Move issue 7815 case to fail_compilation, because it was invalid

It was wrongly accepted by the bug in `StaticIfCondition::include()`.

Bug explanation copied from fail_compilation/fail7815.d:
----
When the Move struct member is analyzed:
1. mixin Helpers!() is instantiated.
2. In Helpers!(), static if and its condition is(Flags!Move)) evaluated.
3. In Flags!Move, string mixin evaluates and CTFE lambda.
4. __traits(derivedMembers, Move) tries to see the member of Move.
   4a. mixin Helpers!() member is analyzed.
   4b. `static if (is(Flags!Move))` in Helpers!() is evaluated
   4c. The Flags!Move instantiation is already in progress, so it cannot be resolved.
   4d. `static if` fails because Flags!Move cannot be determined as a type.
5. __traits(derivedMembers, Move) returns a 1-length tuple("a").
6. The lambda in Flags!Move returns a string "struct Flags {...}", then
   Flags!Move is instantiated to a new struct Flags.
7. Finally Move struct does not have flags member, then the `enum a7815`
   definition will fail in its initializer.

src/cond.c

@@ -354,6 +354,11 @@ int StaticIfCondition::include(Scope *sc, ScopeDsymbol *sds)
         sc->pop();
         --nest;
 
+        // Prevent repeated condition evaluation.
+        // See: fail_compilation/fail7815.d
+        if (inc != 0)
+            return (inc == 1);
+
         if (!e->type->isBoolean())
         {
             if (e->type->toBasetype() != Type::terror)

test/fail_compilation/fail7815.d

@@ -0,0 +1,65 @@
+// REQUIRED_ARGS: -o-
+/*
+TEST_OUTPUT:
+---
+X: tuple("x")
+fail_compilation/fail7815.d(47): Error: no property 'flags' for type 'Move'
+---
+*/
+
+mixin template Helpers()
+{
+    static if (is(Flags!Move))
+    {
+        Flags!Move flags;
+    }
+    else
+    {
+        pragma(msg, "X: ", __traits(derivedMembers, Flags!Move));
+    }
+}
+
+template Flags(T)
+{
+    mixin({
+        int defs = 1;
+        foreach (name; __traits(derivedMembers, Move))
+        {
+            defs++;
+        }
+        if (defs)
+        {
+            return "struct Flags { bool x; }";
+        }
+        else
+        {
+            return "";
+        }
+    }());
+}
+
+struct Move
+{
+    int a;
+    mixin Helpers!();
+}
+
+enum a7815 = Move.init.flags;
+
+/+
+This is an invalid case.
+When the Move struct member is analyzed:
+1. mixin Helpers!() is instantiated.
+2. In Helpers!(), static if and its condition is(Flags!Move)) evaluated.
+3. In Flags!Move, string mixin evaluates and CTFE lambda.
+4. __traits(derivedMembers, Move) tries to see the member of Move.
+   4a. mixin Helpers!() member is analyzed.
+   4b. `static if (is(Flags!Move))` in Helpers!() is evaluated
+   4c. The Flags!Move instantiation is already in progress, so it cannot be resolved.
+   4d. `static if` fails because Flags!Move cannot be determined as a type.
+5. __traits(derivedMembers, Move) returns a 1-length tuple("a").
+6. The lambda in Flags!Move returns a string "struct Flags {...}", then
+   Flags!Move is instantiated to a new struct Flags.
+7. Finally Move struct does not have flags member, then the `enum a7815`
+   definition will fail in its initializer.
++/

test/runnable/xtest46.d

@@ -5899,41 +5899,6 @@ void test7735()
     a7735([]);
 }
 
-/***************************************************/
-// 7815
-
-mixin template Helpers() {
-
-  static if (is(Flags!Move)) {
-    Flags!Move flags;
-  } else {
-    // DMD will happily instantiate the allegedly
-    // non-existent Flags!This here. (!)
-    pragma(msg, __traits(derivedMembers, Flags!Move));
-  }
-}
-
-template Flags(T) {
-  mixin({
-    int defs = 1;
-    foreach (name; __traits(derivedMembers, Move)) {
-        defs++;
-    }
-    if (defs) {
-      return "struct Flags { bool a; }";
-    } else {
-      return "";
-    }
-  }());
-}
-
-struct Move {
-  int a;
-  mixin Helpers!();
-}
-
-enum a7815 = Move.init.flags;
-
 /***************************************************/
 
 struct A7823 {