Specs for: Add __traits(getCmdlineConstant)

dlang/dmd#15257

spec/traits.dd

@@ -63,6 +63,7 @@ $(GNAME TraitsKeyword):
     $(RELATIVE_LINK2 getOverloads, $(D getOverloads))
     $(RELATIVE_LINK2 getParameterStorageClasses, $(D getParameterStorageClasses))
     $(RELATIVE_LINK2 getPointerBitmap, $(D getPointerBitmap))
+    $(RELATIVE_LINK2 getCppNamespaces, $(D getCmdlineConstant))
     $(RELATIVE_LINK2 getCppNamespaces, $(D getCppNamespaces))
     $(RELATIVE_LINK2 getVisibility, $(D getVisibility))
     $(RELATIVE_LINK2 getProtection, $(D getProtection))
@@ -1417,6 +1418,31 @@ bar(T)()
 bar(int n)
 )
 
+$(H3 $(GNAME getCmdlineConstant))
+    $(P The first argument is a string representing the name of the constant, the optional second argument 
+    is a type or false, the optional third argument is an expression and is used as default in case the constant 
+    has not been passed to the compiler.
+
+    The name of the constant must contain the name of the fully qualified module, otherwise it will be added implicitly
+
+    If the second argument contains a type then the constant value will be returned after evaluation if the type of 
+    the expression defined in the constant is equal to or implicitly convertible with the type passed to the second argument
+    If the second argument is not present or false then the value of the constant will be returned as a string without any evaluation
+    )
+
+$(SPEC_RUNNABLE_EXAMPLE_COMPILE
+---
+# compiler defined constans: mymod.test1=some_text, mymod.test2=4
+
+module mymod;
+pragma(msg, __traits(getCmdlineConstant, "test1"));   // Prints "some_text"
+pragma(msg, __traits(getCmdlineConstant, "mymod.test1")); // Prints "some_text"
+pragma(msg, __traits(getCmdlineConstant, "test2", false)); // Prints string "4"
+pragma(msg, __traits(getCmdlineConstant, "test2", int)); // Prints integer 4
+pragma(msg, __traits(getCmdlineConstant, "test3", int, 2)); // Constant not passed to dmd --> fallback default -> Prints integer 2
+---
+)
+
 $(H3 $(GNAME getCppNamespaces))
     $(P The argument is a symbol.
     The result is a *ValueSeq* of strings, possibly empty, that correspond to the namespaces the symbol resides in.