Issue 5081 - Pure functions as initializers for immutable structures

src/cast.c

@@ -576,6 +576,26 @@ MATCH AssocArrayLiteralExp::implicitConvTo(Type *t)
         return Expression::implicitConvTo(t);
 }
 
+MATCH CallExp::implicitConvTo(Type *t)
+{
+#if 0
+    printf("CalLExp::implicitConvTo(this=%s, type=%s, t=%s)\n",
+        toChars(), type->toChars(), t->toChars());
+#endif
+
+    MATCH m = Expression::implicitConvTo(t);
+    if (m)
+        return m;
+
+    /* Allow the result of strongly pure functions to
+     * convert to immutable
+     */
+    if (f && f->isPure() == PUREstrong)
+        return type->invariantOf()->implicitConvTo(t);
+
+    return MATCHnomatch;
+}
+
 MATCH AddrExp::implicitConvTo(Type *t)
 {
 #if 0

src/expression.c

@@ -6859,7 +6859,6 @@ Expression *CallExp::syntaxCopy()
 Expression *CallExp::semantic(Scope *sc)
 {
     TypeFunction *tf;
-    FuncDeclaration *f;
     Type *t1;
     int istemp;
     Objects *targsi = NULL;     // initial list of template arguments

src/expression.h

@@ -935,6 +935,7 @@ struct DotTypeExp : UnaExp
 struct CallExp : UnaExp
 {
     Expressions *arguments;     // function arguments
+    FuncDeclaration *f;         // symbol to call
 
     CallExp(Loc loc, Expression *e, Expressions *exps);
     CallExp(Loc loc, Expression *e);
@@ -954,6 +955,7 @@ struct CallExp : UnaExp
     Expression *toLvalue(Scope *sc, Expression *e);
     int canThrow(bool mustNotThrow);
     Expression *addDtorHook(Scope *sc);
+    MATCH implicitConvTo(Type *t);
 
     int inlineCost(InlineCostState *ics);
     Expression *doInline(InlineDoState *ids);

src/statement.c

@@ -3521,6 +3521,13 @@ Statement *ReturnStatement::semantic(Scope *sc)
         }
         else if (tbret->ty != Tvoid)
         {
+            if (fd->isPureBypassingInference() == PUREstrong &&
+                !exp->type->implicitConvTo(tret) &&
+                exp->type->invariantOf()->implicitConvTo(tret))
+            {
+                exp = exp->castTo(sc, exp->type->invariantOf());
+            }
+
             exp = exp->implicitCastTo(sc, tret);
             if (!((TypeFunction *)fd->type)->isref)
                 exp = exp->optimize(WANTvalue);

test/runnable/xtest46.d

@@ -1875,6 +1875,24 @@ void test99()
 
 /***************************************************/
 
+void test5081()
+{
+    static pure immutable(int[]) x()
+    {
+        return new int[](10);
+    }
+
+    static pure int[] y()
+    {
+        return new int[](10);
+    }
+
+    immutable a = x();
+    immutable b = y();
+}
+
+/***************************************************/
+
 void test100()
 {
    string s;
@@ -3617,6 +3635,7 @@ int main()
     test116();
     test117();
     test118();
+    test5081();
 
     test120();