A different fix to #218 and DMD2682 that does not lead to constant folding regressions.

Fixes run/const_15, run/c/const_16_B.
The price is removing the lvalueness of struct literals. If it turns out too
much code depends on this behavior or we don't want to break with DMD, we
could keep struct literals as lvalues and instead convert struct literals used
as expression initializers into struct initializers.

dmd/expression.c

@@ -3234,10 +3234,13 @@ int StructLiteralExp::isLvalue()
 }
 #endif
 
+/*
+Removed in LDC. See declaration.
 Expression *StructLiteralExp::toLvalue(Scope *sc, Expression *e)
 {
     return this;
 }
+*/
 
 
 int StructLiteralExp::checkSideEffect(int flag)

dmd/expression.h

@@ -502,7 +502,9 @@ struct StructLiteralExp : Expression
     void scanForNestedRef(Scope *sc);
     Expression *optimize(int result);
     Expression *interpret(InterState *istate);
-    Expression *toLvalue(Scope *sc, Expression *e);
+    // LDC: struct literals aren't lvalues! Taking their address can lead to 
+    //      incorrect behavior, see LDC#218, DMD#2682
+    // Expression *toLvalue(Scope *sc, Expression *e);
 
     int inlineCost(InlineCostState *ics);
     Expression *doInline(InlineDoState *ids);

dmd/optimize.c

@@ -46,7 +46,7 @@ Expression *fromConstInitializer(Expression *e1)
     if (e1->op == TOKvar)
     {	VarExp *ve = (VarExp *)e1;
 	VarDeclaration *v = ve->var->isVarDeclaration();
-	if (v && v->isConst() && v->init && !v->init->isStructInitializer())
+	if (v && v->isConst() && v->init)
 	{   Expression *ei = v->init->toExpression();
 	    if (ei && ei->type)
 		e1 = ei;