Fix #18342 - wstring literals cannot be implicitly converted to const…

changelog/dmd.string-postfix-ptr.dd

@@ -0,0 +1,19 @@
+String literals with a postfix can now implicitly convert to a `const` pointer
+
+A string literal with a `StringPostfix` (`c`, `w`, or `d`) can now implicitly
+convert to a `const` or `immutable` pointer of the corresponding character type,
+consistent with how unpostfixed string literals behave:
+
+-------
+const(char)*  p1 = "hello"c;
+const(wchar)* p2 = "hello"w;
+const(dchar)* p3 = "hello"d;
+-------
+
+This makes it easier to pass a string literal with an explicit wide character type to a C API.
+
+----
+GetModuleHandleW("opengl32.dll"); // already possible
+GetModuleHandle("opengl32.dll"w.ptr); // already possible
+GetModuleHandle("opengl32.dll"w); // now also allowed
+---

compiler/src/dmd/dcast.d

@@ -823,6 +823,8 @@ MATCH implicitConvTo(Expression e, Type t)
                     break;
                 }
             }
+            else if (t.ty == Tpointer && e.type.isStaticOrDynamicArray() && e.type.nextOf().isImmutable() && tn.ty == tyn)
+                return m; // e.g. "foo"w -> const(wchar)*
             break;
 
         default:

compiler/test/runnable/implicit.d

@@ -474,6 +474,19 @@ void test15778()
     static assert(!__traits(compiles, (ds[] = "a"w)));
 }
 
+/***********************************/
+// https://github.com/dlang/dmd/issues/18342
+const(char)*  cptr  = "tic"c;
+const(wchar)* wcptr = "tac"w;
+immutable(dchar)* dcptr = "toe"d;
+
+void test18342()
+{
+    assert( cptr[0 .. 4] == "tic\0"c);
+    assert(wcptr[0 .. 4] == "tac\0"w);
+    assert(dcptr[0 .. 4] == "toe\0"d);
+}
+
 /***********************************/
 
 void main()
@@ -490,6 +503,7 @@ void main()
     testDIP29_5();
     testDIP29_6();
     test15778();
+    test18342();
 
     printf("Success\n");
 }

spec/expression.dd

@@ -2215,6 +2215,19 @@ $(H3 $(LEGACY_LNAME2 StringLiteral, string_literals, String Literals))
         $(TROW $(D immutable(dchar)[]))
     )
 
+    $(P A string literal with a $(DDSUBLINK spec/lex, string_postfix, StringPostfix)
+        has a fixed character type and can implicitly convert to a
+        $(D const) or $(D immutable) pointer of that character type:
+    )
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
+    -------------
+    const(char)*  p1 = "hello"c;
+    const(wchar)* p2 = "hello"w;
+    const(dchar)* p3 = "hello"d;
+    -------------
+    )
+
     $(UNDEFINED_BEHAVIOR writing to a string literal. This is not allowed in `@safe` code.)
 
     $(P By default, a string literal is typed as a dynamic array, but the element