fix Issue 15900 - public imports not accessible using FQN

- public imports in imported modules were not accessbile using their
  FQN, this was an oversight when adding the package tree masking to fix
  Bugzilla 313 (see #5426)
- fixed by recursively checking imported scopes for accessible packages
- reuse Module.insearch to not follow import cycles
- Uses the same reduced visibility distinction (only private vs. rest) as the
  unqualified symbol search, b/c extending importedScopes to track rich
  visibility (e.g. package(a.b)) was out of scope for a regression fix.
  This should be implemented when combining the search/import w/
  the symbol visibility mechanism.

src/access.d

@@ -455,7 +455,7 @@ extern (C++) bool checkAccess(Loc loc, Scope* sc, Package p)
         return false;
     for (; sc; sc = sc.enclosing)
     {
-        if (sc.scopesym && sc.scopesym.isPackageAccessible(p))
+        if (sc.scopesym && sc.scopesym.isPackageAccessible(p, Prot(PROTprivate)))
             return false;
     }
     auto name = p.toPrettyChars();

src/dimport.d

@@ -273,14 +273,14 @@ public:
                 {
                     // import a.b.c.d;
                     auto p = pkg; // a
-                    scopesym.addAccessiblePackage(p);
+                    scopesym.addAccessiblePackage(p, protection);
                     foreach (id; (*packages)[1 .. packages.dim]) // [b, c]
                     {
                         p = cast(Package) p.symtab.lookup(id);
-                        scopesym.addAccessiblePackage(p);
+                        scopesym.addAccessiblePackage(p, protection);
                     }
                 }
-                scopesym.addAccessiblePackage(mod); // d
+                scopesym.addAccessiblePackage(mod, protection); // d
             }
 
             mod.semantic();

src/dmodule.d

@@ -1111,6 +1111,18 @@ public:
         return s;
     }
 
+    override bool isPackageAccessible(Package p, Prot protection, int flags = 0)
+    {
+        if (insearch) // don't follow import cycles
+            return false;
+        insearch = true;
+        scope (exit)
+            insearch = false;
+        if (flags & IgnorePrivateImports)
+            protection = Prot(PROTpublic); // only consider public imports
+        return super.isPackageAccessible(p, protection);
+    }
+
     override Dsymbol symtabInsert(Dsymbol s)
     {
         searchCacheIdent = null; // symbol is inserted, so invalidate cache

src/dsymbol.d

@@ -1246,7 +1246,7 @@ private:
     PROTKIND* prots;            // array of PROTKIND, one for each import
 
     import ddmd.root.array : BitArray;
-    BitArray accessiblePackages;// whitelist of accessible (imported) packages
+    BitArray accessiblePackages, privateAccessiblePackages;// whitelists of accessible (imported) packages
 
 public:
     final extern (D) this()
@@ -1482,17 +1482,27 @@ public:
         }
     }
 
-    final void addAccessiblePackage(Package p)
+    final void addAccessiblePackage(Package p, Prot protection)
     {
-        if (accessiblePackages.length <= p.tag)
-            accessiblePackages.length = p.tag + 1;
-        accessiblePackages[p.tag] = true;
+        auto pary = protection.kind == PROTprivate ? &privateAccessiblePackages : &accessiblePackages;
+        if (pary.length <= p.tag)
+            pary.length = p.tag + 1;
+        (*pary)[p.tag] = true;
     }
 
-    final bool isPackageAccessible(Package p)
+    bool isPackageAccessible(Package p, Prot protection, int flags = 0)
     {
-        return p.tag < accessiblePackages.length &&
-            accessiblePackages[p.tag];
+        if (p.tag < accessiblePackages.length && accessiblePackages[p.tag] ||
+            protection.kind == PROTprivate && p.tag < privateAccessiblePackages.length && privateAccessiblePackages[p.tag])
+            return true;
+        foreach (i, ss; importedScopes ? (*importedScopes)[] : null)
+        {
+            // only search visible scopes && imported modules should ignore private imports
+            if (protection.kind <= prots[i] &&
+                ss.isScopeDsymbol.isPackageAccessible(p, protection, IgnorePrivateImports))
+                return true;
+        }
+        return false;
     }
 
     override final bool isforwardRef()

src/dsymbol.h

@@ -296,7 +296,7 @@ class ScopeDsymbol : public Dsymbol
     Dsymbols *importedScopes;   // imported Dsymbol's
     PROTKIND *prots;            // array of PROTKIND, one for each import
 
-    BitArray accessiblePackages;
+    BitArray accessiblePackages, privateAccessiblePackages;
 
 public:
     ScopeDsymbol();
@@ -305,6 +305,7 @@ class ScopeDsymbol : public Dsymbol
     Dsymbol *search(Loc loc, Identifier *ident, int flags = SearchLocalsOnly);
     OverloadSet *mergeOverloadSet(Identifier *ident, OverloadSet *os, Dsymbol *s);
     void importScope(Dsymbol *s, Prot protection);
+    virtual bool isPackageAccessible(Package p, Prot protection, int flags = 0);
     bool isforwardRef();
     static void multiplyDefined(Loc loc, Dsymbol *s1, Dsymbol *s2);
     const char *kind();

test/compilable/imports/g313.d

@@ -0,0 +1,24 @@
+module imports.g313;
+
+// adds public package imports (see Bugzilla 15900)
+public import imports.g313public;
+// same w/ deferred semantics
+static if (true)
+    public import imports.g313staticif;
+mixin("public import imports.g313stringmixin;");
+
+template impD()
+{
+    public import imports.g313templatemixin;
+}
+
+mixin impD!();
+
+void test15900()
+{
+    // publically imported modules should obviously be available in here as well
+    imports.g313public.bug();
+    imports.g313staticif.bug();
+    imports.g313stringmixin.bug();
+    imports.g313templatemixin.bug();
+}

test/compilable/imports/g313public.d

@@ -0,0 +1,3 @@
+void bug()
+{
+}

test/compilable/imports/g313staticif.d

@@ -0,0 +1,3 @@
+void bug()
+{
+}

test/compilable/imports/g313stringmixin.d

@@ -0,0 +1,3 @@
+void bug()
+{
+}

test/compilable/imports/g313templatemixin.d

@@ -0,0 +1,3 @@
+void bug()
+{
+}

test/compilable/test313g.d

@@ -0,0 +1,12 @@
+// REQUIRED_ARGS: -de
+// EXTRA_SOURCES: imports/g313.d
+import imports.g313;
+
+void test15900()
+{
+    // publically imported modules from g313 should be available here
+    imports.g313public.bug();
+    imports.g313staticif.bug();
+    imports.g313stringmixin.bug();
+    imports.g313templatemixin.bug();
+}