disable weird type inference for object constructors (#24455)

closes #24372, refs #20091

This was added in #20091 for some reason but doesn't actually work and
only makes error messages more obscure. So for now, it's disabled.

Can also be backported to 2.0 if necessary.

(cherry picked from commit a610f23)

compiler/semobjconstr.nim

@@ -465,11 +465,14 @@ proc semObjConstr(c: PContext, n: PNode, flags: TExprFlags; expectedType: PType
   if t == nil:
     return localErrorNode(c, result, "object constructor needs an object type")
 
-  if t.skipTypes({tyGenericInst,
-      tyAlias, tySink, tyOwned, tyRef}).kind != tyObject and
-      expectedType != nil and expectedType.skipTypes({tyGenericInst,
-      tyAlias, tySink, tyOwned, tyRef}).kind == tyObject:
-    t = expectedType
+  when false:
+    # attempted type inference for generic object types,
+    # doesn't work since n[0] isn't set and seems underspecified
+    if t.skipTypes({tyGenericInst,
+        tyAlias, tySink, tyOwned, tyRef}).kind != tyObject and
+        expectedType != nil and expectedType.skipTypes({tyGenericInst,
+        tyAlias, tySink, tyOwned, tyRef}).kind == tyObject:
+      t = expectedType
 
   t = skipTypes(t, {tyGenericInst, tyAlias, tySink, tyOwned})
   if t.kind == tyRef:

tests/errmsgs/tuninstobjconstr.nim

@@ -0,0 +1,11 @@
+# issue #24372
+
+type
+  Foo[T] = object
+    x: string
+
+proc initFoo(): Foo[string] =
+  Foo(x: "hello") #[tt.Error
+     ^ cannot instantiate: 'Foo[T]'; the object's generic parameters cannot be inferred and must be explicitly given]#
+
+discard initFoo()