fix issue 15876 - various cases of SEGFAULT when formatting parser er…

…rors

src/dmd/hdrgen.d

@@ -124,7 +124,8 @@ public:
 
     override void visit(ExpStatement s)
     {
-        if (s.exp && s.exp.op == TOK.declaration)
+        if (s.exp && s.exp.op == TOK.declaration &&
+            (cast(DeclarationExp)s.exp).declaration)
         {
             // bypass visit(DeclarationExp)
             (cast(DeclarationExp)s.exp).declaration.accept(this);
@@ -636,7 +637,8 @@ public:
     {
         buf.writestring(Token.toString(s.tok));
         buf.writeByte(' ');
-        s.statement.accept(this);
+        if (s.statement)
+            s.statement.accept(this);
     }
 
     override void visit(ThrowStatement s)
@@ -1847,7 +1849,7 @@ public:
                 buf.writeByte(' ');
             typeToBuffer(d.type, d.ident);
         }
-        else
+        else if (d.ident)
         {
             declstring = (d.ident == Id.string || d.ident == Id.wstring || d.ident == Id.dstring);
             buf.writestring(d.ident.toString());
@@ -2730,19 +2732,21 @@ public:
          * are handled in visit(ExpStatement), so here would be used only when
          * we'll directly call Expression.toChars() for debugging.
          */
-        if (auto v = e.declaration.isVarDeclaration())
+        if (e.declaration)
         {
+            if (auto v = e.declaration.isVarDeclaration())
+            {
             // For debugging use:
             // - Avoid printing newline.
             // - Intentionally use the format (Type var;)
             //   which isn't correct as regular D code.
-            buf.writeByte('(');
-            visitVarDecl(v, false);
-            buf.writeByte(';');
-            buf.writeByte(')');
+                buf.writeByte('(');
+                visitVarDecl(v, false);
+                buf.writeByte(';');
+                buf.writeByte(')');
+            }
+            else e.declaration.accept(this);
         }
-        else
-            e.declaration.accept(this);
     }
 
     override void visit(TypeidExp e)

test/fail_compilation/e15876_1.d

@@ -0,0 +1 @@
+o[{scope(x

test/fail_compilation/e15876_2.d

@@ -0,0 +1 @@
+o[{template

test/fail_compilation/e15876_3.d

@@ -0,0 +1 @@
+d(={for

test/fail_compilation/e15876_4.d

@@ -0,0 +1 @@
+typeof){for

test/fail_compilation/e15876_5.d

@@ -0,0 +1 @@
+p[{alias