fix Issue 13855 - multi-module selective import statements

- allow to add further `mod : sym1, sym2` blocks to import statement
- only allow non-ambiguous variations (requiring look-ahead of 2 tokens)

Author: MartinNowak

changelog/multi_module_selective_imports.dd

@@ -0,0 +1,18 @@
+Allow multiple selective imports from different modules in a single import statement
+
+It is now possible to add more imports after a selective import list,
+given that the imported module has a package or is followed by another
+selective import list.
+
+-------
+import pkg.mod1 : sym1, sym2, pkg.mod2;
+import pkg.mod1 : sym1, mod2 : sym2;
+-------
+
+Modules without package or renamed aliases will still get parsed as part of the selective import list.
+
+-------
+import pkg.mod1 : sym1, mod2; // nope, imports symbol mod2 from pkg.mod1
+import pkg.mod1 : sym1, name=mod2; // nope, parsed as renamed import of symbol mod2 from pkg.mod1
+import pkg.mod1 : sym1, name=pkg1.mod1; // nope, fails to parse
+-------

src/ddmd/parse.d

@@ -3280,9 +3280,15 @@ final class Parser : Lexer
                         _alias = null;
                     }
                     s.addAlias(name, _alias);
+                    if (token.value != TOKcomma)
+                        break;
+                    // recognize import pkg.mod1 : a, b, pkg.mod2;
+                    immutable afterIdent = peekNext2;
+                    if (peekNext2 == TOKdot ||      // pkg . mod2
+                        peekNext2 == TOKcolon)      // mod2 : c
+                        break;  // parse another import
                 }
                 while (token.value == TOKcomma);
-                break; // no comma-separated imports of this form
             }
             aliasid = null;
         }

test/compilable/enh13855.d

@@ -0,0 +1,3 @@
+import imports.pkg313.c313 : bug, imports.c314 : bug; // previously 2 import statements
+import imports.pkg313.c313 : bug, imports.c314; // also allows qualified module w/o selective import
+import imports.c314, imports.pkg313.c313 : bug; // unchanged

test/fail_compilation/enh13855a.d

@@ -0,0 +1,8 @@
+/*
+TEST_OUTPUT:
+----
+fail_compilation/enh13855a.d(7): Error: module imports.c314 import 'enh13855a' not found
+----
+*/
+import imports.c314 : bug, enh13855a; // unqualified module would be ambiguous, parsed as symbol
+import enh13855a, imports.c314 : bug; // works unchanged

test/fail_compilation/enh13855b.d

@@ -0,0 +1,10 @@
+/*
+REQUIRED_ARGS: -vcolumns
+TEST_OUTPUT:
+----
+fail_compilation/enh13855b.d(9,47): Error: ';' expected
+fail_compilation/enh13855b.d(9,52): Error: no identifier for declarator c314
+----
+*/
+import imports.pkg313.c313 : bug, name=imports.c314; // aliased module would require arbitrary lookahead
+import name=imports.c314, imports.pkg313.c313 : bug; // works, unchanged