Tighten covariance rules wrt. in params with -preview=in

Require the `in` storage class on both sides, just like ref/out/lazy.

Doing so makes sure the code compiles with all compilers and for all
targets, independent from whether `in` means `const scope` or `const
scope ref`.

Author: kinke

src/dmd/mtype.d

@@ -6900,8 +6900,8 @@ extern (C++) final class Parameter : ASTNode
      * Params:
      *  returnByRef = true if the function returns by ref
      *  p = Parameter to compare with
-     *  previewIn = Whether `-previewIn` is being used, and thus if
-     *              `in` means `scope`.
+     *  previewIn = Whether `-preview=in` is being used, and thus if
+     *              `in` means `scope [ref]`.
      *
      * Returns:
      *  true = `this` can be used in place of `p`
@@ -6921,8 +6921,8 @@ extern (C++) final class Parameter : ASTNode
                 otherSTC |= STC.scope_;
         }
 
-        enum stc = STC.ref_ | STC.out_ | STC.lazy_;
-        if ((thisSTC & stc) != (otherSTC & stc))
+        const mask = STC.ref_ | STC.out_ | STC.lazy_ | (previewIn ? STC.in_ : 0);
+        if ((thisSTC & mask) != (otherSTC & mask))
             return false;
         return isCovariantScope(returnByRef, thisSTC, otherSTC);
     }

src/dmd/mtype.h

@@ -558,7 +558,7 @@ class Parameter : public ASTNode
     static size_t dim(Parameters *parameters);
     static Parameter *getNth(Parameters *parameters, d_size_t nth);
     const char *toChars() const;
-    bool isCovariant(bool returnByRef, const Parameter *p) const;
+    bool isCovariant(bool returnByRef, const Parameter *p, bool previewIn) const;
 };
 
 struct ParameterList

test/compilable/previewin.d

@@ -24,14 +24,17 @@ struct FooBar
     string toString() const
     {
         string result;
+        // Type is const
+        this.toString((in char[] buf) {
+            static assert(is(typeof(buf) == const(char[])));
+            result ~= buf;
+        });
         // Type inference works
-        this.toString((buf) { result ~= buf; });
-        // Specifying the STC too
         this.toString((in buf) { result ~= buf; });
-        // Some covariance
-        this.toString((const scope buf) { result ~= buf; });
-        this.toString((scope const(char)[] buf) { result ~= buf; });
-        this.toString((scope const(char[]) buf) { result ~= buf; });
+        // No covariance without explicit `in`
+        static assert(!__traits(compiles, { this.toString((buf) { result ~= buf; }); }));
+        static assert(!__traits(compiles, { this.toString((const scope buf) { result ~= buf; }); }));
+        static assert(!__traits(compiles, { this.toString((const scope ref buf) { result ~= buf; }); }));
         return result;
     }