Fix Issue 23326 - Lambda missing parens in generated D import file

Arrow => literals parse with an AssignExpression body, so hdrgen must
parenthesize them when used as a call callee; otherwise the call is
absorbed into the lambda body in .di output.

Author: AayushMainali-Github

compiler/src/dmd/hdrgen.d

@@ -1879,19 +1879,10 @@ void toCBuffer(Dsymbol s, ref OutBuffer buf, ref HdrGenState hgs)
         if (!hgs.errorMsg)
             tf.attributesApply(&printAttribute);
 
-        CompoundStatement cs = f.fbody.isCompoundStatement();
-        Statement s1;
-        if (f.semanticRun >= PASS.semantic3done && cs)
-        {
-            s1 = (*cs.statements)[cs.statements.length - 1];
-        }
-        else
-            s1 = !cs ? f.fbody : null;
-        ReturnStatement rs = s1 ? s1.endsWithReturnStatement() : null;
-        if (rs && rs.exp)
+        if (auto result = arrowFuncLiteralResult(f))
         {
             buf.put(" => ");
-            rs.exp.expressionToBuffer(buf, hgs);
+            result.expressionToBuffer(buf, hgs);
         }
         else
         {
@@ -3863,6 +3854,28 @@ private void expressionToBuffer(Expression e, ref OutBuffer buf, ref HdrGenState
     expressionPrettyPrint(e, buf, hgs);
 }
 
+/**************************************************
+ * Returns the expression result if `f` is printed with `=>` syntax, otherwise `null`.
+ *
+ * Arrow function literals have an AssignExpression body, so they bind less tightly
+ * than postfix operators and must be parenthesized when used as a call callee etc.
+ */
+private Expression arrowFuncLiteralResult(FuncLiteralDeclaration f)
+{
+    if (!f.fbody)
+        return null;
+
+    CompoundStatement cs = f.fbody.isCompoundStatement();
+    Statement s1;
+    if (f.semanticRun >= PASS.semantic3done && cs)
+        s1 = (*cs.statements)[cs.statements.length - 1];
+    else
+        s1 = !cs ? f.fbody : null;
+
+    ReturnStatement rs = s1 ? s1.endsWithReturnStatement() : null;
+    return rs && rs.exp ? rs.exp : null;
+}
+
 // to be called if e could be loweredFrom another expression instead of acessing precedence[e.op] directly
 private PREC expPrecedence(ref HdrGenState hgs, Expression e)
 {
@@ -3877,6 +3890,13 @@ private PREC expPrecedence(ref HdrGenState hgs, Expression e)
             if (ne.loweredFrom)
                 e = ne.loweredFrom;
     }
+    // https://github.com/dlang/dmd/issues/23326
+    // Arrow lambdas are not true primaries; treat like assign-level expressions for paren insertion.
+    if (auto fe = e.isFuncExp())
+    {
+        if (fe.fd && arrowFuncLiteralResult(fe.fd))
+            return PREC.assign;
+    }
     return precedence[e.op];
 }
 

compiler/test/compilable/header23326.d

@@ -0,0 +1,18 @@
+/*
+REQUIRED_ARGS: -o- -Hf${RESULTS_DIR}/compilable/header23326.di
+OUTPUT_FILES: ${RESULTS_DIR}/compilable/header23326.di
+
+TEST_OUTPUT:
+---
+=== ${RESULTS_DIR}/compilable/header23326.di
+// D import file generated from 'compilable/header23326.d'
+enum i = ((x) => x * 2)(3);
+enum j = ((x) => x * 2)(4);
+extern typeof(((x) => x * 2)(5)) d;
+---
+*/
+
+// https://github.com/dlang/dmd/issues/23326
+enum i = (x => x * 2)(3);
+enum j = ((x) => x * 2)(4);
+auto d = (x => x * 2)(5);