Add `intrinsics.procedure_of`

```odin
foo :: proc(x: $T) { fmt.println(x) }
bar :: intrinsics.procedure_of(foo(int(123))) // parameters are never ran at compile time, similar to `size_of`
bar(333) // prints 333
```

base/intrinsics/intrinsics.odin

@@ -295,6 +295,10 @@ simd_rotate_right :: proc(a: #simd[N]T, $offset: int) -> #simd[N]T ---
 // if all listed features are supported.
 has_target_feature :: proc($test: $T) -> bool where type_is_string(T) || type_is_proc(T) ---
 
+
+// Returns the value of the procedure where `x` must be a call expression
+procedure_of :: proc(x: $T) -> T where type_is_proc(T) ---
+
 // WASM targets only
 wasm_memory_grow :: proc(index, delta: uintptr) -> int ---
 wasm_memory_size :: proc(index: uintptr)        -> int ---

src/check_builtin.cpp

@@ -1843,6 +1843,7 @@ gb_internal bool check_builtin_procedure(CheckerContext *c, Operand *operand, As
 	case BuiltinProc_objc_register_class: 
 	case BuiltinProc_atomic_type_is_lock_free:
 	case BuiltinProc_has_target_feature:
+	case BuiltinProc_procedure_of:
 		// NOTE(bill): The first arg may be a Type, this will be checked case by case
 		break;
 
@@ -6157,6 +6158,51 @@ gb_internal bool check_builtin_procedure(CheckerContext *c, Operand *operand, As
 			break;
 		}
 
+	case BuiltinProc_procedure_of:
+		{
+			Ast *call_expr = unparen_expr(ce->args[0]);
+			Operand op = {};
+			check_expr_base(c, &op, ce->args[0], nullptr);
+			if (op.mode != Addressing_Value && !(call_expr && call_expr->kind == Ast_CallExpr)) {
+				error(ce->args[0], "Expected a call expression for '%.*s'", LIT(builtin_name));
+				return false;
+			}
+
+			Ast *proc = call_expr->CallExpr.proc;
+			Entity *e = entity_of_node(proc);
+
+			if (e == nullptr) {
+				error(ce->args[0], "Invalid procedure value, expected a regular/specialized procedure");
+				return false;
+			}
+
+			TypeAndValue tav = proc->tav;
+
+
+			operand->type       = e->type;
+			operand->mode       = Addressing_Value;
+			operand->value      = tav.value;
+			operand->builtin_id = BuiltinProc_Invalid;
+			operand->proc_group = nullptr;
+
+			if (tav.mode == Addressing_Builtin) {
+				operand->mode = tav.mode;
+				operand->builtin_id = cast(BuiltinProcId)e->Builtin.id;
+				break;
+			}
+
+			if (!is_type_proc(e->type)) {
+				gbString s = type_to_string(e->type);
+				error(ce->args[0], "Expected a procedure value, got '%s'", s);
+				gb_string_free(s);
+				return false;
+			}
+
+
+			ce->entity_procedure_of = e;
+			break;
+		}
+
 	case BuiltinProc_constant_utf16_cstring:
 		{
 			String value = {};

src/check_decl.cpp

@@ -88,11 +88,14 @@ gb_internal Type *check_init_variable(CheckerContext *ctx, Entity *e, Operand *o
 			e->type = t_invalid;
 			return nullptr;
 		} else if (is_type_polymorphic(t)) {
-			gbString str = type_to_string(t);
-			defer (gb_string_free(str));
-			error(e->token, "Invalid use of a polymorphic type '%s' in %.*s", str, LIT(context_name));
-			e->type = t_invalid;
-			return nullptr;
+			Entity *e = entity_of_node(operand->expr);
+			if (e->state.load() != EntityState_Resolved) {
+				gbString str = type_to_string(t);
+				defer (gb_string_free(str));
+				error(e->token, "Invalid use of a polymorphic type '%s' in %.*s", str, LIT(context_name));
+				e->type = t_invalid;
+				return nullptr;
+			}
 		} else if (is_type_empty_union(t)) {
 			gbString str = type_to_string(t);
 			defer (gb_string_free(str));
@@ -479,6 +482,9 @@ gb_internal void check_const_decl(CheckerContext *ctx, Entity *e, Ast *type_expr
 			entity = check_selector(ctx, &operand, init, e->type);
 		} else {
 			check_expr_or_type(ctx, &operand, init, e->type);
+			if (init->kind == Ast_CallExpr) {
+				entity = init->CallExpr.entity_procedure_of;
+			}
 		}
 
 		switch (operand.mode) {
@@ -526,6 +532,7 @@ gb_internal void check_const_decl(CheckerContext *ctx, Entity *e, Ast *type_expr
 			return;
 		}
 
