Add minimal test files to reproduce the issue

grails-app/taglib/spock/bug/BarTagLib.groovy

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+package spock.bug
+
+class BarTagLib {
+    static defaultEncodeAs = [taglib:'html']
+    //static encodeAsForTags = [tagName: [taglib:'html'], otherTagName: [taglib:'none']]
+}

test/unit/spock/bug/BarSpec.groovy

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+package spock.bug
+
+import grails.test.mixin.TestFor
+import grails.test.runtime.FreshRuntime
+import org.springframework.context.i18n.LocaleContextHolder
+import spock.lang.Specification
+import spock.util.mop.ConfineMetaClassChanges
+
+
+@ConfineMetaClassChanges([LocaleContextHolder])
+@TestFor(BarTagLib)
+@FreshRuntime
+class BarSpec extends Specification {
+
+
+    def 'bar' () {
+        given:
+            LocaleContextHolder.metaClass.static.setLocale = { Locale locale ->
+
+            }
+		expect:
+			true
+    }
+
+}

test/unit/spock/bug/BazSpec.groovy

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+package spock.bug
+
+import org.springframework.context.i18n.LocaleContextHolder
+import spock.lang.Specification
+import spock.util.mop.ConfineMetaClassChanges
+
+@ConfineMetaClassChanges([LocaleContextHolder])
+class BazSpec extends Specification {
+
+    def 'baz'() {
+        given:
+			LocaleContextHolder.locale = Locale.UK
+        expect:
+			Locale.UK == LocaleContextHolder.locale
+    }
+}

test/unit/spock/bug/FooSpec.groovy

@@ -0,0 +1,16 @@
+package spock.bug
+
+import org.springframework.context.i18n.LocaleContextHolder
+import spock.lang.Specification
+import spock.util.mop.ConfineMetaClassChanges
+
+@ConfineMetaClassChanges(LocaleContextHolder)
+class FooSpec extends Specification {
+
+	def 'foo'() {
+		given:
+			LocaleContextHolder.metaClass.static.getLocale = { Locale.UK }
+		expect:
+			true
+	}
+}