Fix #1229 (#1237)

src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java

@@ -166,9 +166,13 @@ private synchronized void setupMinecraft(ConfigContext configContext) throws Exc
 		extension.setMinecraftProvider(minecraftProvider);
 		minecraftProvider.provide();
 
+		// Realise the dependencies without actually resolving them, this forces any lazy providers to be created, populating the layered mapping factories.
+		project.getConfigurations().getByName(Configurations.MAPPINGS).getDependencies().toArray();
+
 		// Created any layered mapping files.
 		LayeredMappingsFactory.afterEvaluate(configContext);
 
+		// Resolve the mapping files from the configuration
 		final DependencyInfo mappingsDep = DependencyInfo.create(getProject(), Configurations.MAPPINGS);
 		final MappingConfiguration mappingConfiguration = MappingConfiguration.create(getProject(), configContext.serviceFactory(), mappingsDep, minecraftProvider);
 		extension.setMappingConfiguration(mappingConfiguration);

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

@@ -120,4 +120,30 @@ class MojangMappingsProjectTest extends Specification implements GradleProjectTe
 		where:
 		version << STANDARD_TEST_VERSIONS
 	}
+
+	@Unroll
+	def "mojang mappings via lazy provider (gradle #version)"() {
+		setup:
+		def gradle = gradleProject(project: "minimalBase", version: version)
+
+		gradle.buildGradle << '''
+                dependencies {
+                    minecraft "com.mojang:minecraft:1.18-pre5"
+                    mappings project.provider {
+						loom.layered() {
+							officialMojangMappings()
+						}
+					}
+                }
+            '''
+
+		when:
+		def result = gradle.run(task: "build")
+
+		then:
+		result.task(":build").outcome == SUCCESS
+
+		where:
+		version << STANDARD_TEST_VERSIONS
+	}
 }