Aggressive constprop in trevc! to stabilize triangular eigvec (#54635)

We may use aggressive constprop in `trevc!` to eliminate branches, which makes the return type of `eigvec(::UpperTriangular)` concretely inferred. ```julia julia> @inferred eigvecs(UpperTriangular([1 0; 0 1])) 2×2 Matrix{Float32}: 1.0 -0.0 0.0 1.0 ```
JuliaLang · May 31, 2024 · ea8b4af · ea8b4af
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diff --git a/stdlib/LinearAlgebra/src/lapack.jl b/stdlib/LinearAlgebra/src/lapack.jl
@@ -3701,7 +3701,7 @@ for (trcon, trevc, trrfs, elty) in
         # LOGICAL            SELECT( * )
         # DOUBLE PRECISION   T( LDT, * ), VL( LDVL, * ), VR( LDVR, * ),
         #$                   WORK( * )
-        function trevc!(side::AbstractChar, howmny::AbstractChar, select::AbstractVector{BlasInt}, T::AbstractMatrix{$elty},
+        Base.@constprop :aggressive function trevc!(side::AbstractChar, howmny::AbstractChar, select::AbstractVector{BlasInt}, T::AbstractMatrix{$elty},
                         VL::AbstractMatrix{$elty} = similar(T),
                         VR::AbstractMatrix{$elty} = similar(T))
             require_one_based_indexing(select, T, VL, VR)

diff --git a/stdlib/LinearAlgebra/test/triangular.jl b/stdlib/LinearAlgebra/test/triangular.jl
@@ -1105,4 +1105,10 @@ end
     end
 end
 
+@testset "type-stable eigvecs" begin
+    D = Float64[1 0; 0 2]
+    V = @inferred eigvecs(UpperTriangular(D))
+    @test V == Diagonal([1, 1])
+end
+
 end # module TestTriangular