Fix 24056 - const uninitialized data at module scope is not in TLS (#15458)

* Fix 24056 - const uninitialized data at module scope is not in TLS

* Update tests

* Add changelog entry

changelog/dmd.shared-static-ctor-const.dd

@@ -0,0 +1,29 @@
+Global `const` variables can no longer be initialized from a non-shared static constructor
+
+Just like `immutable` data, global `const` data is not placed in Thread Local Storage (TLS), so initializing it in a thread-local static constructor allows you to violate `const`:
+see [issue 24056](https://issues.dlang.org/show_bug.cgi?id=24056) for details.
+Doing this will now result in a deprecation:
+
+---
+int x;
+const int y;
+immutable int z;
+
+static this()
+{
+    x = 1;
+    y = 2; // Deprecation: cannot modify const variable
+    z = 3; // Error: cannot modify immutable variable (same as before)
+}
+---
+
+As a corrective action, move the initialization to a shared static constructor:
+
+---
+const int y;
+
+shared static this()
+{
+    y = 4; // OK
+}
+---

compiler/src/dmd/declaration.d

@@ -188,6 +188,15 @@ bool modifyFieldVar(Loc loc, Scope* sc, VarDeclaration var, Expression e1)
                     MODtoChars(var.type.mod), var.kind(), var.toChars());
                 errorSupplemental(loc, "Use `shared static this` instead.");
             }
+            else if (fd.isStaticCtorDeclaration() && !fd.isSharedStaticCtorDeclaration() &&
+                    var.type.isConst())
+            {
+                // @@@DEPRECATED_2.116@@@
+                // Turn this into an error, merging with the branch above
+                .deprecation(loc, "%s %s `%s` initialization is not allowed in `static this`",
+                    MODtoChars(var.type.mod), var.kind(), var.toChars());
+                deprecationSupplemental(loc, "Use `shared static this` instead.");
+            }
             return result;
         }
         else

compiler/src/dmd/mustuse.d

@@ -18,11 +18,11 @@ import dmd.identifier;
 import dmd.location;
 
 // Used in isIncrementOrDecrement
-private static const StringExp plusPlus, minusMinus;
+private const StringExp plusPlus, minusMinus;
 
 // Loc.initial cannot be used in static initializers, so
 // these need a static constructor.
-static this()
+shared static this()
 {
     plusPlus = new StringExp(Loc.initial, "++");
     minusMinus = new StringExp(Loc.initial, "--");

compiler/test/fail_compilation/fail4923.d

@@ -1,13 +1,17 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail4923.d(4): Error: immutable variable `bar` initialization is not allowed in `static this`
-fail_compilation/fail4923.d(4):        Use `shared static this` instead.
+fail_compilation/fail4923.d(5): Error: immutable variable `bar` initialization is not allowed in `static this`
+fail_compilation/fail4923.d(5):        Use `shared static this` instead.
+fail_compilation/fail4923.d(6): Deprecation: const variable `baz` initialization is not allowed in `static this`
+fail_compilation/fail4923.d(6):        Use `shared static this` instead.
 ---
 */
 #line 1
 immutable int bar;
+const int baz; // https://issues.dlang.org/show_bug.cgi?id=24056
 static this()
 {
     bar = 42;
+    baz = 43;
 }

compiler/test/fail_compilation/fail66.d

@@ -53,7 +53,7 @@ fail_compilation/fail66.d(59): Error: cannot modify `const` expression `x`
 class C4
 {
     static const int x;
-    static this() { x = 5; }
+    shared static this() { x = 5; }
     void foo()
     {
         x = 4;
@@ -67,7 +67,7 @@ fail_compilation/fail66.d(73): Error: cannot modify `const` expression `z5`
 ---
 */
 const int z5;
-static this() { z5 = 3; }
+shared static this() { z5 = 3; }
 void test5()
 {
     z5 = 4;

compiler/test/runnable/template9.d

@@ -1991,7 +1991,7 @@ void test8976()
 // https://issues.dlang.org/show_bug.cgi?id=8940
 
 const int n8940; // or `immutable`
-static this() { n8940 = 3; }
+shared static this() { n8940 = 3; }
 
 void f8940(T)(ref int val)
 {

compiler/test/runnable/test20.d

@@ -1104,7 +1104,7 @@ class C60
     {
     }
 
-    static this()
+    shared static this()
     {
         x = 5;
     }
@@ -1117,7 +1117,7 @@ class C60
 
 const int z60;
 
-static this()
+shared static this()
 {
     z60 = 3;
 }

compiler/test/runnable/test3449.d

@@ -12,11 +12,11 @@ immutable int ig1;
 static this()
 {
     mg1 = 10;
-    cg1 = 10;
 }
 
 shared static this()
 {
+    cg1 = 10;
     ig1 = 10;
 }
 static assert(!__traits(compiles, { static assert(mg1 == 0); }));

compiler/test/runnable/test42.d

@@ -6159,7 +6159,7 @@ void test5332()
 
 const int x11472 = void;
 
-static this() { x11472 = 10; }
+shared static this() { x11472 = 10; }
 
 void test11472()
 {

compiler/test/runnable/testassign.d

@@ -361,7 +361,7 @@ struct CtorTest6174(Data)
 
 const char gc6174;
 const char[1] ga6174;
-static this()
+shared static this()
 {
     gc6174 = 'a';    // OK
     ga6174[0] = 'a'; // line 5, Err
@@ -726,7 +726,7 @@ struct Foo8783
 
 const Foo8783[1] foos8783;
 
-static this()
+shared static this()
 {
     foreach (i; 0 .. foos8783.length)
         foos8783[i].bar[i] = 1; // OK

compiler/test/runnable/testconst.d

@@ -551,7 +551,7 @@ void test38()
 static const int x39;
 const int y39;
 
-static this()
+shared static this()
 {
     x39 = 3;
     y39 = 4;
@@ -613,7 +613,7 @@ class C42
     static const int d;
     static const int e = ctfe() + 2;
 
-    static this()
+    shared static this()
     {
         d = 4;
     }
@@ -1302,7 +1302,7 @@ void test78()
 
 const bool[string] stopWords79;
 
-static this()
+shared static this()
 {
     stopWords79 = [ "a"[]:1 ];
 }

compiler/test/runnable/testswitch.d

@@ -353,15 +353,11 @@ int foo15(int i)
     return y;
 }
 
-static this()
-{
-    X15 = 4;
-    Z15 = 5;
-}
-
 shared static this()
 {
+    X15 = 4;
     Y15 = 4;
+    Z15 = 5;
 }
 
 void test15()