Sema: fix function paramater count mismatch note

expected type 'fn() void', found 'fn(i32) void'
function with 0 parameters cannot cast into a function with 0 parameters
  =>
expected type 'fn() void', found 'fn(i32) void'
function with 1 parameters cannot cast into a function with 0 parameters

Author: jacobly0

src/Sema.zig

@@ -24312,7 +24312,7 @@ fn coerceInMemoryAllowedFns(
 
     if (dest_info.param_types.len != src_info.param_types.len) {
         return InMemoryCoercionResult{ .fn_param_count = .{
-            .actual = dest_info.param_types.len,
+            .actual = src_info.param_types.len,
             .wanted = dest_info.param_types.len,
         } };
     }

test/cases/compile_errors/function_type_coercion.zig

@@ -0,0 +1,21 @@
+fn f(_: i32) void {}
+export fn wrong_param_count() void {
+    _ = @as(fn () void, f);
+}
+export fn wrong_param_type() void {
+    _ = @as(fn (f32) void, f);
+}
+export fn wrong_return_type() void {
+    _ = @as(fn () i32, f);
+}
+
+// error
+// backend=stage2,llvm
+// target=native
+//
+// :3:25: error: expected type 'fn() void', found 'fn(i32) void'
+// :3:25: note: function with 1 parameters cannot cast into a function with 0 parameters
+// :6:28: error: expected type 'fn(f32) void', found 'fn(i32) void'
+// :6:28: note: parameter 0 'i32' cannot cast into 'f32'
+// :9:24: error: expected type 'fn() i32', found 'fn(i32) void'
+// :9:24: note: return type 'void' cannot cast into return type 'i32'