Fix generator-invocation legality check for varargs generators (#47739)

This code was introduced by me back in #31025 to speed up evaluation
of generated functions that didn't make use of all of their arguments to
make generation decisions. However, it neglected to take into account the
possibility that the generator could be varargs. As a result, an unfortunate
coincidence of an unused slot in the correct position could have allowed
expansion of generators that were not supposed to be expandable. This can
cause incorrect inference with all the usual consequences. However, fortunately
this coincidence appears to be pretty rare.

Fixes JuliaDebug/CassetteOverlay.jl#12

(cherry picked from commit 328dd57)

Author: Keno

base/reflection.jl

@@ -1084,13 +1084,25 @@ function may_invoke_generator(method::Method, @nospecialize(atypes), sparams::Si
             end
         end
     end
-    for i = 1:length(at.parameters)
+    non_va_args = method.isva ? method.nargs - 1 : method.nargs
+    for i = 1:non_va_args
         if !isdispatchelem(at.parameters[i])
             if (ast_slotflag(code, 1 + i + nsparams) & SLOT_USED) != 0
                 return false
             end
         end
     end
+    if method.isva
+        # If the va argument is used, we need to ensure that all arguments that
+        # contribute to the va tuple are dispatchelemes
+        if (ast_slotflag(code, 1 + method.nargs + nsparams) & SLOT_USED) != 0
+            for i = (non_va_args+1):length(at.parameters)
+                if !isdispatchelem(at.parameters[i])
+                    return false
+                end
+            end
+        end
+    end
     return true
 end
 

test/staged.jl

@@ -304,3 +304,18 @@ end
     :(global x33243 = 2)
 end
 @test_throws ErrorException f33243()
+
+# https://github.com/JuliaDebug/CassetteOverlay.jl/issues/12
+# generated function with varargs and unfortunately placed unused slot
+@generated function f_vararg_generated(args...)
+    :($args)
+end
+g_vararg_generated() = f_vararg_generated((;), (;), Base.inferencebarrier((;)))
+let tup = g_vararg_generated()
+    @test !any(==(Any), tup)
+    # This is just to make sure that the test is actually testing what we want -
+    # the test only works if there's an unused that matches the position of the
+    # inferencebarrier argument above (N.B. the generator function itself
+    # shifts everything over by 1)
+    @test code_lowered(first(methods(f_vararg_generated)).generator.gen)[1].slotflags[5] == UInt8(0x00)
+end