Make empty mutable struct distinct

Fixes #17149

Author: Keno

src/cgutils.cpp

@@ -2357,7 +2357,7 @@ static jl_cgval_t emit_new_struct(jl_codectx_t &ctx, jl_value_t *ty, size_t narg
     assert(jl_is_concrete_type(ty));
     jl_datatype_t *sty = (jl_datatype_t*)ty;
     size_t nf = jl_datatype_nfields(sty);
-    if (nf > 0) {
+    if (nf > 0 || sty->mutabl) {
         if (jl_justbits(ty)) {
             Type *lt = julia_type_to_llvm(ty);
             unsigned na = nargs < nf ? nargs : nf;
@@ -2475,7 +2475,7 @@ static jl_cgval_t emit_new_struct(jl_codectx_t &ctx, jl_value_t *ty, size_t narg
         }
         return strctinfo;
     }
-    else if (!sty->mutabl) {
+    else {
         // 0 fields, ghost or bitstype
         if (jl_datatype_nbits(sty) == 0)
             return ghostValue(sty);
@@ -2484,11 +2484,6 @@ static jl_cgval_t emit_new_struct(jl_codectx_t &ctx, jl_value_t *ty, size_t narg
         assert(!isboxed);
         return mark_julia_type(ctx, UndefValue::get(lt), false, ty);
     }
-    else {
-        // 0 fields, singleton
-        assert(sty->instance != NULL);
-        return mark_julia_const(sty->instance);
-    }
 }
 
 static Value *emit_exc_in_transit(jl_codectx_t &ctx)

src/datatype.c

@@ -222,7 +222,7 @@ unsigned jl_special_vector_alignment(size_t nfields, jl_value_t *t)
 STATIC_INLINE int jl_is_datatype_make_singleton(jl_datatype_t *d)
 {
     return (!d->abstract && jl_datatype_size(d) == 0 && d != jl_sym_type && d->name != jl_array_typename &&
-            d->uid != 0 && (d->types == jl_emptysvec || !d->mutabl));
+            d->uid != 0 && !d->mutabl);
 }
 
 STATIC_INLINE void jl_allocate_singleton_instance(jl_datatype_t *st)

test/core.jl

@@ -4530,7 +4530,7 @@ undefined_x16090 = (Int,)
 @test_throws TypeError f16090()
 
 # issue #12238
-mutable struct A12238{T} end
+struct A12238{T} end
 mutable struct B12238{T,S}
     a::A12238{B12238{Int,S}}
 end
@@ -5977,3 +5977,11 @@ void24363 = A24363(nothing)
 f24363(a) = a.x
 @test f24363(int24363) === 65535
 @test f24363(void24363) === nothing
+
+# issue 17149
+mutable struct Foo17149
+end
+@test Foo17149() !== Foo17149()
+let a = Foo17149()
+    @test a === a
+end