+
 		if (entity != nullptr) {
 			if (e->type != nullptr) {
 				Operand x = {};

src/check_expr.cpp

@@ -578,6 +578,7 @@ gb_internal bool find_or_generate_polymorphic_procedure(CheckerContext *old_c, E
 	d->defer_use_checked = false;
 
 	Entity *entity = alloc_entity_procedure(nullptr, token, final_proc_type, tags);
+	entity->state.store(EntityState_Resolved);
 	entity->identifier = ident;
 
 	add_entity_and_decl_info(&nctx, ident, entity, d);

src/check_stmt.cpp

@@ -2224,8 +2224,16 @@ gb_internal void check_expr_stmt(CheckerContext *ctx, Ast *node) {
 		}
 		if (do_require) {
 			gbString expr_str = expr_to_string(ce->proc);
+			defer (gb_string_free(expr_str));
+			if (builtin_id) {
+				String real_name = builtin_procs[builtin_id].name;
+				if (real_name != make_string(cast(u8 const *)expr_str, gb_string_length(expr_str))) {
+					error(node, "'%s' ('%.*s.%.*s') requires that its results must be handled", expr_str,
+					      LIT(builtin_proc_pkg_name[builtin_procs[builtin_id].pkg]), LIT(real_name));
+					return;
+				}
+			}
 			error(node, "'%s' requires that its results must be handled", expr_str);
-			gb_string_free(expr_str);
 		}
 		return;
 	} else if (expr && expr->kind == Ast_SelectorCallExpr) {

src/checker.cpp

@@ -1479,6 +1479,10 @@ gb_internal Entity *entity_of_node(Ast *expr) {
 	case_ast_node(cc, CaseClause, expr);
 		return cc->implicit_entity;
 	case_end;
+
+	case_ast_node(ce, CallExpr, expr);
+		return ce->entity_procedure_of;
+	case_end;
 	}
 	return nullptr;
 }

src/checker.hpp

@@ -51,6 +51,12 @@ enum StmtFlag {
 enum BuiltinProcPkg {
 	BuiltinProcPkg_builtin,
 	BuiltinProcPkg_intrinsics,
+	BuiltinProcPkg_COUNT
+};
+
+String builtin_proc_pkg_name[BuiltinProcPkg_COUNT] = {
+	str_lit("builtin"),
+	str_lit("intrinsics"),
 };
 
 struct BuiltinProc {

src/checker_builtin_procs.hpp

@@ -299,6 +299,8 @@ BuiltinProc__type_simple_boolean_end,
 
 BuiltinProc__type_end,
 
+	BuiltinProc_procedure_of,
+
 	BuiltinProc___entry_point,
 
 	BuiltinProc_objc_send,
@@ -614,6 +616,8 @@ gb_global BuiltinProc builtin_procs[BuiltinProc_COUNT] = {
 
 	{STR_LIT(""), 0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
 
+	{STR_LIT("procedure_of"), 1, false, Expr_Expr, BuiltinProcPkg_intrinsics},
+
 	{STR_LIT("__entry_point"), 0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
 
 	{STR_LIT("objc_send"),   3, true,  Expr_Expr, BuiltinProcPkg_intrinsics, false, true},

src/parser.hpp

@@ -458,6 +458,7 @@ AST_KIND(_ExprBegin,  "",  bool) \
 		bool         optional_ok_one; \
 		bool         was_selector; \
 		AstSplitArgs *split_args; \
+		Entity *entity_procedure_of; \
 	}) \
 	AST_KIND(FieldValue,      "field value",              struct { Token eq; Ast *field, *value; }) \
 	AST_KIND(EnumFieldValue,  "enum field value",         struct { \