 +388,7 @@ Lightweight numerical utility functions shared across modules.
 ```@docs
 Utilities
 Utilities.clamp_to_nonneg
+Utilities.promote_typeof
 Utilities.ϵ_numerics
 Utilities.ϵ_numerics_2M_M
 Utilities.ϵ_numerics_2M_N

src/Microphysics2M.jl

@@ -787,7 +787,9 @@ function rain_evaporation(
     (; pdf_r, evap)::CMP.SB2006, aps::CMP.AirProperties, tps::TDI.PS,
     q_tot, q_lcl, q_icl, q_rai, q_sno, ρ, N_rai, T,
 )
-    FT = eltype(q_tot)
+    # the early return below must match the main path's type for any mix of
+    # plain-float and Dual arguments
+    FT = UT.promote_typeof(q_tot, q_lcl, q_icl, q_rai, q_sno, ρ, N_rai, T)
     ϵₘ = UT.ϵ_numerics_2M_M(FT)
     ϵₙ = UT.ϵ_numerics_2M_N(FT)
 

src/Utilities.jl

@@ -9,6 +9,20 @@ module Utilities
 import UnrolledUtilities as UU
 
 export clamp_to_nonneg, ϵ_numerics, ϵ_numerics_2M_M, ϵ_numerics_2M_N, ϵ_numerics_P3_B
+export promote_typeof
+
+"""
+    promote_typeof(args...)
+
+The common promoted type of the arguments' types.
+
+Use it to type early returns and sentinel values from all the arguments the
+main-path result derives from. Typing them from a single argument
+(`FT = eltype(q_tot)`-style) makes the function's return a union when a
+caller mixes plain floats with `ForwardDiff.Dual`s (or float widths) across
+arguments — non-concrete, heap-boxed, and silent.
+"""
+@inline promote_typeof(args...) = promote_type(map(typeof, args)...)
 export unrolled_logsumexp
 export sgs_weight_function, rime_mass_fraction, rime_density
 

test/ad_compat_tests.jl

@@ -70,6 +70,22 @@ function test_ad_compatibility(FT)
         @test CM2.Γ_incl(FT(-0.25), FT(0.5)) isa FT
     end
 
+    @testset "rain_evaporation is concretely typed for mixed arguments ($FT)" begin
+        # the early return (no rain / supersaturated) must have the same type
+        # as the main path when species are Duals over a plain-float q_tot —
+        # a union here heap-boxes every call in the Jacobian hot loop
+        sb = mp.warm_rain.seifert_beheng
+        aps = mp.warm_rain.air_properties
+        z = D(0, 0)
+        # subsaturated (main path) and supersaturated (early return) states
+        for (q_tot, T) in ((FT(0.005), FT(288)), (FT(0.02), FT(288)))
+            t = @inferred CM2.rain_evaporation(
+                sb, aps, tps, q_tot, D(2e-4, 1), z, D(1e-4, 1), z, FT(1.05), D(4e4, 1), T,
+            )
+            @test all(v -> v isa FD.Dual, values(t))
+        end
+    end
+
     @testset "BMT 2M+P3 Jacobian w.r.t. the 8 species ($FT)" begin
         function rhs(x, ρ, T, q_tot, logλ)
             t = BMT.bulk_microphysics_tendencies(