parser, checker: optimize checking generic struct type mismatch (#12179)

vlib/v/checker/checker.v

@@ -932,6 +932,8 @@ pub fn (mut c Checker) generic_insts_to_concrete() {
 						typ.is_public = true
 						typ.kind = parent.kind
 						typ.methods = all_methods
+					} else {
+						util.verror('generic error', 'the number of generic types of interface `$parent.name` is inconsistent with the concrete types')
 					}
 				}
 				ast.SumType {
@@ -968,6 +970,8 @@ pub fn (mut c Checker) generic_insts_to_concrete() {
 						}
 						typ.is_public = true
 						typ.kind = parent.kind
+					} else {
+						util.verror('generic error', 'the number of generic types of sumtype `$parent.name` is inconsistent with the concrete types')
 					}
 				}
 				else {}

vlib/v/checker/tests/generics_inst_non_generic_struct_err.out

@@ -1 +1,6 @@
-generic error: struct `main.Test` is not a generic struct, cannot instantiate to the concrete types
+vlib/v/checker/tests/generics_inst_non_generic_struct_err.vv:5:14: error: struct `Test` is not a generic struct, cannot instantiate to the concrete types
+    3 |
+    4 | fn main() {
+    5 |     println(Test<string>{})
+      |                 ~~~~~~~~
+    6 | }

vlib/v/checker/tests/generics_struct_type_mismatch_err.out

@@ -1 +1,7 @@
-generic error: the number of generic types of struct `main.Example` is inconsistent with the concrete types
+vlib/v/checker/tests/generics_struct_type_mismatch_err.vv:7:20: error: the number of generic types of struct `Example` is inconsistent with the concrete types
+    5 |
+    6 | fn main() {
+    7 |     example := Example<string>{
+      |                       ~~~~~~~~
+    8 |         key: 'key'
+    9 |         value: 'value'

vlib/v/fmt/tests/generics_keep.vv

@@ -14,12 +14,6 @@ fn (_ Foo) simple<T>() T {
 	return T{}
 }
 
-struct NonGenericStruct {}
-
-fn use_as_generic(ngs NonGenericStruct<V>) NonGenericStruct<V> {
-	return NonGenericStruct{}
-}
-
 struct GenericStruct<A, B> {}
 
 fn proper_generics(gs GenericStruct<A, B>) GenericStruct<A, B> {

vlib/v/parser/parse_type.v

@@ -539,6 +539,7 @@ pub fn (mut p Parser) parse_generic_type(name string) ast.Type {
 pub fn (mut p Parser) parse_generic_inst_type(name string) ast.Type {
 	mut bs_name := name
 	mut bs_cname := name
+	start_pos := p.tok.position()
 	p.next()
 	p.in_generic_params = true
 	bs_name += '<'
@@ -561,6 +562,7 @@ pub fn (mut p Parser) parse_generic_inst_type(name string) ast.Type {
 		bs_name += ', '
 		bs_cname += '_'
 	}
+	concrete_types_pos := start_pos.extend(p.tok.position())
 	p.check(.gt)
 	p.in_generic_params = false
 	bs_name += '>'
@@ -575,6 +577,38 @@ pub fn (mut p Parser) parse_generic_inst_type(name string) ast.Type {
 		if parent_idx == 0 {
 			parent_idx = p.table.add_placeholder_type(name, .v)
 		}
+		parent_sym := p.table.get_type_symbol(ast.new_type(parent_idx))
+		match parent_sym.info {
+			ast.Struct {
+				if parent_sym.info.generic_types.len == 0 {
+					p.error_with_pos('struct `$parent_sym.name` is not a generic struct, cannot instantiate to the concrete types',
+						concrete_types_pos)
+				} else if parent_sym.info.generic_types.len != concrete_types.len {
+					p.error_with_pos('the number of generic types of struct `$parent_sym.name` is inconsistent with the concrete types',
+						concrete_types_pos)
+				}
+			}
+			ast.Interface {
+				if parent_sym.info.generic_types.len == 0 {
+					p.error_with_pos('interface `$parent_sym.name` is not a generic interface, cannot instantiate to the concrete types',
+						concrete_types_pos)
+				} else if parent_sym.info.generic_types.len != concrete_types.len {
+					p.error_with_pos('the number of generic types of interfce `$parent_sym.name` is inconsistent with the concrete types',
+						concrete_types_pos)
+				}
+			}
+			ast.SumType {
+				if parent_sym.info.generic_types.len == 0 {
+					p.error_with_pos('sumtype `$parent_sym.name` is not a generic sumtype, cannot instantiate to the concrete types',
+						concrete_types_pos)
+				} else if parent_sym.info.generic_types.len != concrete_types.len {
+					p.error_with_pos('the number of generic types of sumtype `$parent_sym.name` is inconsistent with the concrete types',
+						concrete_types_pos)
+				}
+			}
+			else {}
+		}
+
 		idx := p.table.register_type_symbol(ast.TypeSymbol{
 			kind: .generic_inst
 			name: bs_name