Accept both Cvoid and Ptr{Void} return type in ccall(:memcpy)

POSIX specifies that memcpy returns its first argument, which is
a bit of a useless feature (e.g. the llvm intrinsic just returns
`void`. Nevertheless, since we're intercepting the ccall here, we
should allow it. For convenience, still allow the Cvoid return though.

Fixes #31073

Author: Keno

src/ccall.cpp

@@ -1777,19 +1777,20 @@ static jl_cgval_t emit_ccall(jl_codectx_t &ctx, jl_value_t **args, size_t nargs)
         JL_GC_POP();
         return mark_or_box_ccall_result(ctx, strp, retboxed, rt, unionall, static_rt);
     }
-    else if (is_libjulia_func(memcpy)) {
+    else if (is_libjulia_func(memcpy) && (rt == (jl_value_t*)jl_void_type || jl_is_cpointer_type(rt))) {
         const jl_cgval_t &dst = argv[0];
         const jl_cgval_t &src = argv[1];
         const jl_cgval_t &n = argv[2];
+        Value *destp = emit_unbox(ctx, T_size, dst, (jl_value_t*)jl_voidpointer_type);
         ctx.builder.CreateMemCpy(
-                ctx.builder.CreateIntToPtr(
-                    emit_unbox(ctx, T_size, dst, (jl_value_t*)jl_voidpointer_type), T_pint8),
+                ctx.builder.CreateIntToPtr(destp, T_pint8),
                 ctx.builder.CreateIntToPtr(
                     emit_unbox(ctx, T_size, src, (jl_value_t*)jl_voidpointer_type), T_pint8),
                 emit_unbox(ctx, T_size, n, (jl_value_t*)jl_ulong_type), 1,
                 false);
         JL_GC_POP();
-        return ghostValue(jl_void_type);
+        return rt == (jl_value_t*)jl_void_type ? ghostValue(jl_void_type) :
+            mark_or_box_ccall_result(ctx, destp, retboxed, rt, unionall, static_rt);
     }
 
     jl_cgval_t retval = sig.emit_a_ccall(

test/ccall.jl

@@ -1477,3 +1477,16 @@ test27477() = ccall((:ctest, Pkg27477.libccalltest), Complex{Int}, (Complex{Int}
 end
 
 @test Test27477.test27477() == 2 + 0im
+
+# issue #31073
+let
+    a = ['0']
+    arr = Vector{Char}(undef, 2)
+    ptr = pointer(arr)
+    elsz = sizeof(Char)
+    na = length(a)
+    nba = na * elsz
+    ptr = eval(:(ccall(:memcpy, Ptr{Cvoid}, (Ptr{Cvoid}, Ptr{Cvoid}, UInt), $(arr), $(a), $(nba))))
+    @test isa(ptr, Ptr{Cvoid})
+    @test arr[1] == '0'
+end