wasm: emit a clean error for scalar ?T/!T returns instead of an internal ICE (#27516)

Author: enghitalo

vlib/v/gen/wasm/gen.v

@@ -263,6 +263,17 @@ pub fn (mut g Gen) fn_decl(node ast.FnDecl) {
 		}
 		else {
 			if rt.idx() != ast.void_type_idx {
+				// A scalar `?T`/`!T` return reaches here (it is not a MultiReturn),
+				// so the option/result guards below — which only run inside the
+				// MultiReturn arm or after the match — are dead for it. Guard before
+				// `get_wasm_type`, which would otherwise abort with an internal
+				// "unreachable type" ICE on the option/result-wrapped type.
+				if rt.has_flag(.option) {
+					g.v_error('option types are not implemented', node.return_type_pos)
+				}
+				if rt.has_flag(.result) {
+					g.v_error('result types are not implemented', node.return_type_pos)
+				}
 				wtyp := g.get_wasm_type(rt)
 				if g.is_param_type(rt) {
 					paramdbg << g.dbg_type_name('__rval(0)', rt)

vlib/v/gen/wasm/tests/scalar_option_result_error_test.v

@@ -0,0 +1,32 @@
+import os
+
+// A scalar `?T`/`!T` return type used to abort the wasm backend with an internal
+// `get_wasm_type: unreachable type ... UnknownTypeInfo` ICE, because the
+// option/result guards only ran inside the MultiReturn arm / after the type was
+// already lowered. The backend must instead emit the intended, located
+// "not implemented" user error.
+fn compile_wasm(src string, name string) os.Result {
+	vexe := os.quoted_path(@VEXE)
+	wrkdir := os.join_path(os.vtmp_dir(), 'wasm_scalar_optres_tests')
+	os.mkdir_all(wrkdir) or { panic(err) }
+	source_path := os.join_path(wrkdir, '${name}.v')
+	output_path := os.join_path(wrkdir, '${name}.wasm')
+	os.write_file(source_path, src) or { panic(err) }
+	return os.execute('${vexe} -b wasm -o ${os.quoted_path(output_path)} ${os.quoted_path(source_path)}')
+}
+
+fn test_scalar_option_return_errors_cleanly() {
+	res := compile_wasm('fn f() ?int {\n\treturn 3\n}\n\nfn main() {\n\tx := f() or { 0 }\n\t_ = x\n}\n',
+		'scalar_option')
+	assert res.exit_code != 0, 'expected a compile error, got: ${res.output}'
+	assert res.output.contains('option types are not implemented'), res.output
+	assert !res.output.contains('get_wasm_type: unreachable'), 'leaked the internal ICE: ${res.output}'
+}
+
+fn test_scalar_result_return_errors_cleanly() {
+	res := compile_wasm('fn f() !int {\n\treturn 3\n}\n\nfn main() {\n\tx := f() or { 0 }\n\t_ = x\n}\n',
+		'scalar_result')
+	assert res.exit_code != 0, 'expected a compile error, got: ${res.output}'
+	assert res.output.contains('result types are not implemented'), res.output
+	assert !res.output.contains('get_wasm_type: unreachable'), 'leaked the internal ICE: ${res.output}'
+}