fix UndefVarError for undefined static parameters in statement position (#30886)

Author: JeffBezanson

base/compiler/ssair/queries.jl

@@ -14,7 +14,7 @@ function stmt_effect_free(@nospecialize(stmt), @nospecialize(rt), src, spvals::S
         if head === :static_parameter
             etyp = sparam_type(spvals[e.args[1]])
             # if we aren't certain enough about the type, it might be an UndefVarError at runtime
-            return isa(etyp, Const) || issingletontype(widenconst(etyp))
+            return isa(etyp, Const)
         end
         ea = e.args
         if head === :call

test/core.jl

@@ -5509,6 +5509,11 @@ f_isdefined_tv(::T) where {T} = @isdefined T
 f_isdefined_va(::T...) where {T} = @isdefined T
 @test !f_isdefined_va()
 @test f_isdefined_va(1, 2, 3)
+function f_unused_undefined_sp(::T...) where T
+    T
+    return 0
+end
+@test_throws UndefVarError(:T) f_unused_undefined_sp()
 
 # note: the constant `5` here should be > DataType.ninitialized.
 # This tests that there's no crash due to accessing Type.body.layout.