Fix Issue 20908 - -preview=nosharedaccess requires zero-initializion for aggregates (#15023)

compiler/src/dmd/expressionsem.d

@@ -13375,6 +13375,12 @@ bool checkSharedAccess(Expression e, Scope* sc, bool returnRef = false)
 
         bool visitVar(VarExp e)
         {
+            // https://issues.dlang.org/show_bug.cgi?id=20908
+            // direct access to init symbols is ok as they
+            // cannot be modified.
+            if (e.var.isSymbolDeclaration())
+                return false;
+
             // https://issues.dlang.org/show_bug.cgi?id=22626
             // Synchronized functions don't need to use core.atomic
             // when accessing `this`.

compiler/test/compilable/shared.d

@@ -11,6 +11,17 @@ ref shared(int) f(return shared ref int y)
 }
 
 // https://issues.dlang.org/show_bug.cgi?id=20908
+struct S
+{
+    int i = 2;
+}
+
+union U
+{
+    int i = 1;
+    bool b;
+}
+
 void test20908()
 {
     // shared locals (or struct members) should be able to be initialised:
@@ -39,6 +50,9 @@ void test20908()
         // Multiple indirections
         return ***val;
     }
+
+    shared S s;
+    shared U u;
 }
 
 // Simple tests for `DotVarExp`