fix Issue 15021 - linker error with speculative instantiation and -in…

…line

src/inline.c

@@ -1896,9 +1896,9 @@ static Expression *expandInline(FuncDeclaration *fd, FuncDeclaration *parent,
     // When the function is actually expanded
     if (TemplateInstance *ti = fd->isInstantiated())
     {
-        // change ti to non-speculative instance
+        // change ti to non-speculative root instance
         if (!ti->minst)
-            ti->minst = ti->tempdecl->getModule();
+            ti->minst = ti->tempdecl->getModule()->importedFrom;
     }
 
     if (ps)

src/template.c

@@ -7949,6 +7949,19 @@ bool TemplateInstance::needsCodegen()
         return false;
     }
 
+    /* Even when this is reached to the codegen pass,
+     * a non-root nested template should not generate code,
+     * due to avoid ODR violation.
+     */
+    if (enclosing && enclosing->inNonRoot())
+    {
+        if (tinst)
+            return tinst->needsCodegen();
+        if (tnext)
+            return tnext->needsCodegen();
+        return false;
+    }
+
     /* The issue is that if the importee is compiled with a different -debug
      * setting than the importer, the importer may believe it exists
      * in the compiled importee when it does not, when the instantiation

test/runnable/imports/std15021conv.d

@@ -0,0 +1,13 @@
+module imports.std15021conv;
+
+T to(T, A...)(A args)
+{
+    return toImpl!T(args);
+}
+
+T toImpl(T, S)(S value)
+{
+    import imports.std15021format;
+    enforceValidFormatSpec!(S, char)('c');
+    return "";
+}

test/runnable/imports/std15021format.d

@@ -0,0 +1,12 @@
+module imports.std15021format;
+
+T enforceEx(T)(T value, lazy string msg = "")
+{
+    if (!value) throw new Exception(msg);
+    return value;
+}
+
+void enforceValidFormatSpec(T, Char)(int spec)
+{
+    enforceEx(spec == 's');
+}

test/runnable/link15021.d

@@ -0,0 +1,18 @@
+// PERMUTE_ARGS: -inline -g -debug -unittest
+
+import imports.std15021conv;
+
+class AliasDecl {}
+
+void aliasDecl(AliasDecl ad)
+{
+    AliasDecl* zis;
+
+    static if (is(typeof(to!string(*zis))))
+    {
+        pragma(msg, "hit!");
+        to!string(*zis);
+    }
+}
+
+void main() {}