First version

.editorconfig

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+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[{package.json,yarn.lock}]
+indent_size = 2

.gitignore

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+/index.js
+/index.d.ts
+/node_modules/

index.test.ts

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+import indent from '.';
+
+test('It correctly drops template indentation', () => {
+    expect(indent`
+        Test ${"value"}
+    `).toBe('Test value');
+});
+
+test('It works when there is no placeholders', () => {
+    expect(indent`
+        Test value
+    `).toBe('Test value');
+});
+
+test('It reindent placeholders', () => {
+    expect(indent`
+        <test>
+            ${"test\nvalue"}
+        </test>
+    `).toBe('<test>\n    test\n    value\n</test>');
+});
+
+test('Nested indent test', () => {
+    const list = ['First', 'Second'];
+
+    expect(indent`
+        <test>
+            ${list.map((name) => indent`
+                <value>
+                    ${name}
+                </value>
+            `).join('\n')}
+        </test>
+    `).toBe(`<test>\n    <value>\n        First\n    </value>\n    <value>\n        Second\n    </value>\n</test>`);
+})

index.ts

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+const indentTag = (strings: TemplateStringsArray, ... expressions: any[]) => {
+    let parts: string[] = Array.from(strings);
+
+    // Drop first line feed
+    if ('\n' !== strings[0][0]) {
+        throw new Error('Expect first character to be a line feed');
+    }
+
+    parts[0] = parts[0].slice(1);
+
+    // Remove last line
+    parts[strings.length - 1] = parts[parts.length - 1].replace(/\n[ \t]*$/, '');
+
+    // Dedent using indent from first part
+    const indent = parts[0].match(/^[ \t]*/)![0];
+
+    const deindent = new RegExp(`(^|\n)${indent}`, 'g');
+
+    parts = parts.map((string) => string.replace(deindent, '$1'));
+
+    // Ensure expressions are strings
+    const values = expressions.map((expression) => String(expression));
+
+    // Reindent values
+    const indented = values.map((value, index) => {
+        const part = parts[index];
+
+        const match = part.match(/\n( *)$/);
+
+        // Only indent replacement if there is nothing before it on the line
+        if (!match) {
+            return value;
+        }
+
+        return value.replace(/\n/g, match[0]);
+    })
+
+    return parts.flatMap((part, index) => [part, indented[index]]).join('');
+}
+
+export default indentTag;

jest.config.js

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+/** @type {import('ts-jest/dist/types').InitialOptionsTsJest} */
+module.exports = {
+    preset: 'ts-jest',
+    testEnvironment: 'node',
+};

package.json

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+{
+  "name": "indent-tag",
+  "version": "0.1.0",
+  "description": "A template literal tag that automagically reindents placeholders",
+  "main": "index.js",
+  "types": "index.d.ts",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/jvasseur/indent-tag.git"
+  },
+  "author": "Jérôme Vasseur",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/jvasseur/indent-tag/issues"
+  },
+  "homepage": "https://github.com/jvasseur/indent-tag#readme",
+  "devDependencies": {
+    "@tsconfig/node12": "^1.0.9",
+    "@types/jest": "^27.4.1",
+    "jest": "^27.5.1",
+    "ts-jest": "^27.1.4",
+    "typescript": "^4.6.3"
+  },
+  "scripts": {
+    "build": "tsc",
+    "test": "jest"
+  },
+  "files": [
+    "index.d.ts"
+  ]
+}

tsconfig.json

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+{
+    "extends": "@tsconfig/node12/tsconfig.json",
+    "compilerOptions": {
+        "declaration": true
+    },
+    "include": ["./index.ts"]
+}