Fallback to fabric's experimental version manifest for versions that are not in the launchermeta.

src/main/java/net/fabricmc/loom/configuration/providers/MinecraftProviderImpl.java

@@ -58,6 +58,7 @@ public class MinecraftProviderImpl extends DependencyProvider implements Minecra
 	private File minecraftServerJar;
 	private File minecraftMergedJar;
 	private File versionManifestJson;
+	private File experimentalVersionsJson;
 
 	public MinecraftProviderImpl(Project project) {
 		super(project);
@@ -116,6 +117,7 @@ public class MinecraftProviderImpl extends DependencyProvider implements Minecra
 		minecraftServerJar = new File(getDirectories().getUserCache(), "minecraft-" + minecraftVersion + "-server.jar");
 		minecraftMergedJar = new File(getDirectories().getUserCache(), "minecraft-" + minecraftVersion + "-merged.jar");
 		versionManifestJson = new File(getDirectories().getUserCache(), "version_manifest.json");
+		experimentalVersionsJson = new File(getDirectories().getUserCache(), "experimental_version_manifest.json");
 	}
 
 	private void downloadMcJson(boolean offline) throws IOException {
@@ -151,8 +153,12 @@ public class MinecraftProviderImpl extends DependencyProvider implements Minecra
 			getProject().getLogger().lifecycle("Using custom minecraft manifest");
 		}
 
-		if (!optionalVersion.isPresent()) {
+		if (optionalVersion.isEmpty()) {
 			optionalVersion = mcManifest.versions().stream().filter(versions -> versions.id.equalsIgnoreCase(minecraftVersion)).findFirst();
+
+			if (optionalVersion.isEmpty()) {
+				optionalVersion = findExperimentalVersion(offline);
+			}
 		}
 
 		if (optionalVersion.isPresent()) {
@@ -182,6 +188,29 @@ public class MinecraftProviderImpl extends DependencyProvider implements Minecra
 		}
 	}
 
+	// This attempts to find the version from fabric's own fallback version manifest json.
+	private Optional<ManifestVersion.Versions> findExperimentalVersion(boolean offline) throws IOException {
+		if (offline) {
+			if (!experimentalVersionsJson.exists()) {
+				getProject().getLogger().warn("Skipping download of experimental versions jsons due to being offline.");
+				return Optional.empty();
+			}
+		} else {
+			DownloadUtil.downloadIfChanged(new URL(Constants.EXPERIMENTAL_VERSIONS), experimentalVersionsJson, getProject().getLogger());
+		}
+
+		String expVersionManifest = Files.asCharSource(experimentalVersionsJson, StandardCharsets.UTF_8).read();
+		ManifestVersion expManifest = LoomGradlePlugin.OBJECT_MAPPER.readValue(expVersionManifest, ManifestVersion.class);
+
+		var result = expManifest.versions().stream().filter(versions -> versions.id.equalsIgnoreCase(minecraftVersion)).findFirst();
+
+		if (result.isPresent()) {
+			getProject().getLogger().lifecycle("Using fallback experimental version {}", minecraftVersion);
+		}
+
+		return result;
+	}
+
 	private boolean hasRecentValidManifest() throws IOException {
 		if (getExtension().getCustomManifest() != null) {
 			return false;

src/main/java/net/fabricmc/loom/util/Constants.java

@@ -36,6 +36,7 @@ public class Constants {
 	public static final String LIBRARIES_BASE = "https://libraries.minecraft.net/";
 	public static final String RESOURCES_BASE = "https://resources.download.minecraft.net/";
 	public static final String VERSION_MANIFESTS = "https://launchermeta.mojang.com/mc/game/version_manifest_v2.json";
+	public static final String EXPERIMENTAL_VERSIONS = "https://maven.fabricmc.net/net/minecraft/experimental_versions.json";
 
 	public static final String SYSTEM_ARCH = System.getProperty("os.arch").equals("64") ? "64" : "32";
 

src/test/groovy/net/fabricmc/loom/test/integration/ExperimentalVersionsTest.groovy

@@ -0,0 +1,61 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2021 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.integration
+
+import net.fabricmc.loom.test.util.ProjectTestTrait
+import spock.lang.Specification
+import spock.lang.Unroll
+
+import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
+
+class ExperimentalVersionsTest extends Specification implements ProjectTestTrait {
+    @Override
+    String name() {
+        "minimalBase"
+    }
+
+    @Override
+    def filesReady() {
+        buildGradle() << '''
+            dependencies {
+                minecraft "com.mojang:minecraft:1.18_experimental-snapshot-1"
+                mappings "net.fabricmc:yarn:1.18_experimental-snapshot-1+build.2:v2"
+                modImplementation "net.fabricmc:fabric-loader:0.11.6"
+            }
+        '''
+    }
+
+    @Unroll
+    def "experimental versions (gradle #gradle)"() {
+        when:
+            def result = create("build", gradle)
+        then:
+            result.task(":build").outcome == SUCCESS
+        where:
+            gradle              | _
+            DEFAULT_GRADLE      | _
+            PRE_RELEASE_GRADLE  | _
+    }
+}