add warning if no compiler and message is not compiled

packages/core/src/i18n.test.ts

@@ -282,7 +282,7 @@ describe("I18n", () => {
     })
   })
 
-  it("._ shouldn't use compiled message in production", () => {
+  it("._ should use compiled message in production", () => {
     const messages = {
       Hello: "Salut",
       "My name is {name}": compileMessage("Je m'appelle {name}"),
@@ -304,7 +304,7 @@ describe("I18n", () => {
   it("._ shouldn't double compile message in development", () => {
     const messages = {
       Hello: "Salut",
-      "My name is {name}": ["Je m'appelle {name}"],
+      "My name is {name}": compileMessage("Je m'appelle '{name}'"),
     }
 
     const { setupI18n } = require("@lingui/core")
@@ -338,6 +338,26 @@ describe("I18n", () => {
     })
   })
 
+  it("should print warning if uncompiled message is used", () => {
+    expect.assertions(1)
+
+    const messages = {
+      Hello: "Salut",
+    }
+
+    mockEnv("production", () => {
+      mockConsole((console) => {
+        const { setupI18n } = require("@lingui/core")
+        const i18n = setupI18n({
+          locale: "fr",
+          messages: { fr: messages },
+        })
+
+        i18n._("Hello")
+        expect(console.warn).toBeCalled()
+      })
+    })
+  })
   it("._ should emit missing event for missing translation", () => {
     const i18n = setupI18n({
       locale: "en",

packages/core/src/i18n.ts

@@ -268,9 +268,21 @@ export class I18n extends EventEmitter<Events> {
     let translation = messageForId || message || id
 
     // Compiled message is always an array (`["Ola!"]`).
-    // If message comes as string - it's not compiled, and we need to compile it beforehand.
-    if (isString(translation) && this._messageCompiler) {
-      translation = this._messageCompiler(translation)
+    // If a message comes as string - it's not compiled, and we need to compile it beforehand.
+    if (isString(translation)) {
+      if (this._messageCompiler) {
+        translation = this._messageCompiler(translation)
+      } else {
+        console.warn(`Uncompiled message detected! Message:
+
+> ${translation}
+
+That means you use raw catalog or your catalog doesn't have a translation for the message and fallback was used.
+ICU features such as interpolation and plurals will not work properly for that message. 
+
+Please compile your catalog first. 
+`)
+      }
     }
 
     // hack for parsing unicode values inside a string to get parsed in react native environments