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 +1,19 @@
+Allow multiple selective imports from different modules in a single import statement
+
+It is now possible to add imports from a different module after a selective
+import list, when those import are also selective or when the imported module
+has a qualified name.
+
+-------
+import pkg.mod1 : sym1, mod2 : sym2;
+import pkg.mod1 : sym1, sym2, pkg.mod2;
+-------
+
+Unqualified modules or renamed imports following a selective import will be
+parsed as part of the selective import list, not as separate modules.
+
+-------
+import pkg.mod1 : sym1, mod2;           // selectively imports mod2 from pkg.mod1
+import pkg.mod1 : sym1, name=mod2;      // selectively imports mod2 as name from pkg.mod1
+import pkg.mod1 : sym1, name=pkg1.mod1; // parsing renamed selective imports fails due to qualfier
+-------

src/dmd/parse.d

@@ -3282,9 +3282,15 @@ final class Parser(AST) : 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